Usage of <= hangs the program, but < works fine (K&R C Exercise 1-20)

Question

I'm trying to solve problem from K&R C (Exercise 1-20). I've transformed original getline function into int detab(char s[], int lim, int tab). Problems occur in the following part of function:

       while (i <= ttab)
        {
            s[i] = ' ';

            if (i < ttab)
            {
                ++i;
            }

            //++i; //SPIKE
        }
        //--i; //SPIKE

If I run it as written above - program just hangs and does nothing. If I change if (i < ttab) to the if (i <= ttab) program behaves as I expect it to (i becomes higher by 1 than needed). If I uncomment --i; //SPIKE and --i; //SPIKE and comment out:

            if (i < ttab)
            {
                ++i;
            }

then, I am happy with program behavior but not with code appearance.

#include <stdio.h>
#define MAXLINE 1000 // maximum input line size
#define TAB 5 // assumed tab length

int detab(char line[], int maxline, int TabStop);

//prints one line at a time; replaces tabs with TAB spaces
main()
{
    int len; //current line length
    char line[MAXLINE]; //current input line

    while ((len = detab(line, MAXLINE, TAB)) > 0)
    {
        printf("%s\n%d\n", line, len);
    }

    return 0;
}

// getline: read a line into s, return length, replaces each tab in the s by tablen         spaces
int detab(char s[], int lim, int tab)
{
    int c, i;
    int ttab = tab;

    for (i=0; i<lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
{
    if (c == '\t')
    {
        while (ttab < i)
        {
            ttab += tab;
        }
        while (i <= ttab)
        {
            s[i] = ' ';

            if (i < ttab)
            {
                ++i;
            }


            //++i; //SPIKE
        }
        //--i; //SPIKE
    }

    else
    {
        s[i] = c;
    }

}

if (c == '\n')
{
    s[i] = c;
    ++i;
}
s[i] = '\0';

return i;
}

My intention is to get rid of: --i; //SPIKE and --i; //SPIKE and replace it with:

            if (i < ttab)
            {
                ++i;
            }

But why does it cause hanging and how to fix that?

Answer 1

If you take the ++i and --i out, then wheni==ttab, you loop forever, since i never changes again.
You could loop while i<ttab then add a special check for i==ttab after, or you could use a for loop which increments i, and then does i--at the end.

Answer 2

In your code when i value is same as ttab the code bellow is becomes a infinite loop.

while (i <= ttab) //if i== ttab it will come to loop body
        {
            s[i] = ' ';

            if (i < ttab) //if i == ttab  then i never going to increment 
            {
                ++i;
            }


            //++i; //SPIKE
        }