K&R exercise 1-21

Question

Exercise 1-21. Write a program entab that replaces strings of blanks by the minimum number of tabs and blanks to achieve the same spacing. Use the same tab stops as for detab. When either a tab or a single blank would suffice to reach a tab stop, which should be given preference?

The code I wrote for the exercise:

#include <stdio.h>

#define TAB 8

void entab(int space);

int main()
{
    int c, i;

    i = 0;

    while ((c = getchar()) != EOF)
    {
        if (c == ' ')
            ++i;

        else if (c == '\t')
            i = i + TAB;

        else
        {
            entab(i);
            i = 0;
            putchar(c);
        }
    }

    return 0;
}

void entab(int space)
{
    int i;

    for (i = 1; space >= TAB; ++i)
        if (i == TAB)
        {
            putchar('\t');
            i = 0;
            space = space - TAB;
        }

    for (i = 0; i < space; ++i)
        putchar(' ');
}

But when I give it the input "3 blank spaces + a tab + 5 blank spaces + j" the output I get is longer than the input :

The first line in the picture is the input and the second line is the output

Update:

Rewrote the code based on one of the comments and one of the answers into this:

#include <stdio.h>

#define TAB 8

void entab(int space);

int main()
{
    int c, i, l;

    i = l = 0;

    while ((c = getchar()) != EOF)
    {
        ++l;

        if (c == ' ')
            ++i;

        else if (c == '\t')
            i = i + TAB - (l%8);

        else
        {
            entab(i);
            i = 0;
            putchar(c);
        }

        if (c == '\n')
            l = 0;
    }

    return 0;
}


void entab(int space)
{
    int i;

    for (i = 0; space >= TAB; ++i)
        if (i == TAB)
        {
            putchar('\t');
            i = -1;
            space = space - TAB;
        }

    for (i = 0; i < space; ++i)
        putchar(' ');
}

but now the problem is that it's always one characters less than the input:

Answer 1

The output you are seeing is longer than the input because the program is replacing each space with a tab and some spaces. You should modify the entab function to replace strings of blanks with a tab followed by the appropriate number of spaces.

    #include <stdio.h>

#define TAB 8

void entab(int space);

int main()
{
    int c, i, l;

    i = l = 0;

    while ((c = getchar()) != EOF)
    {
        ++l;

        if (c == ' ')
            ++i;
        else
        {
            entab(i);
            i = 0;
            putchar(c);
        }

        if (c == '\n')
            l = 0;
    }

    return 0;
}

void entab(int space)
{
    int i, num_tabs, num_spaces;

    if (space == 0)
        return;

    num_tabs = space / TAB;
    num_spaces = space % TAB;

    for (i = 0; i < num_tabs; ++i)
        putchar('\t');

    for (i = 0; i < num_spaces; ++i)
        putchar(' ');
}

Answer 2

Couldn't fix the code above so I rewrote it entirely and methinks it works if anyone's interested:

#include <stdio.h>

#define TAB 8

int main()
{
    int i, c, l;

    i = l = 0;

    while ((c = getchar()) != EOF)
    {
        if (c == ' ')
        {
            ++i;
            if ((l%8 + i) == TAB)
            {
                putchar('\t');
                l = i = 0;
            }
        }
        

        else if (c == '\n')
        {
            putchar('\n');
            l = i = 0;
        }

        else if (c == '\t')
        {
            putchar('\t');
            l = i = 0;
        }

        else
        {
            ++l;

            while (i)
            {
                putchar(' ');
                --i;
                ++l;
            }

            putchar(c);
        }
    }

    return 0;
}