How to find the prime numbers in python?

Question

I am a beginner in python. But I am not getting why I am still getting an error. I use to work in MATLAB, now I need to learn python for my Internship. Can someone help me out in fixing this problem?

Below is my code to find the number of prime numbers

def prime_number(num):
    myprime = []
    for x in range(2,num):
        prime = True
        if x == 2:
            myprime.append(x)
        else:
            for y in prime:
                if x%y == 0:
                    prime = False
                    break;
            if prime:
                myprime.append(x)

    return myprime

This is my error:

TypeError                                 Traceback (most recent call last)
<ipython-input-87-cd2bf40e6117> in <module>
----> 1 prime_number(100)

<ipython-input-86-169e0a64e50a> in prime_number(num)
     13         else:
     14 
---> 15             for y in prime:
     16 
     17                 if x%y == 0:

TypeError: 'bool' object is not iterable

Can you also tell me how to fix the indentation or learn more about Indentation in python. I got to know identation in python is very important. I am using python 3 version.

You set prime to `True` then try to iterate through `prime` with `for y in prime:` you cant iterate through a boolean (`True`) since there is nothing to iterate over — Mason Caiby, Aug 15 '19 at 16:56
`prime` is of boolean type. To use for loop you need iterable object like list. Do you want to iterate over `myprime` maybe — bkyada, Aug 15 '19 at 16:56
You cannot iterate through a `boolean`. What do you expect to have in the variable `prime`? — idkman, Aug 15 '19 at 16:57
This might help: https://hackernoon.com/prime-numbers-using-python-824ff4b3ea19 — Adrian, Aug 15 '19 at 17:00

Gage · Answer 1 · 2019-08-15T17:48:32.473

0

Corrected with some help =)

   def prime_number(num):
    myprime = []
    for x in range(1, num):
        if x == 2:
            myprime.append(x)
            continue
        prime = True
        for y in myprime:
            if y <= 1:
                continue
            if x <= 2:
                continue
            if x%y == 0:
                prime = False
                continue
        if prime:
            myprime.append(x)
    return myprime

edited Aug 15 '19 at 17:48

answered Aug 15 '19 at 17:04

Gage

51
1
3

Oh I see what I did. This only finds all odd numbers + 2 =) – Gage Aug 15 '19 at 17:31

score 0 · Answer 2 · answered Aug 15 '19 at 17:12

You confused yourself with variables that are too brief. Been there, done that ...

        for y in prime:

should be

        for y in myprime:

You want to iterate through the list of primes already found. I suggest a global change in variable names:

prime => is_prime
my_prime => prime_list

You'll learn your own naming style as you learn programming.

For the algorithms in general, if you search in your browser for "Python find primes", you'll find references that can explain this much better than we can manage here.

score 0 · Answer 3 · answered Aug 22 '19 at 15:57

This might be a little advanced, but it is my favorite combination of brevity, code clarity, and speed for n below about a billion. Beyond that, C++ is your friend.

Credit to @Robert William Hanks for the brilliant sieve function. Only the driver code at the bottom is mine.

#!/usr/bin/python -Wall
import sys
def rwh_primes1(n):
    # https://stackoverflow.com/questions/2068372/fastest-way-to-list-all-primes- 
    #     below-n-in-python/3035188#3035188
    """ Returns  a list of primes < n """
    sieve = [True] * (n/2)
    for i in xrange(3,int(n**0.5)+1,2):
        if sieve[i/2]:
            sieve[i*i/2::i] = [False] * ((n-i*i-1)/(2*i)+1)
    return [2] + [2*i+1 for i in xrange(1,n/2) if sieve[i]]

if len(sys.argv) > 1:
    N = int(float(sys.argv[1]))
else:
    N = 10000000    # default: 1e7 10,000,000

p = rwh_primes1(N)
print len(p), "primes found <", N
if N < 1000:
    print p

How to find the prime numbers in python?

3 Answers3