issue with for loop in Python

Question

I am trying to learn python and I have tried to convert a code snippet in js to python. I have created a function in js to calculate GCD as follows:

// program to find the GCD of two integers

let gcd

function GCD() {
// take input
const number1 = prompt('Enter a first positive integer: ')
const number2 = prompt('Enter a second positive integer: ')

// looping from 1 to number1 and number2
for (let i = 1; i <= number1 && i <= number2; i++) {

    if( number1 % i == 0 && number2 % i == 0) {
        gcd = i
    }
}

// display the gcd
document.write(`GCD of ${number1} and ${number2} is ${gcd}.`)
}

GCD()

If I supply the first integer as 9 and the second integer of 3, I get the GCD as 3.

I have tried to convert this to a python program as follows:

def gcd():
    gcd = 0

    num1 = int(input("Enter a first positive integer: "))
    num2 = int(input("Enter a second positive integer: "))
    
    for i in range(1, i<=num1 and i<=num2):
        if num1 % i == 0 and num2 % i == 0:
            gcd = i

    print(f"GCD of: {num1} and: {num2} is {gcd}")


gcd()

But I don't know how to get the for loop in python quite right. If I change the for statement to:

def gcd():
    gcd = 0

    num1 = int(input("Enter a first positive integer: "))
    num2 = int(input("Enter a second positive integer: "))

    for i in range(1, num1 and num2):
        if num1 % i == 0 and num2 % i == 0:
            gcd = i

    print(f"GCD of: {num1} and: {num2} is {gcd}")


gcd()

and I give the same input as before of 9 and 3, I get GCD of 1

Thanks, that worked out :) but could you please explain the syntax? is the min I guess to set the min range and the +1 to set it to the correct amount of iterations, for example to get 6 iterations I set range to for I in range(1, 7): — user2371684
– user2371684, Commented Mar 21, 2021 at 10:53

khelwood · Accepted Answer · 2021-03-21 12:23:11Z

2

You can't have an equivalent syntax as i <= number1 && i <= number2 as it is a boolean expression that works as stop condition which is checked at every iteration.

While a for loop in python code generates values with boudaries, to do the same, iterate until the lowest value:

for i in range(1, min(num1, num2) + 1):
    if num1 % i == 0 and num2 % i == 0:
        gcd = i

To use the condition of the JS code, you need a while loop:

i = 1
while i <= num1 and i <= num2:
    if num1 % i == 0 and num2 % i == 0:
        gcd = i
    i += 1

edited Mar 21, 2021 at 12:23

khelwood

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answered Mar 21, 2021 at 10:43

azro

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1 Comment

user2371684 Over a year ago

Thanks.. I need to understand the structure of a for loop in python :)

odaiwa · Accepted Answer · 2021-03-21 10:50:14Z

0

this is a solution based on while loop

def gcd():
    gcd=0
    num1 = int(input("Enter a first positive integer: "))
    num2 = int(input("Enter a second positive integer: "))
    i = 1
    while(i <= num1 and i <= num2):
        if(num1 % i == 0 and num2 % i == 0):
            gcd = i
        i = i + 1
    print("GCD : ", gcd)

gcd()

answered Mar 21, 2021 at 10:50

odaiwa

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