C :: Convert Characters From Array Into ASCII Integers - Incompatible Types Error

The code is supposed to convert characters from an array into their respective ascii integers, and append a 0 if the number is less than 3 digits long. It then supposed to put it all together into one string.

Code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void){

char file[] = "This is a test";
char *ptr = file;
int length = strlen(file);
int i, numbers[length];

[Code] .....