Find prime numbers in a list python

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Find prime numbers in a list python

By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I was having issues in printing a series of prime numbers from one to hundred. I can't figure our what's wrong with my code. You need to check all numbers from 2 to n-1 to sqrt n actually, but ok, let it be n.

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If n is divisible by any of the numbers, it is not prime. If a number is prime, print it. As I've said already, it would be better to check divisors not from 2 to n-1, but from 2 to sqrt n :. You can improve it a little more by incrementing the range you check by 2, and thereby only checking odd numbers. Like so:. As in the first loop odd numbers are selected, in the second loop no need to check with even numbers, so 'i' value can be start with 3 and skipped by 2. So, you are only ever checking if it divisible by 2, giving you all odd numbers.

Instead of trial division, a better approach, invented by the Greek mathematician Eratosthenes over two thousand years ago, is to sieve by repeatedly casting out multiples of primes. Begin by making a list of all numbers from 2 to the maximum desired prime n.

Then repeatedly take the smallest uncrossed number and cross out all of its multiples; the numbers that remain uncrossed are prime. For example, consider the numbers less than Initially, 2 is identified as prime, then 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28 and 30 are crossed out. Next 3 is identified as prime, then 6, 9, 12, 15, 18, 21, 24, 27 and 30 are crossed out. The next prime is 5, so 10, 15, 20, 25 and 30 are crossed out.

And so on.

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The numbers that remain are prime: 2, 3, 5, 7, 11, 13, 17, 19, 23, and An optimized version of the sieve handles 2 separately and sieves only odd numbers.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. This code has a flaw: Since numbers is an unordered set, there is no guarantee that numbers.

Python Program to Check Prime Number

Nevertheless, it works at least for me for some input numbers:. Warning: timeit results may vary due to differences in hardware or version of Python. Personally I think it is a shame that numpy which is so widely used is not part of Python standard library, and that the improvements in syntax and speed seem to be completely overlooked by Python developers.

There's a pretty neat sample from the Python Cookbook here -- the fastest version proposed on that URL is:. You assume that numbers. Sets are unordered and pop removes and returns an arbitrary element, so it cannot be used to select the next prime from the remaining numbers. For truly fastest solution with sufficiently large N would be to download a pre-calculated list of primesstore it as a tuple and do something like:.

If you don't want to reinvent the wheel, you can install the symbolic maths library sympy yes it's Python 3 compatible.

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And use the primerange function. The only substantial change is using a bytearray instead of a list for the boolean, and using compress instead of a list comprehension to build the final list. I'd add this as a comment like moarningsun if I were able. It's instructive to write your own prime finding code, but it's also useful to have a fast reliable library at hand. If you have control over N, the very fastest way to list all primes is to precompute them.

Precomputing is a way overlooked optimization.

Python program to print all Prime numbers in an Interval

For the fastest code, the numpy solution is the best. Since I make the entire list in memory, you need enough space to hold everything, but it seems to scale fairly well. Can someone compare this with the other timings? On my machine it seems pretty comparable to the other Numpy half-sieve. I tested some unutbu's functionsi computed it with hungred millions number.

Complete code on my github repository. First time using python, so some of the methods I use in this might seem a bit cumbersome. Nonetheless this is my suggestion for a pure python prime sieve, based on omitting the multiples of 2, 3 and 5 by using appropriate steps while processing the sieve forward. By using a ctypes. Here is a numpy version of Sieve of Eratosthenes having both good complexity lower than sorting an array of length n and vectorization.

Compared to unutbu times this just as fast as the packages with 46 microsecons to find all primes below a million. I've updated much of the code for Python 3 and threw it at perfplot a project of mine to see which is actually fastest. So why not just write a slow script that generates another source file that has all numbers hardwired in it, and then import that source file when you run your actual program. Of course, this works only if you know the upper bound of N at compile time, but thus is the case for almost all project Euler problems.

PS: I might be wrong though iff parsing the source with hard-wired primes is slower than computing them in the first place, but as far I know Python runs from compiled.

While searching for the next composite, we need to test odd numbers only. While we're at it: instead of the elegant 'D. At least on my machine PGhz.

So I suggest this implementation of this clever algorithm:. The fastest method I've tried so far is based on the Python cookbook erat2 function:. See this answer for an explanation of the speeding-up.Given two positive integer start and end. The task is to write a Python program toprint all Prime numbers in an Interval. Definition: A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself.

The idea to solve this problem is to iterate the val from start to end using a for loop and for every number, if it is greater than 1, check if it divides n.

If we find any other number which divides, print that value.

find prime numbers in a list python

The above solution can be optimized using Sieve of Erathostenes. Please see print prime numbers in a range for details.

find prime numbers in a list python

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Writing code in comment? Please use ide. Python program to print all. If num is divisible by any number. Recommended Posts: Python Program to Print Numbers in an Interval Program to find Prime Numbers Between given Interval Program to print first N Prime numbers Python Program for Efficient program to print all prime factors of a given number Print the nearest prime number formed by adding prime numbers to N Print prime numbers with prime sum of digits in an array Python program to print odd numbers in a List Python program to print even numbers in a list Python program to print all even numbers in a range Python program to print all odd numbers in a range Python program to print positive numbers in a list Python program to print negative numbers in a list Python program to print all positive numbers in a range Python program to print all negative numbers in a range Python program to print all Strong numbers in given list.

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I have a prime function which accepts a list of numbers and checks if all of the numbers in the list are prime. But if you really want to make it more readable, you should factor out the prime check into another function:. Another alternative is to compute a set of primes once and then just check if all of them are in this set:.

So if you know what the largest number is which you need to check, checking against a set that is computed only once is the fastest. You even unnecessarily instantiate this range to a list.

Which of the 2 options is better depends on lista. If there are a lot, smaller elements, the 2nd method is faster, for a few, larger elements, method 1 will be better. I will not get into details on which method to generate all the primes to max listaor the prime check.

Here you can choose the algorithm most suited for your need. There are a lot alternative on the web and SO. Checking whether all elements of lista are primes can also be done with set.

Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Function to check whether entire list is prime numbers Ask Question. Asked 1 year, 10 months ago. Active 1 year, 10 months ago. Viewed 3k times. Jun 7 '18 at Active Oldest Votes. This has the advantage that the range does not get consumed into a list which makes this stop generating values as soon as a value is found not to be prime. Graipher Graipher Note: The second edition of this book is available under the title Cracking Codes with Python.

Topics Covered In This Chapter:. Leonhard Euler, 18 th century mathematician. All of the ciphers described in this book so far have been around for hundreds of years, and all of them with the exception of the one-time pad are easily broken by computers. These ciphers worked very well when hackers had to rely on pencil and paper to hack them, but computers can now manipulate data trillions of times faster than a person with a pencil.

The RSA cipher has several improvements over these old ciphers, and it will be detailed in the next chapter. However, the RSA cipher will require us to learn about prime numbers first. A prime number is an integer that is, a whole number that is greater than 1 and has only two factors: 1 and itself. Remember that the factors of a number are the numbers that can be multiplied to equal the original number.

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The numbers 3 and 7 are factors of The number 12 has factors 2 and 6, but also 3 and 4. Every number has factors of 1 and itself. The numbers 1 and 21 are factors of The numbers 1 and 12 are factors of This is because 1 times any number will always be that same number. But if no other factors exist for that number, then that number is prime.

There are an infinite number of prime numbers. They just keep getting bigger and bigger, just like regular numbers do. The RSA cipher makes use of very large prime numbers in the keys it uses. Because of this, there will be far too many keys to brute-force through. A googol is the number that has a one with a hundred zeros behind it:. A billion billion billion googols has twenty-seven more zeros than a googol:.

But these are tiny numbers. A typical prime number used in our RSA program will have hundreds of digits:. Integers that are not prime numbers are called composite numbers, because they are composed of at least two factors besides 1 and the number itself. Prime numbers are integers greater than 1that have only 1 and themselves as factors.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I was trying to generate all prime numbers in range x to y. I tried simple example first: range 10,11 which means to check if 10 is a prime number: Here is my code:.

Use all to check all elements from 2 upto x-1 met conditions:. But also, attention should be paid to optimized code. As someone said in the the comments in Kasra's answer.

find prime numbers in a list python

Learn more. Finding prime numbers using list comprehention Ask Question. Asked 4 years, 9 months ago. Active 2 years, 1 month ago. Viewed 19k times. How do we do that? Amit Kumar Gupta JavaSa JavaSa 4, 12 12 gold badges 45 45 silver badges 93 93 bronze badges. Active Oldest Votes. Could be better if it ran only upto sqrt x. Nice use of allI now know how many places I could've used it in. Nice solution! Only thing is it shouldn't check to x. I got this one.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I've written some simple programs so I think I get the basics but for whatever reason this program that is meant to list all primes less than or equal to n is not working:.

Which looks almost right but for some reason contains 27, 35, 95, which are composite of course. I've been trying to pick apart the way my loop works but I just don't see where it skips checking for divisibility suddenly. I figured that if someone had a look they could explain to me what about the syntax is causing this.

Thanks a bunch! This is perhaps a more idiomatic solution using a for loop instead of a while loopand works perfectly. The inner one iterates to all numbers from 2 to p. If it reaches a number that divides evenly into pthen it breaks out of the inner loop. The else block executes every time the for loop isn't broken out of printing the prime numbers. As a side note, you only need to iterate through 2 to the square root of psince each factor has a pair. If you don't get a match there won't be any other factors after the square root, and the number will be prime.

Your code has two loops, one inside another. It should help you figure out the code if you replace the inner loop with a function. Then make sure the function is correct and can stand on its own separate from the outer loop.

It is a stand-alone pure function. Incrementing p inside the inner loop was the problem, and this decomposed version doesn't have the problem. Note that in Python, range never includes the upper bound that you pass in. Python has some built-in stuff that is fun. It sets i to values from range 2, n and checks each one, and if a test ever fails it stops checking and returns.

How to generate prime numbers using Python?

If it checks n against every number in the range and not any of them divide n evenly, then the number is prime. Again, you don't need to learn all these tricks right away, but I think they are kind of fun when you do learn them.

5.18. Prime Number List - Python

A while loop executes the body, and then checks if the condition at the top is Trueif it is true, it does the body again. A for loop executes the body once for each item in the iterator. I am going to tell your error only,in line 3 you are incrimenting p but actually what you are missing is your i if your i in previous case is let say 13 then it will check your loop after 13 but it is leaving 2,3,5,7,11 so its an error. This program has two main optimizations, first it only iterates from 2 to the square root of n and it only iterates through odd numbers.

Using these optimizations I was able to find out that the number is prime in only loop iterations. This in my opinion is a more optimised way. This finds all the prime numbers up to 1, in less than 8 seconds on my setup. Learn more. Asked 7 years, 2 months ago. Active 1 year, 6 months ago.


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