Trying to replace #define with const does work

Question

So I originally wrote my program using #define since I know it happens as a preprocessor, but my teacher commented that I should use const. I tried replacing #define in my code but it just broke it and I am not sure why. I understand that const is usable like a variable right so my code should just be calling it and would get the same result no? Here is a screenshot of what my code does working and not working Here is the working:

#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
//Set constant.Preprocessor for speed repace LENGTH
//With what ever number of digits you would want.
#define LENGTH 4

//Converts the input number into an array of digits
int * digitArray(int number) {
    //Creating an array for all digits of the constant length
    static int digits[LENGTH];
    //Loop through the digits and use the mod operator to get each one of them
    for(int i = LENGTH-1; i >= 0; i--) {
        digits[i] = number % 10;
        number = number / 10;
    }
    //Finally, return the array pointer
    return digits;
}

//Compares the correct number array with the guess array of digits, checking for fermi/pica hints and returns
//the number of exact matches of both arrays
int checkGuess(int *num, int *guess) {
    //Create two bool arrays to flag the digits already used
    bool usedNum[LENGTH] = {false};
    bool usedGuess[LENGTH] = {false};
    //First loop to check for number of matches
    int matches = 0;
    int used = 0;
    for(int i = 0; i < LENGTH; i++) {
        //Check if the digit is the same for both arrays at i index
        if(num[i] == guess[i]) {
            //If so there is an exact match
            usedNum[i] = true;
            usedGuess[i] = true;
            matches++;
            used++;
        }
    }

And the not working:

const int LENGTH = 4;

//Converts the input number into an array of digits
int * digitArray(int number) {
    //Creating an array for all digits of the constant length
    static int digits[LENGTH];
    //Loop through the digits and use the mod operator to get each one of them
    for(int i = LENGTH-1; i >= 0; i--) {
        digits[i] = number % 10;
        number = number / 10;
    }
    //Finally, return the array pointer
    return digits;
}

//Compares the correct number array with the guess array of digits, checking for fermi/pica hints and returns
//the number of exact matches of both arrays
int checkGuess(int *num, int *guess) {
    //Create two bool arrays to flag the digits already used
    bool usedNum[LENGTH] = {false};
    bool usedGuess[LENGTH] = {false};
    //First loop to check for number of matches
    int matches = 0;
    int used = 0;
    for(int i = 0; i < LENGTH; i++) {
        //Check if the digit is the same for both arrays at i index
        if(num[i] == guess[i]) {
            //If so there is an exact match
            usedNum[i] = true;
            usedGuess[i] = true;
            matches++;
            used++;
        }
    }

I know this is a kind of a repeat of some other questions but I didn't see any answers that specifically address why it does not work in this instances and how to fix it so the code runs.

Answer 1

Your second snippet is not using a constant value known at compile time, note that const doesn't really mean constant but "read only" in C.

Switch from

const int LENGTH = 4;

to

enum { LENGTH = 4 };  // A real constant

and it should work.

Or you can leave it as is and compile with C99 or C11, then those arrays will be Variable-length-arrays (VLA's) (note that VLA's are optional in C11). But always prefer the first option when you know the number of elements before hand.

And here:

static int digits[LENGTH];

since the array is always filled in the loop, you don't need the static keyword.