initial commit

Author: fsaadatmand

chapter03/3-4a.c

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+/*
+ * Exercise 3-4. In a two's complement number representation, our version of
+ * itoa does not handle the largest negative number, that is, the value of n
+ * equal to -(2^wordsize-1). Explain why not. Modify it to print that value
+ * correctly, regardless of the machine on which it runs. 
+ *
+ *  By Faisal Saadatmand
+ */
+
+/*
+ * ANSWER:
+ * In two's complement number representation, the range of values an int can
+ * hold is from -(2^wordsize-1) to (2^wordsize-1)-1. The most significant bit
+ * is the sign bit; however, it also holds a value for negative numbers; thus,
+ * making the negative limit larger than the positive limit by a value of 1.
+ * When we negate the largest negative number, -(2^wordsize-1), we get a number
+ * that is equal to 2^wordsize-1, which is larger than the largest positive
+ * number, (2^wordsize-1)-1. This will overflow a signed int and cause
+ * unexpected results. To overcome this, we can use an unsigned int for n. The
+ * check for whether n is negative or not is taking care of with the assigned
+ * of n to the int variable sign, in which n is convert to a signed int.
+ *
+ * Alternatively, we could avoid negating n altogether and use the abs function
+ * from stdlib.h to extract the digits. See 3-4a.c.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <limits.h>
+
+#define MAXLEN 1000
+
+void itoa(int, char []);
+void reverse(char []);
+
+/* itoa: convert n to characters in s--abs version */
+void itoa(int n, char s[])
+{
+	int i, sign;
+
+	sign = n;
+	i = 0;
+	do { /* generate digits in revered order */
+		s[i++] = abs(n % 10) + '0'; /* get next digit */
+	} while ((n /= 10) != 0);
+	if (sign < 0)
+		s[i++] = '-';
+	s[i] = '\0';
+	reverse(s);
+}
+
+/* reverse: reverse the order of the characters in the string s */
+void reverse(char s[])
+{
+	int c, i, j;
+
+	for (i = 0, j = strlen(s) - 1; i < j; ++i, --j) {
+		c = s[i];
+		s[i] = s[j];
+		s[j] = c;
+	}
+}
+
+int main(void)
+{
+	char str[MAXLEN];
+
+	itoa(0, str); /* do-while loop is necessary for 0 */
+	printf("%i -> %s\n", 0, str);
+	itoa(INT_MAX, str);
+	printf("%i -> %s\n", INT_MAX, str);
+	itoa(INT_MIN, str);
+	printf("%i -> %s\n", INT_MIN, str);
+	return 0;
+}