Program 6-14: Program to check if the input number is prime or not.

Question

Accepted Answer

> [!TLDR]
> A prime has no divisor other than 1 and itself, so the program tries every candidate divisor from 2 to num−1; the first exact division proves the number composite (and `break`s), while surviving the whole loop proves it prime.

**The idea.** This combines three constructs from the chapter: a `for` loop to generate candidate divisors, the modulus test `num % i == 0` to detect a divisor, and `break` to stop as soon as the verdict is known. A Boolean *flag* (`is_prime`) carries the verdict out of the loop.

```python
# Program 6-14: check whether the input number is prime
num = int(input("Enter a number: "))
is_prime = True
if num <= 1:
    is_prime = False          # 0, 1 and negatives are not prime by definition
for i in range(2, num):
    if num % i == 0:          # found a divisor other than 1 and num
        is_prime = False
        break                 # no need to test further
if is_prime:
    print(num, "is a prime number")
else:
    print(num, "is not a prime number")
```

**Sample runs**

```
Enter a number: 7
7 is a prime number
```

```
Enter a number: 10
10 is not a prime number
```

**Dry run for num = 10**: `i` = 2 → `10 % 2` is 0 → flag set False, `break`. Verdict: not prime (only one division needed). …

Q.Program 6-14: Program to check if the input number is prime or not.

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Q.Program 6-14: Program to check if the input number is prime or not.

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