Exercise 1-20 of the book "C - The Programming Language 2nd Edition" - c

Question

Currently I'm reading the book "C - The Programming Language" and I have a questions to this exercise:
"Write a program 'detab' 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-colmuns. Should n be a variable or a symbolic parameter?"
Let aside the question in the exercise I wrote this program:
#include <stdio.h>
#define COLUMNS 5 /* number of columns for a tab */
int main()
{
char c;
int i;
while ((c = getchar()) != EOF) {
if (c == '\t') {
for (i = 0; i < COLUMNS; i++)
putchar(' ');
}
else
putchar(c);
}
return 0;
}
Then I checked Online for solutions and found this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_BUFFER 1024
#define SPACE ' '
#define TAB '\t'
int CalculateNumberOfSpaces(int Offset, int TabSize)
{
return TabSize - (Offset % TabSize);
}
/* K&R's getline() function from p29 */
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;
}
int main(void)
{
char Buffer[MAX_BUFFER];
int TabSize = 5; /* A good test value */
int i, j, k, l;
while(getline(Buffer, MAX_BUFFER) > 0)
{
for(i = 0, l = 0; Buffer[i] != '\0'; i++)
{
if(Buffer[i] == TAB)
{
j = CalculateNumberOfSpaces(l, TabSize);
for(k = 0; k < j; k++)
{
putchar(SPACE);
l++;
}
}
else
{
putchar(Buffer[i]);
l++;
}
}
}
return 0;
}
My question now is what the difference between my code an the other code is.
I thought the exercise would ask that each \t that occurs should be replaced by n spaces.
Now I don't what the other code does.
Maybe I understood the exercise wrong. If so please explain it to me.
Thanks for your help.
Ok so how about this solution:
#include <stdio.h>
#define TAB 8 /* size of a tab */
int main()
{
char c;
int i,column = 0;
while ((c = getchar()) != EOF) {
if (c == '\t') {
for (i = column; i < TAB; i++){
putchar(' ');
}
column = -1;
}
else{
putchar(c);
}
column++;
}
return 0;
}
New Version:
#include <stdio.h>
#define TAB 8 /* size of a tab */
int main()
{
char c;
int i,column = 0;
while ((c = getchar()) != EOF) {
if(c == '\n')
column = -1;
if(column >= TAB)
column = 0;
if (c == '\t') {
for (i = column; i < TAB; i++){
putchar(' ');
}
column = -1;
}
else{
putchar(c);
}
column++;
}
return 0;
}

Answer 1

You got it wrong. You have to replace each tab by spaces until the next tab stop. That depends exactly on where the cursor is.
So, if there's a tab stop every 5 columns, and you can think of it like:
-----|-----|-----|-----|-----|-----|-----|-----|-----|
Where | is a tab stop and - is a character. Hitting a tab takes you to the next tab stop. So, for example, if you write 3 characters from the beginning of the previous tab stop, and then hit tab, you only have to write 2 spaces.

Answer 2

You don't want to replace every tab by the same number of spaces: some will be replaced by less than that.
Imagine we want TAB to 10 spaces
1234567890123456789012345678901234567890
one<TAB> two<TAB> fortytwo four
^^^^^^^ ^^^^^^^ ^^
7 space 7 space 2