Printing Prime numbers in a range - c

Question

My code is not running the loop. I've tried changing it from for to while loop, tried changing the condition for prime number but still the same response. I've attached my output in this program output image.
#include<stdio.h>
int main(void) {
int a, b, flag = 0;
int i, j;
printf("Enter lower limit of range\n");
scanf("%d", &a);
printf("Enter upper limit of range\n");
scanf("%d", &b);
printf("The prime numbers between %d and %d are\n", a, b);
i = a;
j = 2;
while (i <= b) {
while ( j < i/2 )
{
if (i % j == 0)
{
flag = 1;
}
j++;
}
if (flag == 0)
{
printf("%d\n", i);
}
i++;
}
return 0;
}

Answer 1

You need at least to reset the variable flag to 0 and j to 2 in each iteration of the outer loop. Also the condition j < i /2 should be changed to j <= i / 2 (for example when i is equal to 4).
while (i <= b) {
flag = 0;
j = 2;
while ( flag == 0 && j <= i/2 )
{
if (i % j == 0)
{
flag = 1;
}
j++;
}
if (flag == 0)
{
printf("%d\n", i);
}
i++;
}
Pay attention to that the program can incorrectly consider 2 as a non-prime number and 1 as a prime number.
I would write the loops the following way
for ( ; i <= b; i++ )
{
int prime = i % 2 == 0 ? i == 2 : ( i == 3 ) || ( i != 1 && i % 3 != 0 );
for ( int j = 5; prime && j <= i / j; j += 6 )
{
prime = i % j != 0 && i % ( j + 2 ) != 0;
}
if ( prime )
{
printf("%d\n", i);
}
}
Or a more simple variant
for ( ; i <= b; i++ )
{
int prime = i % 2 == 0 ? i == 2 : i != 1;
for ( int j = 3; prime && j <= i / j; j += 2 )
{
prime = i % j != 0;
}
if ( prime )
{
printf("%d\n", i);
}
}
You should declare the variables a and b as having the type unsigned int instead of int to avoid using negative numbers.
Here is a demonstration program.
#include <stdio.h>
int main( void )
{
unsigned int a = 0, b = 0;
printf( "Enter lower limit of range " );
scanf( "%u", &a );
printf( "Enter upper limit of range " );
scanf( "%u", &b );
printf( "The prime numbers between %u and %u are\n", a, b );
for ( unsigned int i = a; i <= b; i++ )
{
int prime = i % 2 == 0 ? i == 2 : i != 1;
for ( unsigned int j = 3; prime && j <= i / j; j += 2 )
{
prime = i % j != 0;
}
if (prime)
{
printf( "%u ", i );
}
}
putchar( '\n' );
}
The program output might look like
Enter lower limit of range 0
Enter upper limit of range 100
The prime numbers between 0 and 100 are
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