Inserting commas in numbers using for loop not working

Question

I tried making a program where it will put a comma in a certain place between numbers. My explanation may be is vague, but I just wanted to make 12345678.23 into 12,345,678.23. I hope that cleared my explanation tho. Here is my code.

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

void main()
{
  char m[20]="12345678.23";
    int j=11, a, t=1, r=4, s;
    
    
        for(a=0; a=11; a++)
        {
            if(strlen(m)==j)
            {printf("%c", m[a]);
            if(a==t)
            {printf(",");}
            if(a==r)
            {printf(",");}
            }   
        }
        
    
}

This program doesn't work and I don't know why. I hope you guys can help me. Thank you very much!

Answer 1

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

int main(void) // Use the proper signature
{
    char m[] = "123"; // Do not hardcode the size
    char *p; // A pointer to traverse the string
    int mod; // Useful to know when to insert a comma

    p = strchr(m, '.'); // Position of the decimal separator
    if (p == NULL)
    {
        p = strchr(m, '\0'); // Don't have decimal separator
    }
    mod = (p - m) % 3; // Comma must be placed on each mod count
    p = m; // Rewind the pointer
    while (*p != '\0') // While the character is not NUL
    {
        if (*p == '.') // Decimal separator reached
        {
            mod = 3; // Avoid divisibility
        }
        // If position divisbile by 3 but not the first pos
        if ((p != m) && (mod != 3) && (((p - m) % 3) == mod))
        {
            putchar(',');
        }
        putchar(*p);
        p++; // Next character
    }
    putchar('\n');
    return 0;    
}

Output:

12,345,678.23