Prints the sum of primes less than or equal to input numbers.

primesums.c

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

#define INITIAL_ARRAY_CAPACITY 100

int isPrime(int value){
	int i;
	if (value > 2 && value != 4){
		for (i = 2; i < value/2; i++) {
			if (value % i == 0){ 
				return 0;
			}
		}
	}else{
		return 0;
	}
	return 1;
}

/*Datastructure holding a prime number and the sum of itself and the primes lesser than it.*/
struct PrimeSumPair{
	int Prime;
	int Sum;
};

/*Datastructure for a dynamic array containing PrimeSumPairs.*/
struct DynPrimeArray{
	struct PrimeSumPair* values;
	int capacity;
	int length;
	int biggestTestedSoFar;
};

/*Create and zero-initialise a new DynPrimeArray of length size.*/
struct DynPrimeArray* NewPrimeArray(int size){
	int i;
	struct DynPrimeArray *primeArray = malloc(sizeof(struct DynPrimeArray));
	primeArray->values = malloc(sizeof(struct PrimeSumPair)*size);
	for (i = 0; i < size; i++){
		primeArray->values[i].Prime = 0;
		primeArray->values[i].Sum = 0;
	}
	primeArray->length = 0;
	primeArray->capacity = size;
	primeArray->biggestTestedSoFar = 0;	
	return primeArray;
}

/*Add a new value to a DynPrimeArray, extending the array if it's not large enough.*/
void AddValue(struct DynPrimeArray *primes, int prime, int sum){
	if (primes->length == primes->capacity){
		struct PrimeSumPair *newArray = malloc(sizeof(struct PrimeSumPair) * (primes->capacity * 2));
		memcpy(newArray, primes->values, primes->length);
		free(primes->values);
		primes->values = newArray;
	}
	primes->values[primes->length].Prime = prime;
	primes->values[primes->length].Sum = sum;
	primes->length++;
}

/*Find the sum of primes up to the greatest prime that is less than n.*/
int FindSumUpToBiggestPrimeLessThan(int n, struct DynPrimeArray *primes){
	int i;
	for (i = 0; i < primes->length; i++){
		if (primes->values[i].Prime > n){
			break;
		}
	}
	return primes->values[i-1].Sum;
}

/*Print all primes discovered so far and their sums.*/
void printPrimes(struct DynPrimeArray *primes){
		int i;
		for (i = 0; i < primes->length; i++){
			printf("Prime: %d, sum: %d\n", primes->values[i].Prime, primes->values[i].Sum);
		}
}

/*Calculate, if not calculated already, the primes lesser than n and the cumulative primes up to each of these primes.*/
int CalcSumOfPrimesLessThan(int n, struct DynPrimeArray *primes){
	if (n >= primes->biggestTestedSoFar){
		int i;
		for (i = primes->biggestTestedSoFar+1; i <= n; i++){
			if (isPrime(i)){
				primes->values[primes->length].Prime = i;
				if (primes->length == 0){
					primes->values[primes->length].Sum = i + 0;
				}else{
					primes->values[primes->length].Sum = i + primes->values[primes->length-1].Sum;
				}
				primes->length++;
			}
		}
		primes->biggestTestedSoFar = n;		
	}
	return FindSumUpToBiggestPrimeLessThan(n, primes);
}


int main(int argc, char** argv){
	struct DynPrimeArray* primesNSums = NewPrimeArray(INITIAL_ARRAY_CAPACITY);
	int input = 0;
	CalcSumOfPrimesLessThan(10, primesNSums);
	printf("Enter a number: ");
	while(scanf("%d", &input) == 1){
		int result = CalcSumOfPrimesLessThan(input, primesNSums);
		if (input <3){
			result = 0;
		}
		if (isPrime(result)){
			printf("The sum of the primes less or equal to %d is %d. That sum is prime.\n", input, result);			
		} else{
			printf("The sum of the primes less or equal to %d is %d. That sum is not prime.\n", input, result);			
		}
		printf("Enter a number: ");
	}
	free(primesNSums);
	return 0;
}