# C Exercises: Find the sum of all the primes below ten thousand

## C Programming Challenges: Exercise-26 with Solution

A prime number is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself. However, 6 is composite because it is the product of two numbers (2 × 3) that are both smaller than 6.

The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.

Write a C program to find the sum of all prime numbers below ten thousand.

**C Code:**

```
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
char *temp;
unsigned i, j;
size_t num = 10000;
unsigned long long sum = 0ULL;
temp = calloc(num, sizeof *temp);
for (i = 2; i < num; i++) {
if (!temp[i]) {
sum += i;
for (j = i*2; j < num; j += i) {
temp[j] = 1;
}
}
}
free(temp);
printf("%llu\n", sum);
return 0;
}
```

Sample Output:

5736396

**Flowchart:**

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## C Programming: Tips of the Day

** It is more efficient to use if-return-return or if-else-return?**

Since the return statement terminates the execution of the current function, the two forms are equivalent (although the second one is arguably more readable than the first).

The efficiency of both forms is comparable, the underlying machine code has to perform a jump if the if condition is false anyway.

Note that Python supports a syntax that allows you to use only one return statement in your case:

return A+1 if A > B else A-1

Ref : https://bit.ly/2S4P2he

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