Write a program that takes an integer from standard input an

Write a program that takes an integer from standard input and prints all prime numbers that are smaller than it to standard output. Recall that a prime number is a natural number greater than 1 whose only positive divisors are 1 and itself. At the start of the program, prompt the user to input a number by printing \"Input a number greater than 2:\ \". If the user\'s input is less than or equal to 2, then print \"No prime number is smaller than .\ \", where is replaced by the number input by the user. If the user inputs a number greater than 2, then print \"Prime numbers smaller than : \". Then, on the same line, print out a list of all prime numbers that are smaller than the input. In your list, each prime number must be separated by one space and a newline must be printed immediately after the last number in the list. As part of your program, you must make a function that takes an integer as an argument, returns 1 if the integer is prime, and returns 0 otherwise. You can use the following function prototype: int isPrime(int number); Example input/output pairs (excluding the prompt) are provided below: Input: 6; Output: Prime numbers smaller than 6: 2 3 5 Input: 23; Output: Prime numbers smaller than 23: 2 3 5 7 11 13 17 19 Input: 1; Output: No prime number is smaller than 1.

Solution

Hi buddy, I\'ve written 2 programs for you. The first one is the one you asked for and the second one uses sieve method. That is much faster .

Method 1 :

#include <stdio.h>
#include <math.h>

int isPrime(int n){
    //Finding square root of n
    int e = (int)sqrt((float)n);
    //if n is even, return false
    if(n%2==0){
        return 0;
    }
    //Check whether n is divisible by all the odd numbers till e
    for(int i=3;i<=e;i+=2){
        if(n%i==0){
            return 0;
        }
    }
    return 1;
}

int main(void) {
        //Prompting and reading an integer
       printf(\"Input a number greater than 2\ \");
       int n;
       scanf(\"%d\",&n);
      
       //If number less than 2, give msg
       if(n<=2){
            printf(\"No prime number is smaller than %d\ \",n);
            return 0;
       }
      
       printf(\"Input : %d ; Output : 2 \",n);
      
       for(int i=3;i<=n;i++){
            if(isPrime(i)==1){
                printf(\"%d \",i);
            }
       }
       printf(\"\ \");
       return 0;
}

Input : 20

OUTPUT :

Input a number greater than 2
Input : 20 ; Output : 2 3 5 7 11 13 17 19

Method 2 :

#include <stdio.h>

int main(void) {
        //Prompting and reading an integer
       printf(\"Input a number greater than 2\ \");
       int n;
       scanf(\"%d\",&n);
      
       //If number less than 2, give msg
       if(n<=2){
            printf(\"No prime number is smaller than %d\ \",n);
            return;
       }
      
       printf(\"Input : %d ; Output : 2 \",n);
      
       //Initialize an array of n intergers and initialize with 1
       int primes[n+1];
       for(int i=0;i<=n;i++){
            primes[i] = 1;
       }
      
       //I\'m using sieve method to fill populate the array
       for(int i=2;i<=n;i++){
            for(int j=2;j*i<=n;j++){
                //Number i*j can\'t be prime number because both i and j are factors
                primes[i*j] = 0;
            }
       }
      
       //Print the numbers whose prime[i] is 1. (You can skip even numbers (+=2))
       for(int i=3;i<=n;i+=2){
            if(primes[i]==1){
                printf(\"%d \",i);
            }
       }
       printf(\"\ \");
       return 0;
}

Input : 20

OUTPUT :

Input a number greater than 2
Input : 20 ; Output : 2 3 5 7 11 13 17 19