Prime function

Language Help Python
1

def prime():
guess = input('gimme a number: ')

for x in range (2, int(guess)):
    if guess % x == 0:
        print ('this is not prime')

    else:
        print ('this is prime')

prime()

may i know whats wrong with my code

2

Two problems:

It raises TypeError: not all arguments converted during string formatting because the interpreter thinks that this line:

if guess % x == 0:

… is trying to format the string guess somehow, due to the presence of the % symbol.

Solution: make guess an int before the for loop. One way to do so:

guess = int(input("number, please: "))

Problem 2:
say you want to input 4. Here’s what you see on the screen:

gimmie a number: 4
this is not prime
this is prime

Solution: I’ll let you work on it a bit.

3 
import sys

def prime():
    guess = int(input('give me a number: '))

    if guess == 2:
        print('this is prime')

    for x in range (2, guess-1):
        if guess % x != 0:
            print ('this is prime')
            continue
        elif guess % x == 0 :
            print ('this is not prime')
            sys.exit()
        sys.exit()



prime()

i get few lines of ‘this is prime’ , instead of getting one only. may i know what the problem is

4

No need for sys.exit(). Just use break.
Start with this

for x in range (2, guess-1):
       if guess % x == 0 :
            print ('this is not prime')
            break

Run it; what happens? Now, think carefully, what what you want the alternative behavior to look like?

In other words, say the entire for loop (not each individual step) completes without triggering the of condition. What should happen?

5 

def prime():
    guess = int(input('give me a number: '))

    if guess == 2:
        print('this is prime')

    for x in range (2, int((guess**0.5)+1) ):
        if guess % x == 0:
            print('this is not prime')
            break
        else:
            print('this is prime')

prime()

i did this but it doenst work for 45…

6

OK, square root of guess is good.
But, I said, only do this much:

    for x in range (2, int((guess**0.5)+1) ):
        if guess % x == 0:
            print('this is not prime')
            break

… and that does work for 45.

Now, what you have is this:

    for x in range (2, int((guess**0.5)+1) ):        
        if guess % x == 0:
            print('this is not prime')
            break
        else:
            print('this is prime')

… so, when x == 2, the first time around the loop, the if condition is not true, so it goes to else. Is this what you want? Do you want it to go to else every time that if is not true?

Would it not be better to wait until all of the for loop has been completed, then if and only if no divisor has been encountered, print “this is prime”?

One way:

  • Declare a variable prime, set it to True
  • If the if condition becomes True (i.e., guess is not prime), set prime = False before break
  • No elif or else needed: outside of the for loop, just test whether prime is True or False. If True, print the appropriate message; if False, do nothing.