Data Structure
Networking
RDBMS
Operating System
Java
MS Excel
iOS
HTML
CSS
Android
Python
C Programming
C++
C#
MongoDB
MySQL
Javascript
PHP
Physics
Chemistry
Biology
Mathematics
English
Economics
Psychology
Social Studies
Fashion Studies
Legal Studies
- Selected Reading
- UPSC IAS Exams Notes
- Developer's Best Practices
- Questions and Answers
- Effective Resume Writing
- HR Interview Questions
- Computer Glossary
- Who is Who
C program for a number to be expressed as a sum of two prime numbers.
Problem
Find out if a given number can be expressed as sum of two prime numbers or not.
Given a positive integer N, we need to check if the number N can be represented as a sum of two prime numbers.
Solution
Consider an example given below −
20 can be expressed as sum of two prime numbers 3 and 17, 13 and 7.
20= 3+7
20= 13+7
Algorithm
Refer an algorithm given below for expressing a given number as a sum of two prime numbers.
Step 1 − Input the number to be checked at run time.
Step 2 − Repeat from i = 2 to (num/2).
Step 3 − Check i is a prime number.
Step 4 − If i is prime, check if (n - i) is a prime number.
Step 5 − If both (i)and (n - i) are primes, then, the given number can be represented as the sum of primes i and (n - i).
Example
Following is the C program for expressing a given number as a sum of two prime numbers −
#include <stdio.h>
int Sum(int n);
int main(){
int num, i;
printf("Enter number: ");
scanf("%d", &num);
int flag = 0;
for(i = 2; i <= num/2; ++i){
if (sum(i) == 1){
if (sum(num-i) == 1){
printf("
The given %d can be expressed as the sum of %d and %d
", num, i, num - i);
flag = 1;
}
}
}
if (flag == 0)
printf("The given %d cannot be expressed as the sum of two prime numbers
", num);
return 0;
}
//check if a number is prime or not
int sum(int n){
int i, isPrime = 1;
for(i = 2; i <= n/2; ++i){
if(n % i == 0){
isPrime = 0;
break;
}
}
return isPrime;
}Output
When the above program is executed, it produces the following output −
Run 1: Enter number: 34 The given 34 can be expressed as the sum of 3 and 31 The given 34 can be expressed as the sum of 5 and 29 The given 34 can be expressed as the sum of 11 and 23 The given 34 can be expressed as the sum of 17 and 17 Run 2: Enter number: 11 The given 11 cannot be expressed as the sum of two prime numbers
4K+ Views
