Non-deterministic output of simple C program

Question

This is an unfinished program I'm writing to learn C (only checks multiples of 2 currently...)

Ultimately, I want this to be an implementation of the Sieve of Eratosthenes (prime numbers)

The problem I'm having is that the output is non-deterministic: sometimes the output includes 11, other times it does not - This happens for a handful of numbers. I have experimented by changing a few things, such as actually initializing the array of booleans to false.

Any ideas why this might be happening?

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

int main(int argc, char *argv[]) {
        int n = atoi(argv[1]);
        int initialPrimeIterator = 2;
        _Bool compositePrimeNumbers[n];

        printf("Prime Numbers from 2 -> %d\n", n);
        for (int i = initialPrimeIterator; i < n; i += initialPrimeIterator) {
                compositePrimeNumbers[i-1] = true;
        }
        printf("Done...\n");
        
        printf("Printing prime numbers from 2-> %d\n", n);
        for (int i = 2; i < n; i++) {
                if (!compositePrimeNumbers[i]){
                        printf("%d\n", i + 1);
                }
        }
        
        return 0;
}

edit: haha. Just realized I have an array named 'compositePrime...' Should just be 'compositeNumbers'

_Bool compositePrimeNumbers[n]; is uninitialized for numbers which are not multiples of initialPrimeIterator (i += initialPrimeIterator). — KamilCuk
– KamilCuk, Commented Nov 15, 2021 at 6:46
Please don't include line-numbers in code. If you want to point out a specific line then mention it in the question itself and add a comment on that line. — Some programmer dude
– Some programmer dude, Commented Nov 15, 2021 at 6:48
A couple of other unrelated points: The <stdbool.h> header defines the type bool. And you should always check argc before using any element in argv. — Some programmer dude
– Some programmer dude, Commented Nov 15, 2021 at 6:50
Is this supposed to be a treasure-hunt, or do you think you could perhaps supply the inputs for which the outputs are a surprise? — enhzflep
– enhzflep, Commented Nov 15, 2021 at 7:09
You might want to explain your first loop to a rubber duck or suitable substitute (roommate, friend, etc.). All it does is mark all odd indexes as true. — Some programmer dude
– Some programmer dude, Commented Nov 15, 2021 at 7:42

nielsen · Accepted Answer · 2021-11-15 07:50:43Z

3

Since I understand you are aiming at completing the program when overcoming this hurdle, I will not post a completed program, but only point out issues in your current version:

As it has been noted, the array compositePrimeNumbers is uninitialized. Since it must be initialized with all values false which is represented by 0, the quickest way is this:

     memset(compositePrimeNumbers, 0, sizeof(compositePrimeNumbers));

You should not mark the current initialPrimeIterator as a composite number, hence the for-loop should start with the next multiple. Also, n must be included:

    for (int i = 2 * initialPrimeIterator; i <= n; i += initialPrimeIterator) {

(actually, this can be optimized by replacing 2 * initialPrimeIterator with initialPrimeIterator * initialPrimeIterator).

With these changes, I believe you are well on the way to complete the program.

edited Nov 15, 2021 at 7:50

answered Nov 15, 2021 at 7:42

nielsen

8,0911 gold badge18 silver badges35 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Lindydancer · Accepted Answer · 2021-11-15 06:48:03Z

2

In C, a local array is not initialized, probably for performance reasons.

One way to fix this is to loop over it to set each element to false.

answered Nov 15, 2021 at 6:48

Lindydancer

26.3k4 gold badges54 silver badges72 bronze badges

2 Comments

Collin Quiason Rottinghaus Over a year ago

I tried this and still got the same non-deterministic output

Jabberwocky Over a year ago

"probably for performance reasons": you can drop the word "probably".

Collectives™ on Stack Overflow

Non-deterministic output of simple C program

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related