Why %d is showing 0 in my c program output?

Question

The program is to find the largest number amongst all the entered integers. It asks the user to enter a number 10 times or press 0 to quit, whichever is earlier. But, the output is not as expected. I will appreciate it if you can help a newbie.

#include <stdio.h>

int main()
{
    int num[10];
    int max, a;
    for (a = 0; a < 10; a++)
    {
        printf("Enter the integer: ");
        scanf("%d", &num[a]);
        if (num[a] == 0)
            break;
    }

    for(a = 0; a < 10; a++)
    { 
        num[0] = max;
        if(max < num[a])
        {
            num[a] = max;
        }
    }

    printf("This is the largest integer: %d", max); //The output is coming wrong here.

    return 0;
}

You are aware that you neither initialise nor ever write any value at all to the variable `max`, aren't you? What are you expecting to see? Using uninitialised variables is undefined behaviour. You seem to know how to write 0 into `a`. But for `max` you seem to have it reverse. You do know that with `= ` the value on the right of it goes into the variable on the left of it, don't you? — Yunnosch, Sep 26 '22 at 11:22
If all you want to output is the largest integer, then you don't need to use an array. — Cheatah, Sep 26 '22 at 11:34

score 3 · Answer 1 · answered Sep 26 '22 at 11:32

Don't use num[0], you are overwriting it with max variable which is not initialized, so it is 0.
Use max to the minimum type variable (INT_MIN with limits.h header file) or initalize it to max = num[0] after input is captured.

#include <limits.h>

  int max = INT_MIN;

Also you need to change your second for loop as follows so as to update the max variable as you iterate, and not the num[] variables. Loop variable starts with 1 if you already assumed max to be first element before, else loop variable will start with 0.

  for(a = 1; a < 10; a++) // a=0 if max was initialized to INT_MIN above
  { 
    if(num[a]>max)
    {
     max = num[a];
    }
  }

mikyll98 · Answer 2 · 2022-09-26T12:02:45.843

You never assign the max variable.

What you want to do is to check if the value entered is greater than each one you've previously entered, so you would need to use the following condition:

if (num[a] > max)
    max = num[a];

You also need to initialize max to some value (let's say, if you expect to have only positive integers, it could be 0, but have a look at Jigsaw answer for a better solution): int max = 0;.
And eventually add an if-condition that checks if max is 0, that way you know if no values have been entered:

if(max == 0)
    printf("No values have been entered.");

else printf("This is the largest integer: %d", max);

Notice that you can assign the elements of num and update max in the same for loop, therefore the second for becomes completely useless and you can remove it:

#include <stdio.h>

int main()
{
    int num[10];
    int max = 0, a;
    for (a = 0; a < 10; a++)
    {
        printf("Enter the integer: ");
        scanf("%d", &num[a]);
        if (num[a] == 0)
            break;
        if (num[a] > max)
            max = num[a];
    }

    if(max == 0)
        printf("No values have been entered.");
    else printf("This is the largest integer: %d", max);

    return 0;
}

I suggest you to turn on your compilers warning, especially -Wall and -Wextra, so you would notice problems like these:

<source>: In function 'main':
<source>:17:16: warning: 'max' may be used uninitialized [-Wmaybe-uninitialized]
   17 |         num[0] = max;
      |         ~~~~~~~^~~~~
<source>:6:9: note: 'max' was declared here
    6 |     int max, a;
      |         ^~~

Thanks, I solved the issue. I really appreciate that. I was using vscode integrated terminal but it didn't show an error as yours does. Which terminal are you using? — Daksh, Sep 26 '22 at 11:28
@Daksh `gcc` :) here's an example from an online compiler: https://godbolt.org/z/fohx3zT8b — mikyll98, Sep 26 '22 at 11:30
@Daksh You should configure the compiler to be extra picky, see [What compiler options are recommended for beginners learning C?](https://software.codidact.com/posts/282565) This will save you from a lot of pointless troubleshooting chasing down bugs that the compiler has already found. — Lundin, Sep 26 '22 at 11:30
@Daksh you might be interested in that answer: https://stackoverflow.com/a/57173879/19544859 — mikyll98, Sep 26 '22 at 11:32

Vlad from Moscow · Answer 3 · 2022-09-26T12:26:08.377

For starters this for loop

for (a = 0; a < 10; a++)
{
    printf("Enter the integer: ");
    scanf("%d", &num[a]);
if (num[a] == 0)
    break;
}

can enter less than 10 elements in the array num due to the condition

if (num[a] == 0)
    break;

So the next loop has to traverse exactly a elements of the array not 10. So in the next loop you have to use another variable for the index as for example

for( int i = 0; i < a; i++)

The variable max was not initialized

int max, a;

So this for loop invokes undefined behavior where the variable max is assigned to elements of the array or where it is compared with elements of the array.

for(a = 0; a < 10; a++)
{ 
  num[0] = max;
  if(max < num[a])
  {
   num[a] = max;
  }
}

Moreover the variable max is not changed within the for loop. So the loop in whole does not make sense.

Pay attention to that the user can enter 0 at once. So the array will not contain any valid values. In this case the array does not have a maximum value.

Your program can look for example the following way

#include <stdio.h>

int main( void )
{
    enum { N = 10 };
    int num[N];

    int i = 0;

    do
    {
        printf( "Enter the integer (0 - stop): " );
        scanf( "%d", &num[i] );
    } while ( num[i] != 0 && ++i < N );

    int max_i = 0;

    for ( int j = 1; j < i; j++ )
    { 
        if ( num[max_i] < num[j] )
        {
            max_i = j;
        }
    }
  
    if ( i == 0 )
    {
        puts( "You have not entered numbers unequal to zero." );
    }
    else
    {
        printf("This is the largest integer: %d", num[max_i] );
    }

    return 0;
}

score 0 · Answer 4 · answered Sep 26 '22 at 11:27

You neither initialise nor ever write any value at all to the variable max. Using uninitialised variables is undefined behaviour.

You seem to know how to write 0 into a. But for max you seem to have it reversed. Remember that with = the value on the right of it goes into the variable on the left of it.

To fix the problem, turn any something = max; which occurs in your code into max = something;.

Why %d is showing 0 in my c program output?

4 Answers4