Python Program to find Prime Number

Write a Python Program to Find Prime Number using For Loop, While Loop, and Functions. Any natural number that is not divisible by any other except 1 and itself is called Prime.

The list goes like 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, 101, 103, 107, 109 etc. 2 is the only even num.

Python Program to find Prime Number using For Loop

This program allows the user to enter any integer value and checks whether the given number is a Prime or Not using For Loop.

Number = int(input(" Please Enter any Number: "))
count =Number = int(input("Please Enter any Value: "))
count = 0

for i in range(2, (Number//2 + 1)):
    if(Number % i == 0):
        count = count + 1
        break

if (count == 0 and Number != 1):
    print(" %d is a Prime" %Number)
else:
    print(" %d is Not" %Number)

Within the for loop, there is an If statement to check whether the value divisible by i is exactly equal to 0 or not. If the condition is True, then the Count value is incremented, and then the Break Statement is executed. Next, we used another If statement to check whether Count is Zero and Num is Not equal to 1.

The user entered integer in the above example is 365

First Iteration: for i in range(2, 365//2)
It means, for i in range (2, 182.5) – Condition is True
Now, Check the if condition – if (365%2 == 0). As you know, the condition is False
Next, i become 3

Do the same for the remaining For iterations of the to find Prime Number.

Next, it enters into Python If statement. if(count == 0 && Number != 1 ). In all the above iterations, If the condition failed, so Count Value has not incremented from initialized o. And the one that we used is 365 (not zero). So, the condition is True, which means Prime.

Python Program to find Prime Number using While Loop

This program is the same as the above. We just replaced the For loop in the above program with While.

# using While Loop
Number = int(input(" Please Enter any Num: "))
count = 0
i = 2

while(i <= Number//2):
    if(Number % i == 0):
        count = count + 1
        break
    i = i + 1

if (count == 0 and Number != 1):
    print(" %d is a Prime" %Num)
else:
    print(" %d is not" %Num)

 Please Enter any Num: 14
 14 is not
>>> 
 Please Enter any Num: 109
 109 is a Prime

Prime Number using Functions

This program is the same as the first example. However, we separated the logic by defining the new Function.

# using Functions
def finding_factors(Number):
    count = 0

    for i in range(2, (Number//2 + 1)):
        if(Number % i == 0):
            count = count + 1
    return count

Num = int(input(" Please Enter any Num: "))

cnt = finding_factors(Num)

if (cnt == 0 and Num != 1):
    print(" %d is a Prime" %Num)
else:
    print(" %d is not" %Num)

 Please Enter any Num: 44
 44 is not
>>> 
 Please Enter any Num: 139
 139 is a Prime