How can I handle user's inputs and close program when user presses ESC key

I want to make a program that works endlessly until the user presses ESC. And I need to handle the user's input, because I input a value which should be an integer. How can I make this?

#include <stdio.h>

int main(){
    int num,factorial; 
    while(1){
        
        factorial = 1;
        printf("Enter Number : ");
        scanf("%d",&num);
        for(int i = 1 ; i<=num ; i++){ 
            factorial *= i;
        }
        printf("Factorial : %d\n",factorial); 
    }
    return 0;
}

Program should calculate factorial of a given number.

I tried this, but it makes an endless loop.

if(scanf("%d", &sayi) != 1){
    printf("Error Occured.\n");
    continue; 
}

Solution

On Microsoft Windows, it is not possible to detect the ESC key using the functions provided by the C standard library.

However, it is possible to do this with the following platform-specific functions:

When using these functions, you will have to program basic input functionality yourself, though. This includes

deciding when to echo the input back to the user and
deleting the last character when the user presses the backspace key.

Therefore, if you are looking for a simple solution, this is probably not what you want. It would be easier to continue using the functions provided by the C standard library and to make the user do something else to indicate that they are finished, for example to tell the user to enter the number -1 in this case.

If you want something slightly more elegant, for example to make the user enter q or quit instead of -1 when they are finished, then you could use the function fgets instead of scanf, in order to first read one line of input from the user as a string. You can then compare this string with q or quit, and only if this comparison fails do you attempt to convert the string to a number, for example using the function strtol.

Here is an example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

//forward declaration of function
void get_line_from_user( char *buffer, int buffer_size );

int main()
{
    while ( 1 )
    {
        char line[200], *p;
        long num, factorial = 1;

        //prompt user for input
        printf("Enter number, or \"q\" to quit: ");

        //read one line of input from user
        get_line_from_user( line, sizeof line );

        //determine whether user wants to quit
        if ( line[0] == 'q' )
        {
            printf( "Quitting program...\n" );
            exit( EXIT_SUCCESS );
        }

        //attempt to convert input string to a number
        num = strtol( line, &p, 10 );

        //verify that conversion was successful
        if ( p == line )
        {
            printf( "Input error!\n" );
            continue;
        }

        //perform calculations
        for( int i = 2; i <= num; i++ )
        { 
            factorial *= i;
        }

        //print the result
        printf( "Factorial : %ld\n", factorial ); 
    }

    return 0;
}

//This function will read exactly one line of input from the
//user. On failure, the function will never return, but will
//print an error message and call "exit" instead.
void get_line_from_user( char *buffer, int buffer_size )
{
    char *p;

    //attempt to read one line of input
    if ( fgets( buffer, buffer_size, stdin ) == NULL )
    {
        printf( "Error reading from input\n" );
        exit( EXIT_FAILURE );
    }

    //attempt to find newline character
    p = strchr( buffer, '\n' );

    //make sure that entire line was read in (i.e. that
    //the buffer was not too small to store the entire line)
    if ( p == NULL )
    {
        //a missing newline character is ok if the next
        //character is a newline character or if we have
        //reached end-of-file (for example if the input is
        //being piped from a file or if the user enters
        //end-of-file in the terminal itself)
        if ( getchar() != '\n' && !feof(stdin) )
        {
            printf( "Line input was too long!\n" );
            exit( EXIT_FAILURE );
        }
    }
    else
    {
        //remove newline character by overwriting it with
        //null character
        *p = '\0';
    }
}

This program has the following behavior:

Enter number, or "q" to quit: 5
Factorial : 120
Enter number, or "q" to quit: 3
Factorial : 6
Enter number, or "q" to quit: Test
Input error!
Enter number, or "q" to quit: 7
Factorial : 5040
Enter number, or "q" to quit: 5
Factorial : 120
Enter number, or "q" to quit: 6
Factorial : 720
Enter number, or "q" to quit: q
Quitting program...