# Questions tagged [array-manipulation]

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:  ...
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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 element ...
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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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### 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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### 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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### 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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### 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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### 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 ...
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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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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Introduction Consider two non-empty integer arrays, say A = [0 3 2 2 8 4] and B = [7 8 7 2]. To perform alignment addition on them, we do the following: Repeat each array enough times to have total ...
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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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### 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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### Is the matrix centrosymmetric… and so is the code?

Defintion A centrosymmetric matrix is a square matrix that is symmetric about its center. More rigorously, a matrix $A$ of size $n \times n$ is centrosymmetric if, for any \\$i,\: j \in ([1, n] \...
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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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### Yahtzee Small Straight Detection

In the game Yahtzee, players roll five six-sided dice, and attempt to create certain hands to score points. One such hand is a small straight: four consecutive numbers, not necessarily in order. The ...
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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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### The sum is always 15

Write a program or function that takes an array of non-negative integers as input and outputs a set of vectors/arrays with the elements of the input array in order, split so that each vector sums up ...
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### An Array of Challenges #2: Separate a Nested Array

Note: This is #2 in a series of array-manipulation challenges. For the previous challenge, click here. Separating Nested Lists To separate values in a nested list, flatten it, and then wrap each ...
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### There's an echo in my array… echo in my array… my array

Help! I seem to have an annoying echo in some of my arrays, and I'd like to get rid of it. When this occurs, the original array repeats itself somewhere in the middle causing the values to be added ...
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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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### 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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### Hollow out an array

The task This challenge is very simple. Your input is a rectangular 2D array of integers, sized at least 1×1. It can be taken in any reasonable format. Your output shall be the input array, but ...
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### Random Golf of the Day #1: Shuffle an Array

About the Series I will be running a little series of code-golf challenges revolving around the theme of randomness. This will basically be a 9-Hole Golf Course, but spread out over several questions....