Not returning string. This program converts digit like 123 to words like "One Two Three", why in the end I am not getting anything?

Question

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

char *int_to_string( int n );

void main()
{
    int n;

    printf("Enter the number : ");
    scanf("%d",&n);

    printf("\n%d in words is : %s.\n",n,int_to_string(n));
}

char *int_to_string( int n)
{
    char str[100]="";

    if(n<10)
    {
          switch(n)
        {
            case 0: return "Zero";
            case 1: return "One";
            case 2: return "Two";
            case 3: return "Three";
            case 4: return "Four";
            case 5: return "Five";
            case 6: return "Six";
            case 7: return "Seven";
            case 8: return "Eight";
            case 9: return "Nine";
        }
    }

    else
    {
        strcat(str,int_to_string(n/10));
        strcat(str," ");

        return strcat(str,int_to_string(n%10));
    }
}

Function int_to_string() should return a string containing equivalent of the number in words that is passed. it works fine for single digit number ( i.e. 0-9 ) but above that it just gives nothing.

Answer 1

The function has undefined behaviour.

It returns a pointer to local array str that is in general destroyed after exiting the function.

Take into account that it is better to define the parameter as having type unsigned int. Otherwise the function needs to check whether the number is not negative.

You could make the task easier by means of declaring second parameter that will specify a character zero-terminated array that will store the result string.

Or you have to allocate memory dynamically.

Here are shown these two approaches.

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

char * int_to_string( unsigned int n )
{
    if( n < 10 )
    {
        char *p = "";

        switch( n )
        {
        case 0: 
            p = "Zero";
            break;
        case 1: 
            p = "One";
            break;
        case 2: 
            p = "Two";
            break;
        case 3: 
            p = "Three";
            break;
        case 4: 
            p = "Four";
            break;
        case 5: 
            p = "Five";
            break;
        case 6: 
            p = "Six";
            break;
        case 7: 
            p = "Seven";
            break;
        case 8: 
            p = "Eight";
            break;
        case 9: 
            p = "Nine";
            break;
        }
        char *q = malloc( strlen( p ) + 1 );
        strcpy( q, p );
        free( p );
        return q; 
    }
    else
    {
        char *q = int_to_string( n / 10 );
        char *p = int_to_string( n % 10 );

        q = realloc( q, strlen( q ) + strlen( p ) + 2 );
        strcat( q, " " );
        return strcat( q, p );
    }
}

char * int_to_string1( unsigned int n, char *s )
{
    if( n < 10 )
    {
        char *p = "";

        switch( n )
        {
        case 0: 
            p = "Zero";
            break;
        case 1: 
            p = "One";
            break;
        case 2: 
            p = "Two";
            break;
        case 3: 
            p = "Three";
            break;
        case 4: 
            p = "Four";
            break;
        case 5: 
            p = "Five";
            break;
        case 6: 
            p = "Six";
            break;
        case 7: 
            p = "Seven";
            break;
        case 8: 
            p = "Eight";
            break;
        case 9: 
            p = "Nine";
            break;
        }

        return strcat( s, p );
    }
    else
    {
        strcat( int_to_string1( n / 10, s ), " " );

        return int_to_string1( n % 10, s );
    }
}

int main( void )
{
    unsigned int n = 1234567890;
    char *s = int_to_string( n );

    puts( s );

    free( s );

    char s1[100];
    s1[0] = '\0';

    puts( int_to_string1( n, s1 ) );
}

The program output is

One Two Three Four Five Six Seven Eight Nine Zero
One Two Three Four Five Six Seven Eight Nine Zero