#include <ctype.h>

void foo(char *dest, const char *source)
{
    int outindex = 0;
    for (int i = 0; source[i] != '\0'; i++)
    {
        if (isdigit(source[i]))
        {
            dest[outindex] = source[i];
            outindex++;
        }
    }
    dest[outindex] = '\0';
}