1-20.c

/*
 * Exercise 1-20. Write a program detab function that replaces tabs in the
 * input with the proper number of blanks to space to the next tab stop. Assume
 * a fixed set of tab stops, say every n columns. Should n be a variable or a
 * symbolic parameter.
 *
 * By Faisal Saadatmand
 */

/*
 * Answer: Here, n should be a symbolic constant, for the value of n
 * should remain constant throughout the duration of the program. It could also
 * be a const int.
 */

#include <stdio.h>

#define MAXLEN 1000
#define N 4 /* tabstop for every n columns */

/* functions */
int getLine(char [], int);

/* getLine function: read a line into s, return length */
int getLine(char s[], int lim)
{
	int c, i;

	for (i = 0; i < lim - 1 && (c = getchar()) != EOF && c != '\n'; ++i)
		s[i] = c;
	if (c == '\n') {
		s[i] = c;
		++i;
	}
	s[i] = '\0';
	return i;
}

/* detab function: replaces tabs with the proper number of blanks */
void detab(char in[], char out[])
{
	int i; /* index for read line */
	int j; /* index for modified (written) line */
	int nblanks; /* number of blanks to the next tab stop */

	for (i = j = 0; in[i] != '\0'; ++i)
		if (in[i] == '\t') {
			nblanks = N - (j % N);
			while (nblanks-- > 0)
				out[j++] = ' ';
		} else 
			out[j++] = in[i];
	out[j] = '\0';
}

int main(void)
{
	char in[MAXLEN]; /* currently read line */
	char out[MAXLEN]; /* modified line */

	while (getLine(in, MAXLEN) > 0) {
		detab(in, out);
		printf("%s", out);
	}
	return 0;
}