Reverse Array Sum

Question

Your program should take an array as input.

The array:

1. Will always be 1 dimensional
2. Will only contain integers
3. Can be empty

The program should reverse the array, and then add up the elements to the original for example:

Input: [1, 2, 3]

Original: [1, 2, 3]

Reversed: [3, 2, 1]

[1, 2, 3]
 + + +
[3, 2, 1]
[1+3, 2+2, 3+1]

Output: [4, 4, 4]

---

Test Cases:

#In #Out
[8, 92], [100, 100]
[1, 2, 3], [4, 4, 4]
[5, 24, 85, 6], [11, 109, 109, 11]
[], []
[999], [1998]

This is code-golf, the shortest code (in bytes) wins!

• Richard Donovan: J 3 bytes. Program is t. t=:+|.
• Noah Cristino: @RichardDonovan Nice Answer! Can you submit as an answer instead of a comment please :)

Answer 1

Haskell, 20 bytes

5 bytes save by changing to a point free as suggested by nimi

zipWith(+)=<<reverse

Try it online!

• nimi: go pointfree: zipWith(+)=<<reverse.
• Wheat Wizard: @nimi Wow I didn't think to do that but thats pretty smart.
• Noah Cristino: Where do I put the array? I tried arguments, and input on TIO
• Wheat Wizard: @NoahCristino I fixed the TIO. This is a point-free function so you just put the input after the function however Haskell requires a main to compile.
• nimi: @maple_shaft: we use =<< from the function monad which is defined as: (=<<) f g x = f (g x) x. Here, written in infix: (zipWith(+) =<< reverse) x -> zipWith(+) (reverse x) x.

Answer 2

Jelly, 2 bytes

+U

Try it online!

or

+Ṛ

Try it online! (thanks @Mr. Xcoder for the second program)

explanation, though it's pretty self-explanatory

+U Main link
+ Add, vectorizing, 
 U the input, reversed

+Ṛ Main link
+ Add, vectorizing,
 Ṛ the input, reversed, without vectorizing (same thing for depth-1 lists)

For empty array [], this outputs nothing. That is correct. Jelly's representation of an empty list is just simply nothing. Note that Jelly's representation of a list with a single element is just the element itself. Append ŒṘ to the code to see the Python internal representation of the output.

• Noah Cristino: I found 2 issues. 1) When tested on [9] it outputs 18 instead of [18], and 2) when tested on [] it doesn't output anything.
• Erik the Outgolfer: @NoahCristino It's not a full program, and there's already an explanation for that in the answer.
• Noah Cristino: So I guess this is fine, it's just how Jelly outputs it
• hyperneutrino: @NoahCristino Yeah. I added a part to the end of my answer so you can put that atom at the end of the code to see how Python would print it.
• Mr. Xcoder: +Ṛ works too.

Answer 3

JavaScript (ES6), 27 bytes

a=>[...a].map(e=>e+a.pop())

f=
a=>[...a].map(e=>e+a.pop())
console.log(f([8, 92])) // [100, 100]
console.log(f([1, 2, 3])) // [4, 4, 4]
console.log(f([5, 24, 85, 6])) // [11, 109, 109, 11]
console.log(f([])) // []
console.log(f([999])) //[1998]

• Rick Hitchcock: Oh, man, I was almost there, but I didn't think to use the spread operator to do the clone.
• Noah Cristino: Can you add the embedded try it for javascript?

Answer 4

05AB1E, 2 bytes

Â+

Try it online!

• Noah Cristino: Passed all my tests!
• Riley: R+ also works for 2 bytes.

Answer 5

Python 2, 32 bytes

lambda l:map(sum,zip(l,l[::-1]))

Alternative solution without zip (35 bytes):

lambda l:map(int.__add__,l,l[::-1])

Try it online!

Answer 6

Python 2, 40 bytes

lambda l:[i+j for i,j in zip(l,l[::-1])]

Try it online!

The other, shorter Python answer replaces a list comprehension with map. Wish I'd thought to do that faster. ;-;

• Noah Cristino: Passes all my tests!

Answer 7

Japt, 7 bytes

mÈ+Ug~Y

Try it online! with the -Q flag to format the output array.

Explanation

Implicit: U = input array

mÈ

Map the input by the following function...

+Ug

The value, plus the value in the input array at index...

~Y

-(index+1), which gets elements from the end of the array.

• Noah Cristino: Good job! I like the multiple input thing!
• Oliver: I really like the multiple-input Japt interpreter. Nice job!
• ETHproductions: That's really cool :-) I'd add that feature to the "official" interpreter, but with the current design it's sorta unextendable...

Answer 8

Ruby, 25 bytes

->a{[*a].map{|i|i+a.pop}}

Try it online!

Answer 9

Mathematica, 12 bytes

Reverse@#+#&

Try it online!

Answer 10

Python 2, (削除) 33 (削除ここまで) 32 bytes

-1 byte thanks to @xnor

lambda l:[i+l.pop()for i in l*1]

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• Noah Cristino: Passed all my tests good job :)
• xnor: What an unusual method. l*1 saves a byte.
• dhssa: I am having difficulties understanding the par l*1, any elaboration
• ovs: @dhssa l*1 makes a copy of the list l. If we would not make a copy, pop() would delete elements from the list before they were accessed in the for loop.
• dhssa: Thanks for the explanation, I got it now. good trick to know for coding.

Answer 11

Brachylog, 6 bytes

↔;?z+m

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• Noah Cristino: Passes all cases, cool name.

Answer 12

J, 3 bytes

+|.

Reverse, sum.

Try it online!

Answer 13

C# (.NET Core), (削除) 61 (削除ここまで) 60 bytes

-1 byte thanks to TheLethalCoder

a=>a.Reverse().Zip(a,(x,y)=>x+y).ToArray()

Try it online!

Byte count also includes:

using System.Linq;

For explanation - Zip function in LINQ takes two collections and executes given function for all corresponding elements, ie. both first elements together, both second elements etc.

• Noah Cristino: Good answer. Thanks for the comment about the input.
• TheLethalCoder: Hello and welcome to PPCG! You don't need the trailing semi colon in the byte count. I believe you can return the collection straight from the Zip call to so no need for the ToArray(). Nice job!
• Grzegorz Puławski: @TheLethalCoder Thanks, and I added ToArray() since the challenge is about arrays, so I wanted the self-contained lambda to be array -> array.

Answer 14

Vyxal, 2 bytes

Ḃ+

Try it Online!

Ḃ+
Ḃ # bifurcate, i.e. duplicate and reverse
 + # vectorized addition of two lists

Answer 15

CJam, 7 bytes

{_W%.+}

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• Noah Cristino: It outputs "" for [], that's weird. Not your fault just the language.
• Erik the Outgolfer: @NoahCristino That's CJam's representation of [].
• Noah Cristino: Yup, it's hard to have a standard output for my challenge since many languages display arrays differently especially [], and [4], so it's fine.

Answer 16

APL (Dyalog), 3 bytes

⌽+⊢

Try it online!

Explanation

⌽ The argument reversed
+ Plus
⊢ The argument

• Uriel: ninja'd. I need to move to an APL keyboard... xD
• user41805: @Uriel I am always on my APL keyboard :D
• Noah Cristino: Love the simplicity. It outputs ⌽+⊢ with no input
• user41805: @NoahCristino An empty input is specified with ⍬, the empty vector. With no argument, it prints the train by itself

Answer 17

R, (削除) 17 (削除ここまで) 16 bytes

-1 byte thanks to djhurio

rev(l<-scan())+l

Reads from stdin; returns the vector; numeric(0) is the zero-length numeric vector for the empty list.

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• Noah Cristino: For an empty "array" it returns numeric(0)
• Leaky Nun: @NoahCristino an empty vector is represented as numeric(0) in R.
• Noah Cristino: That's fine. @LeakyNun
• djhurio: rev(l<-scan())+l, 16 bytes?
• JayCe: For the record, an R+pryr functional alternative is just one byte longer: pryr::f(rev(x)+x)

Answer 18

Clojure, (削除) 20 (削除ここまで) 17 bytes

3 bytes saved thanks to @MattPutnam

#(map +(rseq %)%)

Seems to be quite competitive with non-golfing languages.

See it online

• MattPutnam: Use rseq instead of reverse.

Answer 19

Pyth, 3 bytes

+V_

Try it here.

• Noah Cristino: Yay passes all my tests!
• Mr. Xcoder: Equivalent: sV_
• Erik the Outgolfer: @Mr.Xcoder nevermind

Answer 20

PowerShell, 26 bytes

($a=$args)|%{+$a[--$i]+$_}

Try it online!

Takes input as command-line arguments.

• Noah Cristino: Can you add a TIO? and a link under the name like this one: codegolf.stackexchange.com/a/135427/61877
• Joey: @NoahCristino: Like this? Haven't used that thing so far, so no idea what I may have done wrong. By the way, if you expect people to use a certain service in their answers, then please state so in the task description.
• Noah Cristino: That's fine. It's not required it just makes the answers more high quality, and easier to test out for future viewers.

Answer 21

C, 49 bytes

f(a,n,b)int*a,*b;{for(b=a+n;a<b--;a++)*a=*b+=*a;}

• Erik the Outgolfer: Shouldn't a,n,b be a,b,n or something?
• orlp: @EriktheOutgolfer No, b isn't a parameter for the function, just an extra definition stuffed in there for golfing reasons. a must be a pointer to integers, and n must be how many integers there are in the array.
• Noah Cristino: Could you please add a TIO link?

Answer 22

PowerShell, (削除) 40 (削除ここまで) 32 bytes

($n=$args)|%{$n[-++$i]+$n[$i-1]}

Try it online!

Takes input as individual command-line arguments, which is allowed as one of the native list format for PowerShell. Then loops through each element (i.e., a shorter way of looping through the indices), adding the element counting from the back (-1 indexed) to the current element (0 indexed, hence the decrement -1). Those are left on the pipeline and output is implicit.

Saved 8 bytes thanks to @briantist

• Noah Cristino: it doesn't output an array.
• AdmBorkBork: @NoahCristino PowerShell input/output is, in general, weird. It is outputting as an array, it's just there's nothing capturing said array, and so when the implicit Write-Output happens, it puts things on stdout one item per line. For example, you can see here that, when captured, the object is indeed an array type.
• Noah Cristino: Good enough then :) atleast it tried
• briantist: I'm on mobile and can't test so easily, but isn't it 1 byte shorter to remove the param and then replace 1..$n with 1..($n=$args)?
• AdmBorkBork: @briantist Not quite, but you did give me a different way of thinking about it, saving a bunch of bytes. Thanks!

Answer 23

Java 8, (削除) 61 (削除ここまで) (削除) 57 (削除ここまで) (削除) 56 (削除ここまで) 53 bytes

a->{for(int l=0,r=a.length;l<r;a[l]=a[--r]+=a[l++]);}

-1 byte and bug-fixed thanks to @Nevay.
-3 bytes thanks to @OliverGrégoire.

(It was a port of (and golfed by (削除) 4 (削除ここまで) 8 bytes) of @jkelm's C# answer, but now it's a different shorter solution thanks to @OliverGrégoire.)

Explanation:

Try it here.

The method modifies the input-array to save bytes, so no need for a return-type.

a->{ // Method with integer-array parameter and no return-type
 for(int l=0, // Left-integer (starting at 0)
 r=a.length; // Right-integer (starting at the length of the input-array)
 l<r; // Loop as long as left is smaller than right
 a[l]= // Change the number at the current left index to:
 a[--r]+=a[l++] // The number at the right index + itself
 // (The += adds the number at the left index also to the right index)
 // (And the --/++ increases/decreases the indexes by 1,
 // until we've reached the middle of the array)
 ); // End of loop
} // End of method

• Nevay: You can save 1 byte by using a->{for(int i=0,l=a.length;i<l/2;a[i]=a[l+~i]+=a[i++]);}.
• Nevay: Besides that the code fails for arrays with an odd length 1,2,3 (returns 4,2,4 instead of 4,4,4), the loop has to run as long as 2*i<l, not i<l/2.
• Kevin Cruijssen: @Nevay Thanks. I knew it should be possible to golf l-i-1, just couldn't come up with it..
• Olivier Grégoire: 53 bytes: a->{for(int l=0,r=a.length;l<r;a[l]=a[--r]+=a[l++]);}.
• Kevin Cruijssen: @OlivierGrégoire Thanks. And your l and r makes sense for your implementation, so I've used those as well (and added an explanation).

Answer 24

K4 / K (oK), 5 bytes

Solution:

x+|x:

Try it online!

Example:

x+|x:,()
,()
x+|x:8 92
100 100
x+|x:,999
,1998

Explanation:

Not quite as elegant as the J solution, but same kinda thing:

x+|x: / the solution
 x: / store input as x
 | / reverse it
 + / add to
x / x

• Noah Cristino: Nice! I like the line by line explanation

Answer 25

Factor, 18 bytes

[ dup reverse v+ ]

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Explanation

 ! { 1 2 3 }
dup ! { 1 2 3 } { 1 2 3 }
reverse ! { 1 2 3 } { 3 2 1 }
v+ ! { 4 4 4 }

Answer 26

tinylisp, ⁴⁴ 39 bytes

(load library
(q((p)(map* a p(reverse p

-5 bytes thanks to @DLosc

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Explanation

An anonymous function that maps the a function (addition) over pairwise elements in the input and the reverse of the input, forming a new list.

• DLosc: map* is your friend here: Try it online!

Answer 27

K (ngn/k), 6 bytes

{x+|x}

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Answer 28

Ohm, 3 bytes

DR+

Try it online!

• Noah Cristino: It outputs floats, but I never said it couldn't, so that's fine, and for [] it doesn't output anything but that's just the language.

Answer 29

Actually, 4 bytes

;R♀+

Try it online!

• Noah Cristino: Perfect, great job.
• Noah Cristino: Erik, solving my problem in like 7 languages xD
• Erik the Outgolfer: @NoahCristino Yeah I'm trying to keep it under 15...;)
• Noah Cristino: I'm going to have to start adding a -1 for every submission xD

Answer 30

anyfix, 3 bytes

"U+

The version on TryItOnline! is an outdated version of anyfix, which contains a few fatal errors such as not being able to add lists because of typos in the source code. Use the code on GitHub instead.

"U+ Program
" Duplicate top of stack
 U Reverse top of stack if it is a list (which it should be...)
 + Add, vectorizing for lists