modified stats exponential distribution procedures to use loc and scale

doc/specs/stdlib_stats_distribution_exponential.md

@@ -15,20 +15,29 @@ Experimental
 ### Description
 
 An exponential distribution is the distribution of time between events in a Poisson point process.
-The inverse scale parameter `lambda` specifies the average time between events (\(\lambda\)), also called the rate of events.
+The inverse `scale` parameter `lambda` specifies the average time between events (\(\lambda\)), also called the rate of events. The location `loc` specifies the value by which the distribution is shifted.
 
-Without argument, the function returns a random sample from the standard exponential distribution \(E(\lambda=1)\).
+Without argument, the function returns a random sample from the unshifted standard exponential distribution \(E(\lambda=1)\) or \(E(loc=0, scale=1)\).
 
-With a single argument, the function returns a random sample from the exponential distribution \(E(\lambda=\text{lambda})\).
+With a single argument of type `real`, the function returns a random sample from the exponential distribution \(E(\lambda=\text{lambda})\). 
 For complex arguments, the real and imaginary parts are sampled independently of each other.
 
-With two arguments, the function returns a rank-1 array of exponentially distributed random variates.
+With one argument of type `real` and one argument of type `integer`, the function returns a rank-1 array of exponentially distributed random variates for (E(\lambda=\text{lambda})\).
+
+With two arguments of type `real`, the function returns a random sample from the exponential distribution \(E(loc=loc, scale=scale)\).
+For complex arguments, the real and imaginary parts are sampled independently of each other.
+
+With two arguments of type `real` and one argument of type `integer`, the function returns a rank-1 array of exponentially distributed random variates for \(E(loc=loc, scale=scale)\).
 
 @note
 The algorithm used for generating exponential random variates is fundamentally limited to double precision.[^1]
 
 ### Syntax
 
+`result = ` [[stdlib_stats_distribution_exponential(module):rvs_exp(interface)]] `([loc, scale] [[, array_size]])`
+
+or
+
 `result = ` [[stdlib_stats_distribution_exponential(module):rvs_exp(interface)]] `([lambda] [[, array_size]])`
 
 ### Class
@@ -40,13 +49,21 @@ Elemental function
 `lambda`: optional argument has `intent(in)` and is a scalar of type `real` or `complex`.
 If `lambda` is `real`, its value must be positive. If `lambda` is `complex`, both the real and imaginary components must be positive.
 
+`loc`: optional argument has `intent(in)` and is a scalar of type `real` or `complex`. 
+
+`scale`: optional argument has `intent(in)` and is a positive scalar of type `real` or `complex`.
+If `scale` is `real`, its value must be positive. If `scale` is `complex`, both the real and imaginary components must be positive.
+
 `array_size`: optional argument has `intent(in)` and is a scalar of type `integer` with default kind.
 
 ### Return value
 
-The result is a scalar or rank-1 array with a size of `array_size`, and the same type as `lambda`.
+If `lambda` is passed, the result is a scalar or rank-1 array with a size of `array_size`, and the same type as `lambda`.
 If `lambda` is non-positive, the result is `NaN`.
 
+If `loc` and `scale` are passed, the result is a scalar or rank-1 array with a size of `array_size`, and the same type as `scale`.
+If `scale` is non-positive, the result is `NaN`.
+
 ### Example
 
 ```fortran
@@ -69,8 +86,14 @@ For a complex variable \(z=(x + y i)\) with independent real \(x\) and imaginary
 
 $$f(x+\mathit{i}y)=f(x)f(y)=\begin{cases} \lambda_{x} \lambda_{y} e^{-(\lambda_{x} x + \lambda_{y} y)} &x\geqslant 0, y\geqslant 0 \\\\ 0 &\text{otherwise}\end{cases}$$
 
+Instead of of the inverse scale parameter `lambda`, it is possible to pass `loc` and `scale`, where \(scale = \frac{1}{\lambda}\) and `loc` specifies the value by which the distribution is shifted.
+
 ### Syntax
 
+`result = ` [[stdlib_stats_distribution_exponential(module):pdf_exp(interface)]] `(x, loc, scale)`
+
+or
+
 `result = ` [[stdlib_stats_distribution_exponential(module):pdf_exp(interface)]] `(x, lambda)`
 
 ### Class
@@ -84,11 +107,17 @@ Elemental function
 `lambda`: has `intent(in)` and is a scalar of type `real` or `complex`.
 If `lambda` is `real`, its value must be positive. If `lambda` is `complex`, both the real and imaginary components must be positive.
 
+`loc`: has `intent(in)` and is a scalar of type `real` or `complex`.
+
+`scale`: has `intent(in)` and is a positive scalar of type `real` or `complex`.
+If `scale` is `real`, its value must be positive. If `scale` is `complex`, both the real and imaginary components must be positive.
+
 All arguments must have the same type.
 
 ### Return value
 
-The result is a scalar or an array, with a shape conformable to the arguments, and the same type as the input arguments. If `lambda` is non-positive, the result is `NaN`.
+The result is a scalar or an array, with a shape conformable to the arguments, and the same type as the input arguments. If non-positive `lambda` or `scale`, the result is `NaN`.
+
 
 ### Example
 
@@ -112,8 +141,14 @@ For a complex variable  \(z=(x + y i)\) with independent real \(x\) and imaginar
 
 $$F(x+\mathit{i}y)=F(x)F(y)=\begin{cases} (1 - e^{-\lambda_{x} x})(1 - e^{-\lambda_{y} y}) &x\geqslant 0, \;\; y\geqslant 0 \\\\ 0 & \text{otherwise} \end{cases}$$
 
+Alternative to the inverse scale parameter `lambda`, it is possible to pass `loc` and `scale`, where \(scale = \frac{1}{\lambda}\) and `loc` specifies the value by which the distribution is shifted.
+
 ### Syntax
 
+`result = ` [[stdlib_stats_distribution_exponential(module):cdf_exp(interface)]] `(x, loc, scale)`
+
+or
+
 `result = ` [[stdlib_stats_distribution_exponential(module):cdf_exp(interface)]] `(x, lambda)`
 
 ### Class
@@ -127,11 +162,16 @@ Elemental function
 `lambda`: has `intent(in)` and is a scalar of type `real` or `complex`.
 If `lambda` is `real`, its value must be positive. If `lambda` is `complex`, both the real and imaginary components must be positive.
 
+`loc`: has `intent(in)` and is a scalar of type `real` or `complex`.
+
+`scale`: has `intent(in)` and is a positive scalar of type `real` or `complex`.
+If `scale` is `real`, its value must be positive. If `scale` is `complex`, both the real and imaginary components must be positive.
+
 All arguments must have the same type.
 
 ### Return value
 
-The result is a scalar or an array, with a shape conformable to the arguments, and the same type as the input arguments. If `lambda` is non-positive, the result is `NaN`.
+The result is a scalar or an array, with a shape conformable to the arguments, and the same type as the input arguments. With non-positive `lambda` or `scale`, the result is `NaN`.
 
 ### Example
 

example/stats_distribution_exponential/example_exponential_cdf.f90

@@ -4,60 +4,80 @@ program example_exponential_cdf
                                                    rexp => rvs_exp
 
   implicit none
-  real, dimension(2, 3, 4) :: x, lambda
+  real, dimension(2, 3, 4) :: x, loc, scale
   real :: xsum
-  complex :: scale
+  complex :: cloc, cscale
   integer :: seed_put, seed_get, i
 
   seed_put = 1234567
   call random_seed(seed_put, seed_get)
 
-  ! standard exponential cumulative distribution at x=1.0
+  ! standard exponential cumulative distribution at x=1.0 with loc=0.0, scale=1.0
+  print *, exp_cdf(1.0, 0.0, 1.0)
+  ! 0.632120550
+
+  ! standard exponential cumulative distribution at x=1.0 with lambda=1.0
   print *, exp_cdf(1.0, 1.0)
   ! 0.632120550
 
   ! cumulative distribution at x=2.0 with lambda=2
   print *, exp_cdf(2.0, 2.0)
   ! 0.981684387
 
-  ! cumulative distribution at x=2.0 with lambda=-1.0 (out of range)
-  print *, exp_cdf(2.0, -1.0) 
+  ! cumulative distribution at x=2.0 with loc=0.0 and scale=0.5 (equivalent of lambda=2)
+  print *, exp_cdf(2.0, 0.0, 0.5)
+  ! 0.981684387
+
+  ! cumulative distribution at x=2.5 with loc=0.5 and scale=0.5 (equivalent of lambda=2)
+  print *, exp_cdf(2.5, 0.5, 0.5)
+  ! 0.981684387
+
+  ! cumulative distribution at x=2.0 with loc=0.0 and scale=-1.0 (out of range)
+  print *, exp_cdf(2.0, 0.0, -1.0)
   ! NaN
 
+  ! cumulative distribution at x=0.5 with loc=1.0 and scale=1.0, putting x below the minimum
+  print *, exp_cdf(0.5, 1.0, 1.0)
+  ! 0.00000000
+
    ! standard exponential random variates array
-  x = reshape(rexp(0.5, 24), [2, 3, 4])
+  x = reshape(rexp(0.0, 2.0, 24), [2, 3, 4])
 
   ! a rank-3 exponential cumulative distribution
-  lambda(:, :, :) = 0.5
-  print *, exp_cdf(x, lambda)
+  loc(:, :, :)   = 0.0
+  scale(:, :, :) = 2.0
+  print *, exp_cdf(x, loc, scale)
   ! 0.301409245  0.335173965  5.94930053E-02  0.113003314
   ! 0.365694344  0.583515942  0.113774836     0.838585377
   ! 0.509324908  0.127967060  0.857194781     0.893231630
   ! 0.355383813  0.470882893  0.574203610     0.799321830
   ! 0.546216846  0.111995399  0.801794767     0.922525287
-  ! 0.937719882  0.301136374  3.44503522E-02  0.134661376 
+  ! 0.937719882  0.301136374  3.44503522E-02  0.134661376
+
 
-  ! cumulative distribution array where lambda<=0.0 for certain elements 
-  print *, exp_cdf([1.0, 1.0, 1.0], [1.0, 0.0, -1.0])
+  ! cumulative distribution array where scale<=0.0 for certain elements
+  print *, exp_cdf([1.0, 1.0, 1.0], [0.0, 0.0, 0.0], [1.0, 0.0, -1.0])
   ! 0.632120550  NaN NaN
 
-  ! `cdf_exp` is pure and, thus, can be called concurrently 
+  ! `cdf_exp` is pure and, thus, can be called concurrently
   xsum = 0.0
   do concurrent (i=1:size(x,3))
-    xsum = xsum + sum(exp_cdf(x(:,:,i), lambda(:,:,i)))
+    xsum = xsum + sum(exp_cdf(x(:,:,i), loc(:,:,i), scale(:,:,i)))
   end do
   print *, xsum
   ! 11.0886612
 
-  ! complex exponential cumulative distribution at (0.5,0.5) with real part of
-  ! lambda=0.5 and imaginary part of lambda=1.0
-  scale = (0.5, 1.0)
-  print *, exp_cdf((0.5, 0.5), scale)
+  ! complex exponential cumulative distribution at (0.5, 0.0, 2) with real part of
+  ! scale=2 and imaginary part of scale=1.0
+  cloc   = (0.0, 0.0)
+  cscale = (2, 1.0)
+  print *, exp_cdf((0.5, 0.5), cloc, cscale)
   ! 8.70351046E-02
 
-  ! As above, but with lambda%im < 0 
-  scale = (1.0, -2.0)
-  print *, exp_cdf((1.5, 1.0), scale)
+  ! As above, but with scale%im < 0
+  cloc   = (0.0, 0.0)
+  cscale = (1.0, -2.0)
+  print *, exp_cdf((1.5, 1.0), cloc, cscale)
   ! NaN
 
 end program example_exponential_cdf

example/stats_distribution_exponential/example_exponential_pdf.f90

@@ -4,59 +4,74 @@ program example_exponential_pdf
                                                     rexp => rvs_exp
 
   implicit none
-  real, dimension(2, 3, 4) :: x, lambda
+  real, dimension(2, 3, 4) :: x, loc, scale
   real :: xsum
-  complex :: scale
+  complex :: cloc, cscale
   integer :: seed_put, seed_get, i
 
   seed_put = 1234567
   call random_seed(seed_put, seed_get)
 
-  ! probability density at x=1.0 in standard exponential
+  ! probability density at x=1.0 with loc=0 and scale=1.0
+  print *, exp_pdf(1.0, 0.0, 1.0)
+  ! 0.367879450
+
+  ! probability density at x=1.0 with lambda=1.0
   print *, exp_pdf(1.0, 1.0)
   ! 0.367879450
 
   ! probability density at x=2.0 with lambda=2.0
-  print *, exp_pdf(2.0, 2.0) 
+  print *, exp_pdf(2.0, 2.0)
+  ! 3.66312787E-02
+
+  ! probability density at x=2.0 with loc=0.0 and scale=0.5 (lambda=2.0)
+  print *, exp_pdf(2.0, 0.0, 0.5)
+  ! 3.66312787E-02
+
+  ! probability density at x=1.5 with loc=0.5 and scale=0.5 (lambda=2.0)
+  print *, exp_pdf(2.5, 0.5, 0.5)
   ! 3.66312787E-02
 
-  ! probability density at x=2.0 with lambda=-1.0 (out of range)
-  print *, exp_pdf(2.0, -1.0) 
+  ! probability density at x=2.0 with loc=0.0 and scale=-1.0 (out of range)
+  print *, exp_pdf(2.0, 0.0, -1.0)
   ! NaN
 
-  ! standard exponential random variates array  
-  x = reshape(rexp(0.5, 24), [2, 3, 4])
+  ! standard exponential random variates array
+  x = reshape(rexp(0.0, 2.0, 24), [2, 3, 4])
 
   ! a rank-3 exponential probability density
-  lambda(:, :, :) = 0.5
-  print *, exp_pdf(x, lambda)
+  loc(:, :, :)   = 0.0
+  scale(:, :, :) = 2.0
+  print *, exp_pdf(x, loc, scale)
   ! 0.349295378      0.332413018     0.470253497     0.443498343      0.317152828
   ! 0.208242029      0.443112582     8.07073265E-02  0.245337561      0.436016470
   ! 7.14025944E-02   5.33841923E-02  0.322308093     0.264558554      0.212898195
   ! 0.100339092      0.226891592     0.444002301     9.91026312E-02   3.87373678E-02
-  ! 3.11400592E-02   0.349431813     0.482774824     0.432669312     
+  ! 3.11400592E-02   0.349431813     0.482774824     0.432669312
 
-  ! probability density array where lambda<=0.0 for certain elements 
-  print *, exp_pdf([1.0, 1.0, 1.0], [1.0, 0.0, -1.0])
+  ! probability density array where scale<=0.0 for certain elements (loc = 0.0)
+  print *, exp_pdf([1.0, 1.0, 1.0], [0.0, 0.0, 0.0], [1.0, 0.0, -1.0])
   ! 0.367879450  NaN NaN
 
-  ! `pdf_exp` is pure and, thus, can be called concurrently 
+  ! `pdf_exp` is pure and, thus, can be called concurrently
   xsum = 0.0
   do concurrent (i=1:size(x,3))
-    xsum = xsum + sum(exp_pdf(x(:,:,i), lambda(:,:,i)))
+    xsum = xsum + sum(exp_pdf(x(:,:,i), loc(:,:,i), scale(:,:,i)))
   end do
   print *, xsum
   ! 6.45566940
 
-  ! complex exponential probability density function at (1.5,1.0) with real part
-  ! of lambda=1.0 and imaginary part of lambda=2.0
-  scale = (1.0, 2.)
-  print *, exp_pdf((1.5, 1.0), scale)
+  ! complex exponential probability density function at (1.5, 0.0, 1.0) with real part
+  ! of scale=1.0 and imaginary part of scale=0.5
+  cloc   = (0.0, 0.0)
+  cscale = (1.0, 0.5)
+  print *, exp_pdf((1.5, 1.0), cloc, cscale)
   ! 6.03947677E-02
 
-  ! As above, but with lambda%re < 0 
-  scale = (-1.0, 2.)
-  print *, exp_pdf((1.5, 1.0), scale)
+  ! As above, but with scale%re < 0
+  cloc   = (0.0, 0.0)
+  cscale = (-1.0, 2.0)
+  print *, exp_pdf((1.5, 1.0), cloc, cscale)
   ! NaN
 
 end program example_exponential_pdf

example/stats_distribution_exponential/example_exponential_rvs.f90

@@ -3,30 +3,38 @@ program example_exponential_rvs
   use stdlib_stats_distribution_exponential, only: rexp => rvs_exp
 
   implicit none
-  complex :: scale
+  complex :: cloc, cscale
   integer :: seed_put, seed_get
 
   seed_put = 1234567
   call random_seed(seed_put, seed_get)
 
-  print *, rexp()         !single standard exponential random variate
-
-! 0.358690143
-
-  print *, rexp(2.0)       !exponential random variate with lambda=2.0
-
-! 0.816459715
-
-  print *, rexp(0.3, 10)   !an array of 10 variates with lambda=0.3
-
-!  1.84008647E-02  3.59742008E-02  0.136567295  0.262772143  3.62352766E-02
-!  0.547133625  0.213591918  4.10784185E-02  0.583882213  0.671128035
-
-  scale = (2.0, 0.7)
-  print *, rexp(scale)
-!single complex exponential random variate with real part of lambda=2.0;
-!imagainary part of lambda=0.7
-
-! (1.41435969,4.081114382E-02)
+  ! single standard exponential random variate
+  print *, rexp()
+  ! 0.358690143
+
+  ! exponential random variate with loc=0 and scale=0.5 (lambda=2)
+  print *, rexp(0.0, 0.5)
+  ! 0.122672431
+
+  ! exponential random variate with lambda=2
+   print *, rexp(2.0)
+  ! 0.204114929
+
+  ! exponential random variate with loc=0.6 and scale=0.2 (lambda=5)
+  print *, rexp(0.6, 0.2)
+  ! 0.681645989
+
+  ! an array of 10 variates with loc=0.0 and scale=3.0 (lambda=1/3)
+  print *, rexp(0.0, 3.0, 10)
+  ! 1.36567295       2.62772131      0.362352759       5.47133636       2.13591909
+  ! 0.410784155      5.83882189      6.71128035        1.31730068       1.90963650
+
+  ! single complex exponential random variate with real part of scale=0.5 (lambda=2.0);
+  ! imagainary part of scale=1.6 (lambda=0.625)
+  cloc   = (0.0, 0.0)
+  cscale = (0.5, 1.6)
+  print *, rexp(cloc, cscale)
+  ! (0.426896989,2.56968451)
 
 end program example_exponential_rvs

src/stdlib_stats_distribution_exponential.fypp

@@ -9,7 +9,7 @@ module stdlib_stats_distribution_exponential
     implicit none
     private
 
-    integer :: ke(0:255)
+    integer  :: ke(0:255)
     real(dp) :: we(0:255), fe(0:255)
     logical  :: zig_exp_initialized = .false.
 
@@ -26,14 +26,26 @@ module stdlib_stats_distribution_exponential
     !! ([Specification](../page/specs/stdlib_stats_distribution_exponential.html#
     !! rvs_exp-exponential-distribution-random-variates))
     !!
-        module procedure rvs_exp_0_rsp                 !0 dummy variable
+        module procedure rvs_exp_0_rsp                        !0 dummy variable
+
+        ! new interfaces using loc and scale
 
         #:for k1, t1 in RC_KINDS_TYPES
-        module procedure rvs_exp_${t1[0]}$${k1}$       !1 dummy variable
+        module procedure rvs_exp_${t1[0]}$${k1}$              !1 dummy variable
         #:endfor
 
         #:for k1, t1 in RC_KINDS_TYPES
-        module procedure rvs_exp_array_${t1[0]}$${k1}$ !2 dummy variables
+        module procedure rvs_exp_array_${t1[0]}$${k1}$        !2 dummy variables
+        #:endfor
+
+        ! original interfaces using lambda
+
+        #:for k1, t1 in RC_KINDS_TYPES
+        module procedure rvs_exp_lambda_${t1[0]}$${k1}$       !1 dummy variable
+        #:endfor
+
+        #:for k1, t1 in RC_KINDS_TYPES
+        module procedure rvs_exp_array_lambda_${t1[0]}$${k1}$ !2 dummy variables
         #:endfor
     end interface rvs_exp
 
@@ -46,9 +58,17 @@ module stdlib_stats_distribution_exponential
     !! ([Specification](../page/specs/stdlib_stats_distribution_exponential.html#
     !! pdf_exp-exponential-distribution-probability-density-function))
     !!
+        ! new interfaces using loc and scale
+
         #:for k1, t1 in RC_KINDS_TYPES
         module procedure pdf_exp_${t1[0]}$${k1}$
         #:endfor
+
+        ! original interfaces using lambda
+
+        #:for k1, t1 in RC_KINDS_TYPES
+        module procedure pdf_exp_lambda_${t1[0]}$${k1}$
+        #:endfor
     end interface pdf_exp
 
 
@@ -60,9 +80,17 @@ module stdlib_stats_distribution_exponential
     !! ([Specification](../page/specs/stdlib_stats_distribution_exponential.html#
     !! cdf_exp-exponential-distribution-cumulative-distribution-function))
     !!
+        ! new interfaces using loc and scale
+
         #:for k1, t1 in RC_KINDS_TYPES
         module procedure cdf_exp_${t1[0]}$${k1}$
         #:endfor
+
+        ! original interfaces using lambda
+
+        #:for k1, t1 in RC_KINDS_TYPES
+        module procedure cdf_exp_lambda_${t1[0]}$${k1}$
+        #:endfor
     end interface cdf_exp
 
 
@@ -114,7 +142,7 @@ contains
     #:for k1, t1 in REAL_KINDS_TYPES
     impure function rvs_exp_0_${t1[0]}$${k1}$( ) result(res)
     !
-    ! Standard exponential random variate (lambda=1)
+    ! Standard exponential random variate (lambda=1; scale=1/lambda=1)
     !
         ${t1}$ :: res, x
         ${t1}$, parameter :: r = 7.69711747013104972_${k1}$
@@ -153,18 +181,18 @@ contains
 
 
     #:for k1, t1 in REAL_KINDS_TYPES
-    impure elemental function rvs_exp_${t1[0]}$${k1}$(lambda) result(res)
+    impure elemental function rvs_exp_${t1[0]}$${k1}$(loc, scale) result(res)
     !
     ! Exponential distributed random variate
     !
-        ${t1}$, intent(in) :: lambda
+        ${t1}$, intent(in) :: loc, scale
         ${t1}$ :: res
 
-        if (lambda <= 0._${k1}$) then
+        if (scale <= 0._${k1}$) then
             res = ieee_value(1._${k1}$, ieee_quiet_nan)
         else
             res = rvs_exp_0_${t1[0]}$${k1}$(  )
-            res = res / lambda
+            res = res * scale + loc
         end if
     end function rvs_exp_${t1[0]}$${k1}$
 
@@ -174,13 +202,13 @@ contains
 
 
     #:for k1, t1 in CMPLX_KINDS_TYPES
-    impure elemental function rvs_exp_${t1[0]}$${k1}$(lambda) result(res)
-        ${t1}$, intent(in) :: lambda
+    impure elemental function rvs_exp_${t1[0]}$${k1}$(loc, scale) result(res)
+        ${t1}$, intent(in) :: loc, scale
         ${t1}$ :: res
         real(${k1}$) :: tr, ti
 
-        tr = rvs_exp_r${k1}$(lambda % re)
-        ti = rvs_exp_r${k1}$(lambda % im)
+        tr = rvs_exp_r${k1}$(loc%re, scale%re)
+        ti = rvs_exp_r${k1}$(loc%im, scale%im)
         res = cmplx(tr, ti, kind=${k1}$)
     end function rvs_exp_${t1[0]}$${k1}$
 
@@ -190,14 +218,14 @@ contains
 
 
     #:for k1, t1 in REAL_KINDS_TYPES
-    impure function rvs_exp_array_${t1[0]}$${k1}$(lambda, array_size) result(res)
-        ${t1}$, intent(in) :: lambda
+    impure function rvs_exp_array_${t1[0]}$${k1}$(loc, scale, array_size) result(res)
+        ${t1}$, intent(in) :: loc, scale
         integer, intent(in) :: array_size
         ${t1}$ :: res(array_size), x, re
         ${t1}$, parameter :: r = 7.69711747013104972_${k1}$
         integer :: jz, iz, i
 
-        if (lambda <= 0._${k1}$) then
+        if (scale <= 0._${k1}$) then
             res = ieee_value(1._${k1}$, ieee_quiet_nan)
             return
         end if
@@ -228,7 +256,7 @@ contains
                     end if
                end do L1
             endif
-            res(i) = re / lambda
+            res(i) = re * scale + loc
         end do
     end function rvs_exp_array_${t1[0]}$${k1}$
 
@@ -238,16 +266,16 @@ contains
 
 
     #:for k1, t1 in CMPLX_KINDS_TYPES
-    impure function rvs_exp_array_${t1[0]}$${k1}$(lambda, array_size) result(res)
-        ${t1}$, intent(in) :: lambda
+    impure function rvs_exp_array_${t1[0]}$${k1}$(loc, scale, array_size) result(res)
+        ${t1}$, intent(in) :: loc, scale
         integer, intent(in) :: array_size
         ${t1}$ :: res(array_size)
         integer :: i
         real(${k1}$) :: tr, ti
 
         do i = 1, array_size
-            tr = rvs_exp_r${k1}$(lambda % re)
-            ti = rvs_exp_r${k1}$(lambda % im)
+            tr = rvs_exp_r${k1}$(loc%re, scale%re)
+            ti = rvs_exp_r${k1}$(loc%im, scale%im)
             res(i) = cmplx(tr, ti, kind=${k1}$)
         end do
     end function rvs_exp_array_${t1[0]}$${k1}$
@@ -258,19 +286,19 @@ contains
 
 
     #:for k1, t1 in REAL_KINDS_TYPES
-    elemental function pdf_exp_${t1[0]}$${k1}$(x, lambda) result(res)
+    elemental function pdf_exp_${t1[0]}$${k1}$(x, loc, scale) result(res)
     !
     ! Exponential Distribution Probability Density Function
     !
-        ${t1}$, intent(in) :: x, lambda
+        ${t1}$, intent(in) :: x, loc, scale
         real(${k1}$) :: res
 
-        if (lambda <= 0._${k1}$) then
+        if (scale <= 0._${k1}$) then
             res = ieee_value(1._${k1}$, ieee_quiet_nan)
-        else if (x < 0._${k1}$) then
+        else if (x < loc) then
             res = 0._${k1}$
         else
-            res = exp(- x * lambda) * lambda
+            res = exp(- (x - loc) / scale) / scale
         end if
     end function pdf_exp_${t1[0]}$${k1}$
 
@@ -280,12 +308,12 @@ contains
 
 
     #:for k1, t1 in CMPLX_KINDS_TYPES
-    elemental function pdf_exp_${t1[0]}$${k1}$(x, lambda) result(res)
-        ${t1}$, intent(in) :: x, lambda
+    elemental function pdf_exp_${t1[0]}$${k1}$(x, loc, scale) result(res)
+        ${t1}$, intent(in) :: x, loc, scale
         real(${k1}$) :: res
 
-        res = pdf_exp_r${k1}$(x % re, lambda % re)
-        res = res * pdf_exp_r${k1}$(x % im, lambda % im)
+        res = pdf_exp_r${k1}$(x%re, loc%re, scale%re)
+        res = res * pdf_exp_r${k1}$(x%im, loc%im, scale%im)
     end function pdf_exp_${t1[0]}$${k1}$
 
     #:endfor
@@ -294,19 +322,19 @@ contains
 
 
     #:for k1, t1 in REAL_KINDS_TYPES
-    elemental function cdf_exp_${t1[0]}$${k1}$(x, lambda) result(res)
+    elemental function cdf_exp_${t1[0]}$${k1}$(x, loc, scale) result(res)
     !
     ! Exponential Distribution Cumulative Distribution Function
     !
-        ${t1}$, intent(in) :: x, lambda
+        ${t1}$, intent(in) :: x, loc, scale
         real(${k1}$) :: res
 
-        if (lambda <= 0._${k1}$) then
+        if (scale <= 0._${k1}$) then
             res = ieee_value(1._${k1}$, ieee_quiet_nan)
-        else if (x < 0._${k1}$) then
+        else if (x < loc) then
             res = 0._${k1}$
         else
-            res = 1.0_${k1}$ - exp(- x * lambda)
+            res = 1.0_${k1}$ - exp(- (x - loc) / scale)
         end if
     end function cdf_exp_${t1[0]}$${k1}$
 
@@ -316,14 +344,147 @@ contains
 
 
     #:for k1, t1 in CMPLX_KINDS_TYPES
-    elemental function cdf_exp_${t1[0]}$${k1}$(x, lambda) result(res)
-        ${t1}$, intent(in) :: x, lambda
+    elemental function cdf_exp_${t1[0]}$${k1}$(x, loc, scale) result(res)
+        ${t1}$, intent(in) :: x, loc, scale
         real(${k1}$) :: res
 
-        res = cdf_exp_r${k1}$(x % re, lambda % re)
-        res = res * cdf_exp_r${k1}$(x % im, lambda % im)
+        res = cdf_exp_r${k1}$(x%re, loc%re, scale%re)
+        res = res * cdf_exp_r${k1}$(x%im, loc%im, scale%im)
     end function cdf_exp_${t1[0]}$${k1}$
 
     #:endfor
 
+
+
+
+    !
+    ! below: wrapper functions for interfaces using lambda (for backward compatibility)
+    !
+
+
+
+
+    #:for k1, t1 in REAL_KINDS_TYPES
+    impure elemental function rvs_exp_lambda_${t1[0]}$${k1}$(lambda) result(res)
+    !
+    ! Exponential distributed random variate
+    !
+        ${t1}$, intent(in) :: lambda
+        ${t1}$ :: res
+
+        res = rvs_exp_${t1[0]}$${k1}$(0._${k1}$, 1.0_${k1}$/lambda)
+    end function rvs_exp_lambda_${t1[0]}$${k1}$
+
+    #:endfor
+
+
+
+
+    #:for k1, t1 in CMPLX_KINDS_TYPES
+    impure elemental function rvs_exp_lambda_${t1[0]}$${k1}$(lambda) result(res)
+        ${t1}$, intent(in) :: lambda
+        ${t1}$ :: res
+        real(${k1}$) :: tr, ti
+
+        tr = rvs_exp_r${k1}$(0._${k1}$, 1.0_${k1}$/lambda%re)
+        ti = rvs_exp_r${k1}$(0._${k1}$, 1.0_${k1}$/lambda%im)
+        res = cmplx(tr, ti, kind=${k1}$)
+    end function rvs_exp_lambda_${t1[0]}$${k1}$
+
+    #:endfor
+
+
+
+
+    #:for k1, t1 in REAL_KINDS_TYPES
+    impure function rvs_exp_array_lambda_${t1[0]}$${k1}$(lambda, array_size) result(res)
+        ${t1}$, intent(in) :: lambda
+        integer, intent(in) :: array_size
+        ${t1}$ :: res(array_size)
+
+        res = rvs_exp_array_${t1[0]}$${k1}$(0._${k1}$, 1.0_${k1}$/lambda, array_size)
+    end function rvs_exp_array_lambda_${t1[0]}$${k1}$
+
+    #:endfor
+
+
+
+
+    #:for k1, t1 in CMPLX_KINDS_TYPES
+    impure function rvs_exp_array_lambda_${t1[0]}$${k1}$(lambda, array_size) result(res)
+        ${t1}$, intent(in) :: lambda
+        integer, intent(in) :: array_size
+        ${t1}$ :: res(array_size)
+        integer :: i
+        real(${k1}$) :: tr, ti
+
+        do i = 1, array_size
+            tr = rvs_exp_r${k1}$(0._${k1}$, 1.0_${k1}$/lambda%re)
+            ti = rvs_exp_r${k1}$(0._${k1}$, 1.0_${k1}$/lambda%im)
+            res(i) = cmplx(tr, ti, kind=${k1}$)
+        end do
+    end function rvs_exp_array_lambda_${t1[0]}$${k1}$
+
+    #:endfor
+
+
+
+
+    #:for k1, t1 in REAL_KINDS_TYPES
+    elemental function pdf_exp_lambda_${t1[0]}$${k1}$(x, lambda) result(res)
+    !
+    ! Exponential Distribution Probability Density Function
+    !
+        ${t1}$, intent(in) :: x, lambda
+        real(${k1}$) :: res
+
+        res = pdf_exp_${t1[0]}$${k1}$(x, 0._${k1}$, 1.0_${k1}$/lambda)
+    end function pdf_exp_lambda_${t1[0]}$${k1}$
+
+    #:endfor
+
+
+
+
+    #:for k1, t1 in CMPLX_KINDS_TYPES
+    elemental function pdf_exp_lambda_${t1[0]}$${k1}$(x, lambda) result(res)
+        ${t1}$, intent(in) :: x, lambda
+        real(${k1}$) :: res
+
+        res = pdf_exp_r${k1}$(x%re, 0._${k1}$, 1.0_${k1}$/lambda%re)
+        res = res * pdf_exp_r${k1}$(x%im, 0._${k1}$, 1.0_${k1}$/lambda%im)
+    end function pdf_exp_lambda_${t1[0]}$${k1}$
+
+    #:endfor
+
+
+
+
+    #:for k1, t1 in REAL_KINDS_TYPES
+    elemental function cdf_exp_lambda_${t1[0]}$${k1}$(x, lambda) result(res)
+    !
+    ! Exponential Distribution Cumulative Distribution Function
+    !
+        ${t1}$, intent(in) :: x, lambda
+        real(${k1}$) :: res
+
+        res = cdf_exp_${t1[0]}$${k1}$(x, 0._${k1}$, 1.0_${k1}$/lambda)
+    end function cdf_exp_lambda_${t1[0]}$${k1}$
+
+    #:endfor
+
+
+
+
+    #:for k1, t1 in CMPLX_KINDS_TYPES
+    elemental function cdf_exp_lambda_${t1[0]}$${k1}$(x, lambda) result(res)
+        ${t1}$, intent(in) :: x, lambda
+        real(${k1}$) :: res
+
+        res = cdf_exp_r${k1}$(x%re, 0._${k1}$, 1.0_${k1}$/lambda%re)
+        res = res * cdf_exp_r${k1}$(x%im, 0._${k1}$, 1.0_${k1}$/lambda%im)
+    end function cdf_exp_lambda_${t1[0]}$${k1}$
+
+    #:endfor
+
 end module stdlib_stats_distribution_exponential

test/stats/test_distribution_exponential.fypp

@@ -51,6 +51,8 @@ contains
 
         print *, ""
         print *, "Test exponential random generator with chi-squared"
+
+        ! using interface for lambda
         freq = 0
         do i = 1, num
             j = 1000 * (1 - exp(- expon_rvs(1.0)))
@@ -66,13 +68,31 @@ contains
         write(*,*) "Chi-squared for exponential random generator is : ", chisq
         call check((chisq < 1143.9), &
             msg="exponential randomness failed chi-squared test", warn=warn)
+
+        ! using interface for loc and scale
+        freq = 0
+        do i = 1, num
+            j = 1000 * (1 - exp(- expon_rvs(0.0, 1.0)))
+            freq(j) = freq(j) + 1
+        end do
+        chisq = 0.0_dp
+        expct = num / array_size
+        do i = 0, array_size - 1
+           chisq = chisq + (freq(i) - expct) ** 2 / expct
+        end do
+        write(*,*) "The critical values for chi-squared with 1000 d. of f. is" &
+            //" 1143.92"
+        write(*,*) "Chi-squared for exponential random generator is : ", chisq
+        call check((chisq < 1143.9), &
+            msg="exponential randomness failed chi-squared test", warn=warn)
+
     end subroutine test_exponential_random_generator
 
 
 
     #:for k1, t1 in RC_KINDS_TYPES
     subroutine test_expon_rvs_${t1[0]}$${k1}$
-        ${t1}$ :: res(10), scale
+        ${t1}$ :: res(10), lambda, loc, scale
         integer, parameter :: k = 5
         integer :: i
         integer :: seed, get
@@ -116,20 +136,35 @@ contains
 
         print *, "Test exponential_distribution_rvs_${t1[0]}$${k1}$"
         seed = 593742186
-        call random_seed(seed, get)
 
+        ! set args
         #:if t1[0] == "r"
             #! for real type
-        scale = 1.5_${k1}$
+        lambda = 1.5_${k1}$
+        loc    = 0._${k1}$
+        scale  = 1.0_${k1}$/lambda
         #:else
             #! for complex type
-        scale = (0.7_${k1}$, 1.3_${k1}$)
+        lambda = (0.7_${k1}$, 1.3_${k1}$)
+        loc    = (0._${k1}$, 0._${k1}$)
+        scale  = cmplx(1.0_${k1}$/lambda%re, 1.0_${k1}$/lambda%im, kind=${k1}$)
         #:endif
 
+        ! tests using interface for lambda
+        call random_seed(seed, get)
+        do i = 1, k
+            res(i) = expon_rvs(lambda)      ! 1 dummy
+        end do
+        res(6:10) = expon_rvs(lambda, k)    ! 2 dummies
+        call check(all(abs(res - ans) < ${k1}$tol),                            &
+            msg="exponential_distribution_rvs_${t1[0]}$${k1}$ failed", warn=warn)
+
+        ! tests using interface for loc and scale
+        call random_seed(seed, get)
         do i = 1, k
-            res(i) = expon_rvs(scale)      ! 1 dummy
+            res(i) = expon_rvs(loc, scale)
         end do
-        res(6:10) = expon_rvs(scale, k)    ! 2 dummies
+        res(6:10) = expon_rvs(loc, scale, k)
         call check(all(abs(res - ans) < ${k1}$tol),                            &
             msg="exponential_distribution_rvs_${t1[0]}$${k1}$ failed", warn=warn)
     end subroutine test_expon_rvs_${t1[0]}$${k1}$
@@ -142,7 +177,7 @@ contains
     #:for k1, t1 in RC_KINDS_TYPES
     subroutine test_expon_pdf_${t1[0]}$${k1}$
 
-        ${t1}$ :: x1, x2(3,4), scale
+        ${t1}$ :: x1, x2(3,4), lambda, loc, scale
         integer :: seed, get
         real(${k1}$) :: res(3,5)
         #:if t1[0] == "r"
@@ -185,21 +220,38 @@ contains
 
         print *, "Test exponential_distribution_pdf_${t1[0]}$${k1}$"
         seed = 123987654
-        call random_seed(seed, get)
+
+        ! set args
         #:if t1[0] == "r"
             #! for real type
-        scale = 1.5_${k1}$
+        lambda = 1.5_${k1}$
+        loc    = 0._${k1}$
+        scale  = 1.0_${k1}$/lambda
         #:else
             #! for complex type
-        scale = (0.3_${k1}$, 1.6_${k1}$)
+        lambda = (0.3_${k1}$, 1.6_${k1}$)
+        loc    = (0._${k1}$, 0._${k1}$)
+        scale  = cmplx(1.0_${k1}$/lambda%re, 1.0_${k1}$/lambda%im, kind=${k1}$)
         #:endif
 
-        x1 = expon_rvs(scale)
-        x2 = reshape(expon_rvs(scale, 12), [3,4])
-        res(:,1) = expon_pdf(x1, scale)
-        res(:, 2:5) = expon_pdf(x2, scale)
+        ! tests using interface for lambda
+        call random_seed(seed, get)
+        x1 = expon_rvs(lambda)
+        x2 = reshape(expon_rvs(lambda, 12), [3,4])
+        res(:,1) = expon_pdf(x1, lambda)
+        res(:, 2:5) = expon_pdf(x2, lambda)
+        call check(all(abs(res - reshape(ans, [3,5])) < ${k1}$tol),            &
+            msg="exponential_distribution_pdf_${t1[0]}$${k1}$ failed", warn=warn)
+
+        ! tests using interface for loc and scale
+        call random_seed(seed, get)
+        x1 = expon_rvs(loc, scale)
+        x2 = reshape(expon_rvs(loc, scale, 12), [3,4])
+        res(:,1) = expon_pdf(x1, loc, scale)
+        res(:, 2:5) = expon_pdf(x2, loc, scale)
         call check(all(abs(res - reshape(ans, [3,5])) < ${k1}$tol),            &
             msg="exponential_distribution_pdf_${t1[0]}$${k1}$ failed", warn=warn)
+
     end subroutine test_expon_pdf_${t1[0]}$${k1}$
 
     #:endfor
@@ -210,7 +262,7 @@ contains
     #:for k1, t1 in RC_KINDS_TYPES
     subroutine test_expon_cdf_${t1[0]}$${k1}$
 
-        ${t1}$ :: x1, x2(3,4), scale
+        ${t1}$ :: x1, x2(3,4), lambda, loc, scale
         integer :: seed, get
         real(${k1}$) :: res(3,5)
         #:if t1[0] == "r"
@@ -252,21 +304,37 @@ contains
 
         print *, "Test exponential_distribution_cdf_${t1[0]}$${k1}$"
         seed = 621957438
-        call random_seed(seed, get)
 
+        ! set args
         #:if t1[0] == "r"
             #! for real type
-        scale = 2.0_${k1}$
+        lambda = 2.0_${k1}$
+        loc    = 0._${k1}$
+        scale  = 1.0_${k1}$/lambda
         #:else
-        scale = (1.3_${k1}$, 2.1_${k1}$)
+        lambda = (1.3_${k1}$, 2.1_${k1}$)
+        loc    = (0._${k1}$, 0._${k1}$)
+        scale  = cmplx(1.0_${k1}$/lambda%re, 1.0_${k1}$/lambda%im, kind=${k1}$)
         #:endif
 
-        x1 = expon_rvs(scale)
-        x2 = reshape(expon_rvs(scale, 12), [3,4])
-        res(:,1) = expon_cdf(x1, scale)
-        res(:, 2:5) = expon_cdf(x2, scale)
+        ! tests using interface for lambda
+        call random_seed(seed, get)
+        x1 = expon_rvs(lambda)
+        x2 = reshape(expon_rvs(lambda, 12), [3,4])
+        res(:,1) = expon_cdf(x1, lambda)
+        res(:, 2:5) = expon_cdf(x2, lambda)
+        call check(all(abs(res - reshape(ans,[3,5])) < ${k1}$tol),             &
+            msg="exponential_distribution_cdf_${t1[0]}$${k1}$ failed", warn=warn)
+
+        ! tests using interface for loc and scale
+        call random_seed(seed, get)
+        x1 = expon_rvs(loc, scale)
+        x2 = reshape(expon_rvs(loc, scale, 12), [3,4])
+        res(:,1) = expon_cdf(x1, loc, scale)
+        res(:, 2:5) = expon_cdf(x2, loc, scale)
         call check(all(abs(res - reshape(ans,[3,5])) < ${k1}$tol),             &
             msg="exponential_distribution_cdf_${t1[0]}$${k1}$ failed", warn=warn)
+
     end subroutine test_expon_cdf_${t1[0]}$${k1}$
 
     #:endfor