Animated Sieve of Eratosthenes prime number finder program in C.

sieve_of_eratosthenes.c

// Sieve of Eratosthenes:
// https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

#include <stdio.h>
#include <stdlib.h>
// https://github.com/tapio/rlutil
#include "rlutil.h"

int N, *numbers, nrOfDigits;

void printNumbers(int selectedIndex, int removedIndex) {
	int lineLength = N <= 20 ? 5 : N <= 100 ? 10 : ((1 + (N - 1) / 100) * 5);
	locate(1, 4);
	for (int i = 0; i < N; ++i) {
		if (i > 0 && i % lineLength == 0)
			printf("\n");
		if (numbers[i] == 0 || i < 1)
			setColor(DARKGREY);
		if (i == selectedIndex)
			setBackgroundColor(BLUE);
		else if (i == removedIndex) {
			setBackgroundColor(RED);
			setColor(GREY);
		} else if (1 <= i && i < selectedIndex && numbers[i] != 0)
			setBackgroundColor(GREEN);
		printf("[%*d]", nrOfDigits, numbers[i]);
		resetColor();
		printf(" ");
	}
	printf("\n");
}

void eliminate() {
	for (int i = 1; i < N; ++i)
		if (numbers[i] != 0) {
			locate(1, 3);
			printf("Removing multiples of %d:", numbers[i]);
			for (int j = i + 1; j < N; ++j)
				if (numbers[j] != 0 && numbers[j] % numbers[i] == 0) {
					printNumbers(i, j);
					numbers[j] = 0;
					msleep(250);
				}
			printNumbers(i, -1);
			msleep(750);
		}
}

void listPrimes() {
	int printed = 0;
	printf("\nPrime numbers found: {");
	for (int i = 1; i < N; ++i)
		if (numbers[i] != 0)
			printf("%s%d", printed++ ? ", " : "", numbers[i]);
	printf("}\n");
}

int main() {
	int max;
	cls();

	printf("Enter N: ");
	scanf("%d", &N);
	hidecursor();

	numbers = (int*) malloc(sizeof(int) * N);
	for (int i = 0; i < N; ++i) {
		numbers[i] = i + 1;
	}
	max = N;
	nrOfDigits = 1;
	while ((max /= 10) != 0)
		nrOfDigits++;
	eliminate();

	printNumbers(N, N);
	listPrimes();
	free(numbers);
	showcursor();
	return 0;
}