C Program (Prime Number in a given range) - c

Question

I have started learning C language. I wrote this program to find all prime numbers between the given range but I am unable to get the expected output.
Can anyone tell me what's wrong with this program please?
#include <stdio.h>
#include <conio.h>
void main() {
int min, max, i, j, count = 0;
printf("Enter Your First Number\n");
scanf("%d", &min);
printf("Enter Your Last Number\n");
scanf("%d", &max);
for(i=min; i<=max; i++) {
for(j=1; j<=i; j++) {
if(i % j == 0) {
count++;
}
}
if(count==2) {
printf("%d\t",i);
}
}
getch();
}

Answer 1

I just suggest getting rid of that count variable.
How do you know if a number N is prime? If for every j in the range (2 to N-1) you have N%j != 0.
So:
In the inner loop, use j from 2 to N-1 (instead of from 1 to N as you used tio do). In fact N%1 and N%N will be 0
The first time you find a j so that N % j == 0 break. You are sure it's not prime
Why incrementing count? For a prime number the j counter will be equal to i (because you looped until j<i, and the last j++ made j
equal to i). So just check for j == i and print the prime number i
#include <stdio.h>
#include <conio.h>
int main( void )
{
int min, max, i, j, count = 0;
printf("Enter Your First Number\n");
scanf("%d", &min);
printf("Enter Your Last Number\n");
scanf("%d", &max);
for(i=min; i<=max; i++)
{
// Was for(j=1; j<=i; j++)
for(j=2; j<i; j++)
{
if(i % j == 0)
{
//Was count++;
break;
}
}
//Was if(count==2)
if(j == i)
{
printf("%d\t",i);
}
}
getch();
return 0;
}

Answer 2

Here you are.
#include <stdio.h>
int main( void )
{
printf( "Enter the range of numbers (two unsigned integer numbers): " );
unsigned int first = 0, last = 0;
scanf( "%u %u", &first, &last );
if ( last < first )
{
unsigned int tmp = first;
first = last;
last = tmp;
}
do
{
int prime = first % 2 == 0 ? first == 2 : first != 1;
for ( unsigned int i = 3; prime && i <= first / i; i += 2 )
{
prime = first % i != 0;
}
if ( prime ) printf( "%u ", first );
} while ( first++ != last );
putchar( '\n' );
return 0;
}
The program output might look like
Enter the range of numbers (two unsigned integer numbers): 0 100
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97
As for your program then you need re-initialize the variable count before the inner loop
for(i=min; i<=max; i++) {
count = 0;
for(j=1; j<=i; j++) {
if(i % j == 0) {
count++;
}
}
And the inner loop is inefficient.

Answer 3

Need to reset the value of count. It starts at count=0, then for any inputs, the loops will count up. The For each outer loop index, it will go like this:
1 (1%1=0 --> count++, count = 1)
2 (2%1=0 --> count++, and 2%2=0 --> count++, count = 3)
3 (3%1=0 --> count++, and 3%3=0 --> count++, count = 5)
etc... until max is reached.

Answer 4

You can use a simple isprime function to check whether a number is prime or not and then call the function for the given interval.
To find whether a number is prime or not , we can use a simple primality test to check it.
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
bool isprime(int n)
{
if(n <= 1) return false;
if(n <= 3) return true;
if(n%2 == 0 || n%3 == 0) return false;
for(int i = 5;i*i <= n;i += 6)
{
if(n%i == 0 || n%(i + 2) == 0)
{
return false;
}
}
return true;
}
int main()
{
int a,b;
printf("Enter the first number :");
scanf("%d",&a);
printf("Enter the second number :");
scanf("%d",&b);
for(int i = a;i <= b;i++)
{
if(isprime(i)) printf("%d ",i);
}
return 0;
}

Answer 5

There is a simple change you should do:
#include <stdio.h>
#include <conio.h>
void main() {
int min, max, i, j, count;
printf("Enter Your First Number\n");
scanf("%d", &min);
printf("Enter Your Last Number\n");
scanf("%d", &max);
for(i=min; i<=max; i++)
{
count=1;
for(j=2; j<=i; j++)
{
if(i % j == 0) {
count++;
}
}
if(count==2) {
printf("%d\t",i);
}
}
}

Answer 6

My answer may be a bit late, but since it's the same issue, i'll write it here in case it helps someone else coming to this thread in the future.
My code is written from the POV of a beginner (No complex functions or data types are used) as this is a code that mostly they will get stuck on.
Working:
User inputs the range.
Using the for loop, each number in the range is sent to the isprime function which returns TRUE or FALSE after checking the condition for being a prime number.
if TRUE : program prints the number.
if FALSE : program skips the number using continue function.
#include<stdio.h>
int isprime(int num);
int main() {
int min, max;
printf("Input the low number: ");
scanf("%d", &min);
printf("Input the high number: ");
scanf("%d", &max);
for(int i = min; i<=max; i++) {
if(isprime(i) == 1) {
printf("%d ", i);
}
else if(isprime(i) == 0){
continue;
}
}
return 0;
}
int isprime(int num) {
int count = 0;
for(int i=2; i<=(num/2); i++) {
if(num % i == 0 ) {
count ++;
}
else{
continue;
}
}
if(count>0){
return 0;
}
else if (count == 0){
return 1;
}
}