The Code exits after the input and output statements

Question

After printing the 2nd printf the code exit and says Run Failed and says exit on level 2 ( whatever the input was )

void main()
{
    int testcases;
    int n_lines ,m_length;
    int test, lines, length;
    char n_m_matrix[n_lines][m_length];
    printf("Enter the no. of Test Cases ");
    scanf("%d",&testcases);
    printf("%d",testcases);  
    for(test=0;test>testcases;test++)
    {
         printf("Enter the lines and length of the test cases ");
         scanf("%d%d",&n_lines,&m_length);
        for(lines=0;lines<n_lines;lines++)
        {
            for(length=0;length<m_length;length++)
           {
               scanf("%c",&n_m_matrix[lines][length]);
           }
       }
   }

}

Use `int main()` instead of `void main()`. It's *ancient*. – iBug Dec 23 '17 at 06:33 — iBug, Dec 23 '17 at 06:33

Barmar · Answer 1 · 2017-12-23T07:05:33.857

You need to move the declaration of n_m_matrix into the loop so that it's after you input the variables that hold the dimensions.

As others mentioned, you also have a typo in test > testcases, it should be <.

And you should read an extra character after entering the dimensions, to read the newline. Otherwise it will leave the newline after the dimensions in the input buffer, and this will be read as the first %c input when reading the contents.

You might also consider using fgets() to read each line, instead of doing it character by character. The way you've written it, if there are newlines at the end of each line, they will be inserted into the array. It's not clear whether that's wanted. If it is, make sure you include them in the line length input.

int main()
{
    int testcases;
    int n_lines ,m_length;
    int test, lines, length;
    printf("Enter the no. of Test Cases ");
    scanf("%d",&testcases);
    printf("%d\n",testcases);  
    for(test=0; test < testcases; test++)
    {
        printf("Enter the lines and length of the test cases ");
        scanf("%d%d",&n_lines,&m_length);
        getc(); // Skip over the newline
        char n_m_matrix[n_lines][m_length];
        for(lines=0;lines<n_lines;lines++)
        {
            for(length=0;length<m_length;length++)
            {
                scanf("%c",&n_m_matrix[lines][length]);
            }
        }
    }

}

The space after `%d%d` is OK [if the input comes from a file](https://stackoverflow.com/questions/19499060/), but not if it is typed. Also, the innermost pair of loops will treat newlines as a valid character for storing in the array, which might or might not be OK. — Jonathan Leffler, Dec 23 '17 at 07:02
@JonathanLeffler Thanks, I had a feeling something was wrong with that. I've added something about the newlines in the rest of the input. — Barmar, Dec 23 '17 at 07:06

user2736738 · Answer 2 · 2017-12-23T07:20:08.503

To clear confusion among the serveral problem the first one which invokes Undefined behavior in the code is, using uninitialized variable as VLA's size.

Also the for loop logic is wrong and character input might create some problem the way it is handled. (when entered with \n).

Correct code would be (considering that you don't want the whitespaces inputted into the array. Because if you do then there are other better choices like fgets etc).

#include<stdio.h>
#include<stdlib.h>
#define MAXSIZE 1024
int main(void)
{
    int testcases;
    int n_lines ,m_length;
    int test, lines, length;

    printf("Enter the no. of Test Cases ");
    if( scanf("%d",&testcases) != 1){
        fprintf(stderr, "%s\n","Error in input" );
        exit(1);
    }
    if( testcases <= 0 ){
        fprintf(stderr,"%s\n","Enter nonzero integer for testcases");
        exit(1);
    }
    printf("%d",testcases);  
    for(test = 0; test < testcases; test++)
    {
        printf("Enter the lines and length of the test cases ");
        if(scanf("%d%d",&n_lines,&m_length)!=1){
            fprintf(stderr, "%s\n","Error in input" );
            exit(1);           
        }
        if( n_lines <= 0 || m_length <= 0 || !(n_lines <= MAXSIZE && m_length <= MAXSIZE)){
            fprintf(stderr, "%s\n","Error in input n_lines and m_length" );
            exit(1);           
        }

        char n_m_matrix[n_lines][m_length];
        for(lines = 0; lines < n_lines; lines++)
        {
            for(length = 0; length < m_length; length++)
            {
                if( scanf(" %c",&n_m_matrix[lines][length]) != 1)
                {
                    fprintf(stderr, "%s\n","Eror in input" );
                    exit(1);
                }

            }
        }
        // work with VLA here.
    }
    return 0;
}

Here if you need much larger array sizes than this go for dynamic memory allocation. That will satisfy larger memory requirement in array.

There isn't a good way to determine the limit other than understanding the problem space. But you do need to limit the size of VLA otherwise you blow your stack with no chance of recovery. At that point, dynamic memory allocation becomes sensible. — Jonathan Leffler, Dec 23 '17 at 07:12
@JonathanLeffler.: I see..yes that's where Dynamic Mem allocation would be helpful. — user2736738, Dec 23 '17 at 07:14

score 0 · Answer 3 · answered Dec 23 '17 at 06:38

0

You need to use dynamic memory allocation, malloc method. The matrix n_m_matrix dimension is dynamically decided based on user input.

Allocate memory to n_m_matrix using malloc
In for loop correct logical error test>testcase to test<testcase
Put below code out of for loop
printf("Enter the lines and length of the test cases "); scanf("%d%d",&n_lines,&m_length);

answered Dec 23 '17 at 06:38

Raviprakash

2,410
6
34
56

1

You don't need to use `malloc`, you just need to declare the array after setting the variables. – Barmar Dec 23 '17 at 06:39
@Barmar The value of `n_lines` and `m_length` dynamically changes(based on user input), so static memory allocation is not possible. [Refer this QA](https://stackoverflow.com/questions/2128728/allocate-matrix-in-c) – Raviprakash Dec 23 '17 at 06:44
You can simply move the declaration of the array inside the `for` loop so it's after the input. – Barmar Dec 23 '17 at 06:46
It's possible if your compiler supports the VLA feature. That means it has to be a C99 or C11 compiler, which of course MCVS is not (it supports C90, with some but not all C99/C11 extensions and, AFAIK, it does not support VLA). – Jonathan Leffler Dec 23 '17 at 07:10

The Code exits after the input and output statements

3 Answers3