How do i write this code in C program? i am stuck on it for the past 3 hrs.Suggestions are welcomed

Posted by Different-Affect-690@reddit | learnprogramming | View on Reddit | 12 comments

Print all prime numbers between a and b using functions

A prime number is a whole number that has exactly two factors which are itself and 1, whereas a composite number has factors in addition to 1 and itself.

Given 2 numbers a and b, you need to write a function which will print all prime numbers between these two given numbers (inclusive).

If there are no prime numbers between the given range, print -1.

Input Format

First line contains two integers a 
and
 b.

Output Format

Print the prime numbers separated 
by
 space.

Sample Input:

2 10

Sample Output:

2 3 5 7

[-]

Awkward-Chair2047@reddit

Every time you copy/paste someone else's code, you are cheating yourself of a great learning opportunity. Search for a pseudo-code approach to solving problem. This approach works across programming languages.

[-]

Classic-Rate-5104@reddit

You ask us doing your homework?

[-]

EfficientMongoose317@reddit

Don’t overthink it. The problem is basically:

Make a function to check if a number is prime
Loop from a to b
Print numbers where the function returns true

Something like this:

#include <stdio.h>

int isPrime(int n) {
    if (n < 2)
        return 0;

    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0)
            return 0;
    }

    return 1;
}

int main() {
    int a, b;
    int found = 0;

    scanf("%d %d", &a, &b);

    for (int i = a; i <= b; i++) {
        if (isPrime(i)) {
            printf("%d ", i);
            found = 1;
        }
    }

    if (!found)
        printf("-1");

    return 0;
}

The important logic is:

prime numbers have no divisors except 1 and themselves
You only need to check divisibility till sqrt(n)
The function returns 1 for prime and 0 otherwise

[-]

smichaele@reddit

This is r/learnprogramming. Writing the program for him doesn't help. Everyone else was guiding OP to an answer.

[-]

smichaele@reddit

This is r/learnprogramming. Writing the program for him doesn't help. Everyone else was guiding OP to an answer.

[-]

Ormek_II@reddit

Can you write a function to Check if a single number is Prime?

[-]

Sad_School828@reddit

You iterate from A to B, inclusive. These are your divisors.

For each divisor you iterate from 1 to (current_divisor - 1) and these are your dividends.

For each dividend, you test divisor modulo dividend. If the modulo is ever non-zero except at 1, the divisor is not prime so you can just continue.

If the modulo result is always zero except at 1, then the number is prime so you add it to the output before continuing.

You could alternately dink around providing an implementation of the Sieve of Eratosthenes, but I'm guessing your professor would take one look at it and accuse you of either using AI or getting somebody else to write your code.

[-]

Different-Affect-690@reddit (OP)

yup u are right but here is my final code
void printPrimes(int a, int b)

{

int j;

int foundAny = 0;

for (int i = a; i <= b; i++) {

if (i < 2) {

continue;

}

for (j = 2; j < i; j++) {

if (i % j == 0) {

break;

}

if (i == j) {

printf("%d ", i);

foundAny = 1;

}

if (foundAny == 0) {

printf("-1");

}

int main()

{

int a,b;

scanf("%d%d", &a,&b);

printPrimes(a, b);

return 0;

}

[-]

HashDefTrueFalse@reddit

Do some research on how you might write a function to check if a number is prime, as that's the core question. Then do that for each in the range.

Since you'll come across it in the first 30 seconds, I don't think there's any harm in mentioning that you'll probably end up "sieving" them, a method based on the observation that multiples of primes cannot be prime, thus you can eliminate all multiples of a prime within the range as possibilities once you've found one.

The C code could look all kinds of ways depending on how you want to check for eliminations.

[-]

peterlinddk@reddit

Can you write it in another language?

Can you desribe step by step what the program is supposed to do?

Do you know how to find the next prime number?

Do you know how to check if a number is a prime number or not?

Start by doing the exercise on paper: if given the input of 5 and 13, which numbers should be output, and how would you calculate which ones?

Write down the rules to follow, and then convert that list of rules to C code.

[-]

Mortomes@reddit

What have you done in the past 3 hours?

[-]

Different-Affect-690@reddit (OP)

well the only thing i wrote

void printPrimes(int a, int b)

{

for(int i=2;i<=b+1;i++)

printf("%d",b);

}

after that i havent wrote anything at all