K&R Exercise 1-21 - Mental incomprehension

Question

The "impossible" K&R exercise.

"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, say every n columns. Should n be a variable or a symbolic parameter?"

The problem I'm having is, I'm unsure about how to even do this correctly. I know it's not very explanatory, but that's pretty much the problem here. Most of the examples I've seen have counted a number of blanks, and replaced those series with a tab, but this isn't what its asking, I reckon I understand what its asking, but currently feel unable to do this.

Could anyone help :)

Edit: The code I've written so far can be found here.

Answer 1

If your question is "What is this asking me to do?" I think I can help by paraphrasing the original question (posing the same question in a different way).

Write a program that takes as input text with spaces and produces as output visually equivalent text using tabs to the maximum extent possible.

For example, with tabstops every 8 characters, and showing spaces as '.' and tabs as '-';

input;
".foo:...bar;......#comment"
output;
".foo:-bar;-..#comment"

input;
".......-foo:.....bar;......#comment"
output;
"-foo:-.bar;-...#comment"

Write the program so that tabstop parameter n can be varied, i.e. allow values of n other than 8. Be prepared to justify your decision to make n a constant, or alternatively a variable.

Edit I had a look at your code and I think it is more complex than it needs to be. My advice is to do it a character at a time. There's no need to buffer a whole line. Maintain a column count as you read each character ('\n' resets it to zero, '\t' bumps it by 1 or more, other characters increment it). When you see a space (or tab), don't emit anything right away, start your entabbing process, emit zero or more tabs and then spaces later (at '\n' or a non whitespace character, whichever comes first).

A final hint is that a state machine can make this kind of algorithm a lot easier to write, validate, test and read.

Edit 2 In a shameless attempt to get the OP to accept my answer, I have now gone ahead and actually coded a solution myself, based on the hints I offered above and my comment in the discussion.

// K&R Exercise 1-21, entab program, for Stackoverflow.com
#include <stdio.h>
#define N 4     // Tabstop value. Todo, make this a variable, allow
                //  user to modify it using command line

int main()
{
    int col=0, base_col=0, entab=0;

    // Loop replacing spaces with tabs to the maximum extent
    int c=getchar();
    while( c != EOF )
    {

        // Normal state
        if( !entab )
        {

            // If whitespace goto entab state
            if( c==' ' || c=='\t' )
            {
                entab = 1;
                base_col = col;
            }

            // Else emit character
            else
                putchar(c);
        }

        // Entab state
        else
        {

            // Trim trailing whitespace
            if( c == '\n' )
            {
                entab = 0;
                putchar( '\n' );
            }

            // If not whitespace, exit entab state
            else if( c!=' ' && c!='\t' )
            {
                entab = 0;

                // Emit tabs to get close to current column position
                //  eg base_col=1, N=4, col=10
                //  base_col + 3 = 4 (1st time thru loop)
                //  base_col + 4 = 8 (2nd time thru loop)
                while( (base_col + (N-base_col%N)) <= col )
                {
                    base_col += (N-base_col%N);
                    putchar( '\t' );
                }

                // Emit spaces to close onto current column position
                // eg base_col=1, N=4, col=10
                //  base_col -> 8, and two tabs emitted above
                //  base_col + 1 = 9 (1st time thru this loop)
                //  base_col + 1 = 10 (2nd time thru this loop)
                while( (base_col + 1) <= col )
                {
                    base_col++;
                    putchar( ' ' );
                }

                // Emit buffered character after tabs and spaces
                putchar( c );
            }
        }

        // Update current column position for either state
        if( c == '\t' )
            col += (N - col%N); // eg col=1, N=4, col+=3
        else if( c == '\n' )
            col=0;
        else
            col++;

        // End loop
        c = getchar();
    }
    return 0;
}