Traversing Two Dimensional Array

Question

How come Line 1 and Line 2 have more than ten x(s) when the loop should end at ten given the condition of ii < 10?

I am trying to understand how these two dimensional arrays work, any advice is appreciated.

#include <stdio.h>
int main()
{
    char name[3][10];

    for(int i = 0; i < 3; i++){
        for(int ii = 0; ii < 10; ii++){
            name[i][ii] = 'x';
        }
    }


    printf("Line 1: %s.\n", name[0]);

    printf("Line 2: %s.\n", name[1]);

    printf("Line 3: %s.\n", name[2]);


    return 0;
}

FIX:

Thanks to everyone's comments I managed to fix the code. The problem was that I was not reserving one space for the NULL terminator at the end of the array. Since I wanted ten x(s), the solution is to increase the size of the two dimensional array so that it has space for the NULL terminators.

#include <stdio.h>
int main()
{
    char name[4][11];

    for(int i = 0; i < 3; i++){
        for(int ii = 0; ii < 10; ii++){
            name[i][ii] = 'x';
        }
    }


    printf("Line 1: %s.\n", name[0]);

    printf("Line 2: %s.\n", name[1]);

    printf("Line 3: %s.\n", name[2]);


    return 0;
}

You must reserve one character for your NUL terminator, taht is name[i][9] must be 0. — tadman
– tadman, Commented Apr 7, 2021 at 22:07
Possible dupe target : When does printf(“%s”, char*) stop printing? — J...
– J..., Commented Apr 7, 2021 at 22:12
You might want to fix it, then post your fix as an answer so you can learn from this mistake. Happens to the best of us! — tadman
– tadman, Commented Apr 7, 2021 at 22:14
for(int ii = 0; ii < 10; ii++){ name[i][ii] = 'x'; } --> for(int ii = 0; ii < 9; ii++){name[i][ii] = 'x'; } name[i][ii] = '\0'; — 4386427
– 4386427, Commented Apr 7, 2021 at 22:17
You didn't need to increase the size from 3 to 4. Only the size from 10 to 11 needed to be increased for the null character — risingStark
– risingStark, Commented Apr 7, 2021 at 22:30

4386427 · Accepted Answer · 2021-04-08 06:29:19Z

As it has been discussed in comments, the problem with your code was that you didn't add a NUL to terminate the strings.

However, I'll post an answer as your fix isn't really a fix

Two things to mention:

You don't need to extend the dimension of the array to [4][11] but only to [3][11]. It's only the number of chars in each string that need to be increased to make room for the termination. Changing [3] will give you room for an extra string and that is not what you are looking for. So use [3][11]
Your fix still doesn't add the NUL terminator. Remember that a local variable isn't automatic initialized so you need to do it yourself. In other words - you need some code that explicit sets name[i][10] to NUL.

This is how memory is looking in your original code, your fix and how you really want it.

Below is two ways to get what you want.

Option 1:

char name[3][11];

for(int i = 0; i < 3; i++){
    for(int ii = 0; ii < 10; ii++){
        name[i][ii] = 'x';
    }

    // Add NUL to the end to get a correct C style string
    name[i][ii] = '\0';
}

Option 2:

// Explicit initialize the whole array to zero
char name[3][11] = { 0 };

for(int i = 0; i < 3; i++){
    for(int ii = 0; ii < 10; ii++){
        name[i][ii] = 'x';
    }
}

I prefer option 1 but option 2 will also do the job.

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