# Questions tagged [array]

A competition to solve a particular problem through the usage and manipulation of arrays.

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### Xorting an array

Conceptually, this challenge is really simple. You're given a list of non-negative integers ai. If possible, find a non-negative integer ...
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### 2048-like array shift

Assume we want to shift an array like it is done in the 2048 game: if we have two equal consecutive elements in array, merge them into twice the value element. Shift must return a new array, where ...
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### N-dimensional N^N array filled with N

In: Enough memory and a positive integer N Out: N-dimensional N^N array filled with N, where N^N means N terms of N-by-N-by-N-by... Examples: 1: [1] ...
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### Lossy Sorting (Implement Dropsort)

Dropsort, designed by David Morgan-Mar, is an example of a linear-time "sorting algorithm" that produces a list that is, in fact, sorted, but contains only some of the original elements. Any ...
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### Count trailing truths

Inspired by, and in memory of, my dear friend and colleague, Dan Baronet, 1956 – 2016. R.I.P. He found the shortest possible APL solution to this task: Task Given a Boolean list, count the number of ...
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### The forbidden built-in

In the standard loopholes, the following is forbidden: Claiming that your answer is written in "MyOwnLanguage", where the command x means "read a sequence of ...
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### Be the First 1 (leave only the first Truthy)

Intro Every year, Dyalog Ltd. holds a student competition. The challenge there is to write good APL code. This is a language agnostic code-golf edition of this year's eighth problem. I have explicit ...
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### Increment an Array

Given a nonempty array of positive integers, "increment" it once as follows: If all the array elements are equal, append a 1 to the end of the array. For example: ...
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### There are two new sheriffs in town – Identifying DJMcMego pairs!

We have some new sheriffs moderators in town, Mego and DJMcMayhem. We need a challenge to properly honour them for their new positions, so there we go. Here's something that has caught my attention ...
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### Surrounded Countries

Countries own a series of territories on a 1D world. Each country is uniquely identified by a number. Ownership of the territories can be represented by a list as follows: ...
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### Detect heat waves

Background The Royal Netherlands Meteorological Institute defines a heat wave* as a series of at least 5 consecutive days of ≥25°C weather (“summery weather”), such that at least 3 of those days are ≥...
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### Covering a Skyline with brush strokes

Given a non-negative integer skyline height list, answer how many uninterrupted 1-unit-high horizontal brush strokes are needed to cover it. [1,3,2,1,2,1,5,3,3,4,2]...
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### Cover up zeroes in a list

Inspired by this SO question As input you will be given a non-empty list of integers, where the first value is guaranteed to be non-zero. To construct the output, walk from the start of the list, ...
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### Moving modest minimum

Inspired by a question over at Stack Overflow. The title here is entirely my fault. The challenge Given a list of positive integers containing at least two entries, replace each number by the ...
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### Find the first duplicated element

Given an array a that contains only numbers in the range from 1 to a.length, find the first duplicate number for which the second occurrence has the minimal index. In other words, if there are more ...
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### At least h with at least h

Input A list of nonnegative integers. Output The largest nonnegative integer h such that at least h of the numbers in the ...
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### Duck, Duck, Josephus

Given an Integer array: Start from the first number Jump forward n positions where n is the value of the current position Delete the current position, making what was the next position the current ...
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### Implementing a stack

I can't believe we don't have this already.. It's one of the most important data-structures in programming, yet still simple enough to implement it in a code-golf: Challenge Your task is to ...
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### The shortest way to find one unique value when all other values are the same

In this question at Code Review they tried to find the fastest way to return the unique element in an array where all the elements are the same except one. But what is the shortest code that ...
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### Weapons of Math Instruction

Last time when I tried to come up with something easy that wasn't a duplicate, it ended up being way too hard.. So hopefully this time it's indeed something newcomers can try as well. Input: An ...
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### First code golf decathlon [closed]

Tasks All competitors try to solve the following list of 10 tasks: math Read a positive integer n from input and return the sum of the cubes of the first n non-negative integers. For input ...
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### Into how many pieces can you cut this string?

Consider a piece of string (as in "rope", not as in "a bunch of characters"), which is folded back and forth on the real line. We can describe the shape of the string with a list of points it passes ...
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### Zigzagify a Matrix

As part of its compression algorithm, the JPEG standard unrolls a matrix into a vector along antidiagonals of alternating direction: Your task is to take a matrix (not necessarily square) and return ...
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### Am I an insignificant array?

An insignificant array is an array of positive integers, where the absolute differences between consecutive elements are all smaller than or equal to 1. For example, the following array is ...
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### Sever-sort an array

Challenge Given a non-empty array of integers, e.g.: [5, 2, 7, 6, 4, 1, 3] First sever it into arrays where no item is larger than the previous (i.e. non-...
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### Unflatten an Array

This challenge was inspired by a question on Mathematica.SE. Say you've got a nested list/array of some arbitrary structure (the lists at each level don't necessarily have the same length). For ...
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### Remove every N-th N

The task In this challenge, your input is a non-empty list of positive integers, given in the native format of your language. Your output is that same list, in the same format, with some elements ...
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### Implement the "Lazy Sort"

I'm supposed to sort a list of numbers, but I'm super lazy. It's really hard to figure how to swap all the numbers around until all of them are in increasing order, so I came up with my own algorithm ...
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### An Array of Challenges #1: Alternating Arrays

Alternating Arrays An alternating array is a list of any length in which two (not necessarily different) values are alternating. That is to say, all even-indexed items are equal, and all odd-indexed ...
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### In Search of a Soulmate

Given a nonempty finite list of integers, output a truthy value if there are exactly two equal entries and all other entries are distinct, and a falsey value otherwise. Examples ...
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### Hot moo shuffle: milk an array

Background Milking the deck is the name given to the following card shuffling method: 'Pinch' the deck to remove the top and bottom cards simultaneously. (With lots of imagination, this action ...
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### Is it true? Ask Jelly!

Background Inspired by Octave's (and, by extension, MATL's) very convenient interpretation of truthy/falsy matrices, Jelly got the Ȧ (Octave-style all) atom. Ȧ takes an array as input and returns 1 ...
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### Difference of the square of the sum

Find the difference between the square of the sums and sum of the squares. This is the mathematical representation: $\left(\sum n\right)^2-\sum n^2$ Your program/method should take two inputs, ...
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### Skip like a rabbit!

Given a list of non-negative integers in any reasonable format, iterate over it, skipping as many elements as every integer you step on says. Here is a worked example: ...
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### Recursive Stalin Sort [duplicate]

There is a "sorting" algorithm often called Stalin Sort, where instead of sorting an array, you just remove any items that are out of order. In this challenge, you'll implement part of a ...
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### Consolidate an Array

The task is simple: consolidate an array of ints. Consolidating this array consists of the following: All instances of 0 need to be moved to the end of the array. There should be no 0s between the ...
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### Cheating a multiple choice test

Introduction While studying, I tried to come up with several ways to cheat a multiple choice test. It basically is a compressed version of the multiple choice answers. The method goes as following: ...
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### Remove leading and trailing zeroes

Given a non-empty list/array containing only non-negative integers like this: [0, 0, 0, 8, 1, 4, 3, 5, 6, 4, 1, 2, 0, 0, 0, 0] Output the list with trailing and ...
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### Remove surrounding zeroes of a 2d array

This is a 2-dimensional version of this question. Given a non-empty 2-dimensional array/matrix containing only non-negative integers:  \begin{bmatrix} {\color{Red}0} & {\color{Red}0} & {\...
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### Is there a bump?

Given a list of positive integers determine if there is an element that is either greater than its two neighbors or less than its two neighbors (a "bump"). To be clear a bump can never be the first ...
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### Print a physical multiplication table

Rectangles have this nice property - an $n \times m$ rectangle consists of exactly $n \times m$ characters! A.. more interesting property is that the rectangles can be aligned nicely in a ...
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### An Array of Challenges #2: Separate a Nested Array

Note: This is #2 in a series of array challenges. For the previous challenge, click here. Separating Nested Lists To separate values in a nested list, flatten it, and then wrap each value so it is at ...
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### Zero sum covers

Introduction Consider a nonempty list L of integers. A zero-sum slice of L is a contiguous subsequence of L whose sum equals 0. For example, [1, -3, 2] is a zero-sum slice of [-2, 4, 1, -3, 2, 2, -1, ...
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### Look, up in the sky! It's a super duper array!

Inspired by this question from our rivals friends over at Code Review. Definitions A super array is an array where each new element in the array is larger than the sum of all the previous elements. <...
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### Recreate Minecraft's lighting

Minecraft has a fairly unique lighting system. Each block's light value is either one less than the brightest one surrounding it, or it is a light source itself. Your task is to write a method that ...
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### P Pr Pre Pref Prefi Prefix Prefixe Prefixes

Given some finite list, return a list of all its prefixes, including an empty list, in ascending order of their length. (Basically implementing the Haskell function ...
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### Determine the depth of an array

A simple challenge for your Monday evening (well, or Tuesday morning in the other half of the world...) You're given as input a nested, potentially ragged array of positive integers: ...
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### Boustrophedonise

Related but very different. A boustrophedon is a text where every other line of writing is flipped or reversed, with reversed letters. In this challenge, we will just reverse every other line, but ...
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### Fold a List in Half

We are going to fold a list of integers. The procedure to do so is as follows, If the list is of even length, make a list of half of its length where the nth item of the new list is the sum of the ...
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