In this example, you
will learn to print all prime numbers between two numbers entered by the user.

To
understand this example, you should have the knowledge of the following ** C
programming** topics:

·
*C if...else Statement*

·
*C for Loop*

·
*C break and continue*

## Display
Prime Numbers Between Two Intervals

```
#include <stdio.h>
int main() {
int low, high, i, flag;
printf("Enter two numbers(intervals): ");
scanf("%d %d", &low, &high);
printf("Prime numbers between %d and %d are: ", low, high);
// iteration until low is not equal to high
while (low < high) {
flag = 0;
// ignore numbers less than 2
if (low < = 1) {
++low;
continue;
}
// if low is a non-prime number, flag will be 1
for (i = 2; i < = low / 2; ++i) {
if (low % i == 0) {
flag = 1;
break;
}
}
if (flag == 0)
printf("%d ", low);
// to check prime for the next number
// increase low by 1
++low;
}
return 0;
}
```

**Output**

Enter two numbers(intervals): 20

50

Prime numbers between 20 and 50 are: 23 29 31 37 41 43 47

In this program, the `while`

loop is iterated
( `high-low-1`

) times.

In
each iteration, whether `low`

is
a prime number or not is checked, and the value of `low`

is incremented by `1`

until `low`

is equal to `high`

.

Visit
this page to learn more about how to check whether a number is prime or
not.

If the user enters the larger number first, the above program doesn't work as intended. You can solve this issue by swapping the numbers.

## Display
Prime Numbers when Larger Number is Entered first

```
#include <stdio.h>
int main() {
int low, high, i, flag, temp;
printf("Enter two numbers(intervals): ");
scanf("%d %d", &low, &high);
// swap numbers if low is greather than high
if (low > high) {
temp = low;
low = high;
high = temp;
}
printf("Prime numbers between %d and %d are: ", low, high);
while (low < high) {
flag = 0;
// ignore numbers less than 2
if (low < = 1) {
++low;
continue;
}
for (i = 2; i < = low / 2; ++i) {
if (low % i == 0) {
flag = 1;
break;
}
}
if (flag == 0)
printf("%d ", low);
++low;
}
return 0;
}
```

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