Ascii to integer using no library functions - c

Question

My teacher gave us an assignment where we create a function that reads ASCII digit characters and converts them to a number without using any library functions such as atoi. Through some research i came up with this in my own file:
#include <stdio.h>
#include <sttdef.h>
int main() {
char testString[] = "123";
int convertedResult = 0;
int i;
for(i = 0; testString[i] != '\0'; i++){
convertedResult = convertedResult*10 + testString[i] - '0';
printf("%i\n",convertedResult);
if (testString[i] == '\0') {
break;
}
}
return 0;
}
While this works on its own i have to use the main file he gave us to call on this specific function.
char *asciiToInteger(char *inputString, int *integerPtr) {
return inputString;
}
I'm a bit confused as to how to proceed from here? attatched picture is main

Answer 1

#include <stdio.h>
#include <stddef.h>
char * asciiToInteger(char *inputString, int *integerPtr){
int convertedResult =0;
for(int i = 0; inputString[i] != '\0'; i++){
convertedResult = convertedResult*10 + inputString[i] - '0';
}
*integerPtr=convertedResult;
return inputString;
}
int main() {
char testString[] = "123";
int integerPtr;
asciiToInteger(testString, &integerPtr) ;
printf("%d\n",integerPtr);
return 0;
}

Answer 2

Your code has a couple of problems:
It assumes the entire string is digits
It checks for the end of string twice
I think a better implementation would be:
#include <stdio.h>
const char *asciiToInteger(const char *inputString, int *value)
{
int result = 0;
while (isdigit((unsigned int) *inputString))
{
result *= 10;
result += *inputString++ - '0';
}
*value = result;
return inputString;
}
This returns a pointer to the first non-converted character, which might be to the end of string marker if the string is all digits. I added const on the strings of course, since this converter is just reading from the strings.

Answer 3

When you get an assignment like this the first step is to make sure you understand what the function is supposed to do. Your question has no such description so that is the place to start.
From the behavior of the main function it seems to be something like:
If the first character in the input string is not a digit return NULL
If the first character in the input string is a digit convert all leading digits to an integer stored in the object pointed to by integerPtr and return a pointer to the character following the converted digits.
Examples:
inputString = "a123b67" --> return NULL
inputString = "123b67" --> *integerPtr = 123 and return a pointer to the 'b' in the input
That could look something like this:
char *asciiToInteger(char *inputString, int *integerPtr) {
if (*inputString < '0' || *inputString > '9')
return NULL; // no leading digit
*integerPtr = 0;
do
{
*integerPtr = *integerPtr * 10 + *inputString - '0';
++inputString;
} while (*inputString >= '0' && *inputString <= '9');
return inputString;
}
Notice that the code above can't handle negative integers.