Storing a variable integer with a char Array in C

Question

I want to extract, edit and re-concatenate a string that contains an integer. How does this get done?

char str_test[] = "CAT ";
int count = 10;
//(1) something here to combine str_test and count stored in some variable
char str_new[] = ????;
//(2) something else here to extract the 10 and +1? 
//such that if I print str_new, it gives me "CAT 11"

????

Answer 1

You can use sprintf to write a formatted number to a string, and you can use snprintf to find out how many characters are required.

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


/*  Return a newly allocated string containing String followed by Number
    converted to decimal.  The caller is responsible for freeing the memory.
*/
static char *ConcatenateIntToString(const char *String, int Number)
{
    //  Get the number of non-null characters in String.
    size_t Length = strlen(String);

    /*  Allocate space for:
            the characters in String,
            the characters needed to format Number with "%d", and
            a terminating null byte.
    */
    char *Result = malloc(
        Length
        + snprintf(NULL, 0, "%d", Number)
        + 1);

    //  Test whether the allocation succeeded.
    if (!Result)
    {
        fprintf(stderr, "Error, unable to allocate memory.\n");
        exit(EXIT_FAILURE);
    }

    //  Copy the characters from String.
    memcpy(Result, String, Length);

    //  Append the formatted number and a null terminator.
    sprintf(Result + Length, "%d", Number);

    //  Return the new string.
    return Result;
}


int main(void)
{
    char *NewString = ConcatenateIntToString("CAT ", 11);
    printf("The new string is %s.\n", NewString);
    free(NewString);
}