Question mark at the end of the output

Question

Trying to implement detab function which is described in K&R book http://clc-wiki.net/wiki/K%26R2_solutions:Chapter_1:Exercise_20 an faced a problem: when outputting a replaced string there's a question mark in the end of the output. Why?

#include <stdio.h>
#define MAXLINE 9999

void get_text(char output[]);
void detab(char input[], char output[], int tab_size);

int main()
{
    char input[MAXLINE];
    char final[MAXLINE];
    get_text(input);
    detab(input, final, 4);
    for (int i = 0; final[i] != '\0'; ++i)
    {
        putchar(final[i]);
    }
    putchar('\n');
    return 0;
}

void get_text(char output[])
{
    int c;
    int i = 0;
    for (i = 0; i < MAXLINE && (c = getchar()) != EOF; ++i)
    {
        output[i] = c;
    }
    output[i + 1] = '\0';
}

void detab(char input[], char output[], int tab_size)
{
    int c = 0;
    int r = 0;
    for (int i = 0; input[i] != '\0'; ++i)
    {
        c = input[i];
        if(c == '\t')
        {
            for (int t = 0; t < tab_size; ++t)
            {
                output[r] = '.';
                r++;
            }
        }
        else
        {
            output[r] = c;
            r++;
        }
    }
    output[r] = '\0';
}

And here is the output when I'm passing the file with following content 'asdasdads tasdasdasdasdasd sadasdasd':
asdasdads....tasdasdasdasdasd....sadasdasd? (? at the end). Why there is a question mark at the end?

Just out of curiosity, is there a specific reason not to use such stdio functions as `gets` or `printf`? — kuroi neko, Jan 11 '14 at 23:24
@kuroi neko, or is there any reason to write those functions in C while there are python or ruby? :) — Dima Knivets, Jan 11 '14 at 23:25
+1 for not using `gets`. @kuroineko; Why are you suggesting him to use [`gets`](http://stackoverflow.com/a/21068539/2455888)? — haccks, Jan 11 '14 at 23:27
@haccks Just for the vintage feel of good ole K&R exercises :). But you're right, it was not a really smart move to advise a new fellow C programmer to use something that has long been deprecated for very good reasons. — kuroi neko, Jan 11 '14 at 23:34
I seem this is so does not take into account the tab stop position. — BLUEPIXY, Jan 11 '14 at 23:37

score 3 · Accepted Answer · answered Jan 11 '14 at 23:21

output[i + 1] = '\0';

You don't need to add 1 here, it was already done in the loop. (First i is incremented and then i < MAXLINE && (c = getchar()) != EOF is tested, so i is one higher than in the last loop iteration already)

score 2 · Answer 2 · answered Jan 11 '14 at 23:21

I'd guess that's in place of an unprintable character. The characters in final aren't initialized to anything in particular, and you're leaving a gap at the end of the detabbed string when you say:

output[i + 1] = '\0';

output[i], one past the output text, is still uninitialized.

You want:

output[i] = '\0';

Question mark at the end of the output

2 Answers2