Linked Questions

137 votes
29 answers
12k views

Making Future Posts Runnable Online with Stack Snippets [closed]

Stack Snippets were recently added to PPCG! Reminiscent of JSFiddle, Stack Snippets allow HTML, CSS, and JavaScript to be run directly in posts! Here is a very simple Stack Snippet: ...
Calvin's Hobbies's user avatar
49 votes
71 answers
7k views

An abundance of integers!

An Abundant number is any number where the sum of its proper divisors is greater than the original number. For example, the proper divisors of 12 are: ...
DJMcMayhem's user avatar
  • 59.3k
43 votes
57 answers
8k views

Catalan Numbers

The Catalan numbers (OEIS) are a sequence of natural numbers often appearing in combinatorics. The nth Catalan number is the number of Dyck words (balanced strings of parenthesis or brackets such as <...
a spaghetto's user avatar
  • 11.2k
33 votes
68 answers
6k views

Count the divisors of a number

Introduction This is a very simple challenge: simply count the divisors of a number. We've had a similar but more complicated challenge before, but I'm intending this one to be entry-level. The ...
jqkul's user avatar
  • 2,272
37 votes
61 answers
5k views

Sum of primes between given range

Write the shortest code for finding the sum of primes between \$a\$ and \$b\$ (inclusive). Input \$a\$ and \$b\$ can be taken from command line or stdin (space seperated) Assume \$1 \le a \le b \le ...
st0le's user avatar
  • 2,218
136 votes
11 answers
14k views

(-a) × (-a) = a × a

We all know that \$(-a) \times (-a) = a \times a\$ (hopefully), but can you prove it? Your task is to prove this fact using the ring axioms. What are the ring axioms? The ring axioms are a list of ...
Wheat Wizard's user avatar
  • 98.8k
34 votes
51 answers
4k views

Prime counting function

Introduction The Prime Counting Function, also known as the Pi function \$\pi(x)\$, returns the amount of primes less than or equal to x. Challenge Your program will take an integer x which you can ...
Pavel's user avatar
  • 9,377
97 votes
15 answers
12k views

Create a programming language that only appears to be unusable

Robbers' challenge thread is here. Cops' challenge: Design a programming language that appears to be unusable for programming, but admits computation (or at least completion of the task) through some ...
feersum's user avatar
  • 31.4k
35 votes
54 answers
4k views

Generating a random alphanumeric string of length N

What is the shortest way of generating a random string with a given length and with only alphanumeric characters allowed? example of a random string: with N = 9 output would be ...
m13r's user avatar
  • 455
36 votes
41 answers
4k views

Is it a Mersenne Prime?

A number is a Mersenne Prime if it is both prime and can be written in the form 2n-1, where n is a positive integer. Your task is to, given any positive integer, determine whether or not it is a ...
FlipTack's user avatar
  • 14.7k
49 votes
35 answers
2k views

PPCG Jeopardy: Cops

How well do you know the site? Let's find out. This is a cops-and-robbers challenge. Robber's thread. As a cop, you need to: Find a non-deleted, non-closed challenge on this site to answer. The ...
Nathan Merrill's user avatar
87 votes
6 answers
42k views

Hard code golf: Regex for divisibility by 7

Matthias Goergens has a 25,604-character (down from the original 63,993-character) regex to match numbers divisible by 7, but that includes a lot of fluff: redundant parentheses, distribution (...
Charles's user avatar
  • 2,771
27 votes
43 answers
2k views

A prime test that's LITERALLY prime

Write a program that will test the primality of a specified number, and give the output as a Boolean value (True is prime). Your prime test can (but doesn't have to) be valid for the number 1. Here'...
Noah L's user avatar
  • 861
39 votes
30 answers
5k views

Can the number reach 1 by repeatedly subtracting the largest prime less than it?

Challenge: Given a number, take the largest prime strictly less than it, subtract it from this number, do this again to this new number with the biggest prime less than it, and continue doing this ...
xenia's user avatar
  • 8,142
33 votes
30 answers
5k views

Ah, why is there a bug in my program?

This challenge is inspired by @LuisMendo's MATL answer to my "binary multiples" challenge. Task Pick a code-golf challenge that is open and that has been posted before this one; let me call it the "...
RGS's user avatar
  • 14.1k

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