57
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I'm honestly surprised that this hasn't been done already. If you can find an existing thread, by all means mark this as a duplicate or let me know.

Input

Your input is in the form of any positive integer greater than or equal to 1.

Output

You must output the sum of all integers between and including 1 and the number input.

Example

 In: 5
     1+2+3+4+5 = 15
Out: 15

OEIS A000217 — Triangular numbers: a(n) = binomial(n+1,2) = n(n+1)/2 = 0 + 1 + 2 + ... + n.

Leaderboard

Run the code snippet below to view a leaderboard for this question's answers. (Thanks to programmer5000 and steenbergh for suggesting this, and Martin Ender for creating it.)

var QUESTION_ID=133109,OVERRIDE_USER=69148;function answersUrl(e){return"https://api.stackexchange.com/2.2/questions/"+QUESTION_ID+"/answers?page="+e+"&pagesize=100&order=desc&sort=creation&site=codegolf&filter="+ANSWER_FILTER}function commentUrl(e,s){return"https://api.stackexchange.com/2.2/answers/"+s.join(";")+"/comments?page="+e+"&pagesize=100&order=desc&sort=creation&site=codegolf&filter="+COMMENT_FILTER}function getAnswers(){jQuery.ajax({url:answersUrl(answer_page++),method:"get",dataType:"jsonp",crossDomain:!0,success:function(e){answers.push.apply(answers,e.items),answers_hash=[],answer_ids=[],e.items.forEach(function(e){e.comments=[];var s=+e.share_link.match(/\d+/);answer_ids.push(s),answers_hash[s]=e}),e.has_more||(more_answers=!1),comment_page=1,getComments()}})}function getComments(){jQuery.ajax({url:commentUrl(comment_page++,answer_ids),method:"get",dataType:"jsonp",crossDomain:!0,success:function(e){e.items.forEach(function(e){e.owner.user_id===OVERRIDE_USER&&answers_hash[e.post_id].comments.push(e)}),e.has_more?getComments():more_answers?getAnswers():process()}})}function getAuthorName(e){return e.owner.display_name}function process(){var e=[];answers.forEach(function(s){var r=s.body;s.comments.forEach(function(e){OVERRIDE_REG.test(e.body)&&(r="<h1>"+e.body.replace(OVERRIDE_REG,"")+"</h1>")});var a=r.match(SCORE_REG);a&&e.push({user:getAuthorName(s),size:+a[2],language:a[1],link:s.share_link})}),e.sort(function(e,s){var r=e.size,a=s.size;return r-a});var s={},r=1,a=null,n=1;e.forEach(function(e){e.size!=a&&(n=r),a=e.size,++r;var t=jQuery("#answer-template").html();t=t.replace("{{PLACE}}",n+".").replace("{{NAME}}",e.user).replace("{{LANGUAGE}}",e.language).replace("{{SIZE}}",e.size).replace("{{LINK}}",e.link),t=jQuery(t),jQuery("#answers").append(t);var o=e.language;/<a/.test(o)&&(o=jQuery(o).text()),s[o]=s[o]||{lang:e.language,user:e.user,size:e.size,link:e.link}});var t=[];for(var o in s)s.hasOwnProperty(o)&&t.push(s[o]);t.sort(function(e,s){return e.lang>s.lang?1:e.lang<s.lang?-1:0});for(var c=0;c<t.length;++c){var i=jQuery("#language-template").html(),o=t[c];i=i.replace("{{LANGUAGE}}",o.lang).replace("{{NAME}}",o.user).replace("{{SIZE}}",o.size).replace("{{LINK}}",o.link),i=jQuery(i),jQuery("#languages").append(i)}}var ANSWER_FILTER="!t)IWYnsLAZle2tQ3KqrVveCRJfxcRLe",COMMENT_FILTER="!)Q2B_A2kjfAiU78X(md6BoYk",answers=[],answers_hash,answer_ids,answer_page=1,more_answers=!0,comment_page;getAnswers();var SCORE_REG=/<h\d>\s*([^\n,]*[^\s,]),.*?(\d+)(?=[^\n\d<>]*(?:<(?:s>[^\n<>]*<\/s>|[^\n<>]+>)[^\n\d<>]*)*<\/h\d>)/,OVERRIDE_REG=/^Override\s*header:\s*/i;
body{text-align:left!important}#answer-list,#language-list{padding:10px;width:290px;float:left}table thead{font-weight:700}table td{padding:5px} /* font fix */ body {font-family: Arial,"Helvetica Neue",Helvetica,sans-serif;} /* #language-list x-pos fix */ #answer-list {margin-right: 200px;}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <link rel="stylesheet" type="text/css" href="//cdn.sstatic.net/codegolf/all.css?v=83c949450c8b"> <div id="answer-list"> <h2>Leaderboard</h2> <table class="answer-list"> <thead> <tr><td></td><td>Author</td><td>Language</td><td>Size</td></tr></thead> <tbody id="answers"> </tbody> </table> </div><div id="language-list"> <h2>Winners by Language</h2> <table class="language-list"> <thead> <tr><td>Language</td><td>User</td><td>Score</td></tr></thead> <tbody id="languages"> </tbody> </table> </div><table style="display: none"> <tbody id="answer-template"> <tr><td>{{PLACE}}</td><td>{{NAME}}</td><td>{{LANGUAGE}}</td><td>{{SIZE}}</td><td><a href="{{LINK}}">Link</a></td></tr></tbody> </table> <table style="display: none"> <tbody id="language-template"> <tr><td>{{LANGUAGE}}</td><td>{{NAME}}</td><td>{{SIZE}}</td><td><a href="{{LINK}}">Link</a></td></tr></tbody> </table>

\$\endgroup\$
  • 5
    \$\begingroup\$ Closely related \$\endgroup\$ – FryAmTheEggman Jul 18 '17 at 20:36
  • \$\begingroup\$ @FryAmTheEggman Sorry - had a bit of a brain fart there. I see what you mean. \$\endgroup\$ – GarethPW Jul 18 '17 at 20:45
  • 2
    \$\begingroup\$ @Aaron you got ninja'd by Husk, which was just posted with a 1 byte solution \$\endgroup\$ – Skidsdev Jul 18 '17 at 21:35
  • 7
    \$\begingroup\$ I suggest a stack snippet. \$\endgroup\$ – programmer5000 Jul 19 '17 at 11:42
  • 1
    \$\begingroup\$ Related: minecraftforum.net/forums/off-topic/… \$\endgroup\$ – Jerry Jeremiah Jul 27 '17 at 12:20

146 Answers 146

43
\$\begingroup\$

Pyth, 2 bytes

sS

Try it online! Implicit input. S is 1-indexed range, and s is the sum.

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  • 98
    \$\begingroup\$ Finally, Pyth(on) code sounds like a snake. \$\endgroup\$ – totallyhuman Jul 18 '17 at 21:15
  • 2
    \$\begingroup\$ This is the perfect challenge for Pyth... \$\endgroup\$ – Mr. Xcoder Jul 18 '17 at 22:09
  • \$\begingroup\$ I was going to answer this, but I guess not \$\endgroup\$ – Stan Strum Sep 7 '17 at 3:41
31
\$\begingroup\$

Husk, 1 byte

Σ

Try it online!

Builtin! Σ in Husk is usually used to get the sum of all elements of a list, but when applied to a number it returns exactly n*(n+1)/2.

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  • \$\begingroup\$ Out of curiosity, does this occur because the number is cast to a range and then summed, or is this actually hardcoded? \$\endgroup\$ – FryAmTheEggman Jul 18 '17 at 21:33
  • 3
    \$\begingroup\$ @FryAmTheEggman this is actually hardcoded, and is similar to the behavior of another builtin, Π, which can compute the product of all elements of a list or the factorial of a single number \$\endgroup\$ – Leo Jul 18 '17 at 21:37
  • 4
    \$\begingroup\$ Σ is a two byte unicode character on my machine. I guess you use code page 1253? msdn.microsoft.com/en-us/library/cc195055.aspx \$\endgroup\$ – gmatht Jul 19 '17 at 2:32
  • 5
    \$\begingroup\$ @gmatht Husk's code page \$\endgroup\$ – Jonathan Allan Jul 19 '17 at 3:02
20
\$\begingroup\$

Piet, 161 bytes / 16 codels

You can interpret it with this Piet interpreter or upload the image on this website and run it there. Not sure about the byte count, if I could encode it differently to reduce size.

Scaled up version of the source image:

rapapaing-image

Explanation

The highlighted text shows the current stack (growing from left to right), assuming the user input is 5:

1st transition Input a number and push it onto stack

5

2nd transition Duplicate this number on the stack

5 5

3rd transition Push 1 (the size of the dark red area) onto stack

5 5 1

4th transition Add the top two numbers

5 6

5th transition Multiply the top two numbers

30

6th transition The black area makes sure, that the cursor moves down right to the light green codel. That transition pushes 2 (the size of dark green) onto stack

30 2

7th transition Divide the second number on the stack by the first one

15

8th transition Pop and output the top number (interpreted as number)

[empty]

final trap By inserting a white area, the transition is a nop, the black traps our cursor. This ends execution of the program.

Original file (far too small for here): Original source image

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  • \$\begingroup\$ We transitioned from an intelligible text (e.g. C) to unintelligible text (e.g. Jelly) to images... What next? :P \$\endgroup\$ – frarugi87 Jul 24 '17 at 7:32
  • \$\begingroup\$ +1 I haven't actually seen a Piet answer with an explanation before \$\endgroup\$ – MilkyWay90 Apr 1 at 23:01
19
\$\begingroup\$

Brain-Flak, 16 bytes

({({}[()])()}{})

Try it online!

This is one of the few things that brain-flak is really good at.

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18
\$\begingroup\$

Mathematica, 9 bytes

#(#+1)/2&

Mathematica, 10 bytes

(#^2+#)/2&

Mathematica, 11 bytes

Tr@Range@#&

Mathematica, 12 bytes

i~Sum~{i,#}&

Mathematica, 14 bytes

(by @user71546)

1/2/Beta[#,2]&

Mathematica, 15 bytes

Tr[#&~Array~#]&

Mathematica, 16 bytes

Binomial[#+1,2]&

Mathematica, 17 bytes

(by @Not a tree)

⌊(2#+1)^2/8⌋&

Mathematica, 18 bytes

PolygonalNumber@#&

Mathematica, 19 bytes

#+#2&~Fold~Range@#&

Mathematica, 20 bytes

(by @Not a tree)

f@0=0;f@i_:=i+f[i-1]
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  • 4
    \$\begingroup\$ It seems a shame to skip 13, 14 and 17… \$\endgroup\$ – Not a tree Jul 19 '17 at 0:48
  • 3
    \$\begingroup\$ It seems like a next challenge....or at least help me to complete the list. \$\endgroup\$ – J42161217 Jul 19 '17 at 0:51
  • 2
    \$\begingroup\$ I still don't have anything for 13 or 14 bytes (apart from just un-golfing your shorter answers), but here are another 26 with larger byte-counts. \$\endgroup\$ – Not a tree Jul 19 '17 at 7:16
  • 1
    \$\begingroup\$ @MarkS. on 10.4 works fine \$\endgroup\$ – J42161217 Jul 25 '17 at 10:56
  • 1
    \$\begingroup\$ @Notatree For your list, here is a candidate for 35: Array[Boole[#2>=#]&,{#,#}]~Total~2& \$\endgroup\$ – Mark S. Aug 12 '17 at 5:07
17
\$\begingroup\$

JavaScript (ES6), 10 bytes

n=>n*++n/2

Example

let f =

n=>n*++n/2

console.log(f(5))

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  • 2
    \$\begingroup\$ n*-~n/2 also works, but only for n < 2**31 \$\endgroup\$ – Patrick Roberts Dec 3 '17 at 18:53
14
\$\begingroup\$

Oasis, 3 bytes

n+0

Try it online!

How it works

n+0
  0    a(0)=0
n+     a(n)=n+a(n-1)
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  • 27
    \$\begingroup\$ And here I was, all of my life thinking that n+0 is n... \$\endgroup\$ – Wojowu Jul 19 '17 at 7:22
11
\$\begingroup\$

x86_64 machine language (Linux), 9 8 bytes

0:   8d 47 01                lea    0x1(%rdi),%eax
3:   f7 ef                   imul   %edi
5:   d1 e8                   shr    %eax
7:   c3                      retq 

To Try it online! compile and run the following C program.

#include<stdio.h>
const char f[]="\x8d\x47\x01\xf7\xef\xd1\xe8\xc3";
int main(){
  for( int i = 1; i<=10; i++ ) {
    printf( "%d %d\n", i, ((int(*)())f)(i) );
  }
}

Thanks to @CodyGray and @Peter for -1.

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  • 1
    \$\begingroup\$ You should probably use shr instead of sar, to treat your output as unsigned (no change in code size). (Spotted by @CodyGray and pointed out in his 7-byte add+loop answer). \$\endgroup\$ – Peter Cordes Jul 22 '17 at 8:36
  • 1
    \$\begingroup\$ This looks optimal for performance in an implementation of the closed-form formula, but you can save a byte by using the one-operand form of mul %edi or imul %edi (each 2B) instead of the 3B two-operand form. It clobbers EDX with the high-half result, but that's fine. Multi-operand imul was introduced later than the one-operand form, and has a 2-byte opcode with a 0F escape byte. Any of the three options will always produce the same result in eax, it's only the high half that depends on signed vs. unsigned. \$\endgroup\$ – Peter Cordes Jul 22 '17 at 8:36
10
\$\begingroup\$

C# (.NET Core), 10 bytes

n=>n++*n/2

Try it online!

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10
\$\begingroup\$

Java (OpenJDK 8), 10 bytes

n->n++*n/2

Try it online!

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10
\$\begingroup\$

Octave, 22 19 bytes

Because arithmetic operations are boring...

@(n)nnz(triu(e(n)))

Try it online!

Explanation

Given n, this creates an n×n matrix with all entries equal to the number e; makes entries below the diagonal zero; and outputs the number of nonzero values.

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9
\$\begingroup\$

Python 2, 24 16 bytes

-8 bytes thanks to FryAmTheEggman.

lambda n:n*-~n/2

Try it online!

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8
\$\begingroup\$

Jelly, 2 bytes

RS

Try it online!

Explanation

RS

    implicit input
 S  sum of the...
R   inclusive range [1..input]
    implicit output

Gauss sum, 3 bytes

‘×H

Explanation

‘×H

     implicit input
  H  half of the quantity of...
‘    input + 1...
 ×   times input
     implicit output
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  • \$\begingroup\$ This also works in Anyfix :P (not on TIO) \$\endgroup\$ – HyperNeutrino Jul 18 '17 at 21:08
8
\$\begingroup\$

APL, 3 bytes

+/⍳

Try it online!

+/ - sum (reduce +), - range.

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  • \$\begingroup\$ This depends on the indexing. If indexing is set to 0, then you'd need an additional 2 bytes 1+ \$\endgroup\$ – Werner Jul 18 '17 at 21:27
  • 2
    \$\begingroup\$ @Werner indexing is default 1 so I didn't specify. its common here to specify only when using ⎕IO←0 (and it does not included in byte count) \$\endgroup\$ – Uriel Jul 18 '17 at 21:29
8
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Haskell, 13 bytes

This is the shortest (I thinkthought):

f n=sum[1..n]

Try it online!

Direct, 17 13 bytes

f n=n*(n+1)/2

Thanks @WheatWizard for -4 bytes!

Try it online!

Pointfree direct, 15 bytes

(*)=<<(/2).(+1)

Thanks @nimi for the idea!

Try it online!

Pointfree via sum, 16 bytes

sum.enumFromTo 1

Try it online!

Recursively, 22 18 bytes

f 0=0;f n=n+f(n-1)

Thanks @maple_shaft for the idea & @Laikoni for golfing it!

Try it online!

Standard fold, 19 bytes

f n=foldr(+)0[1..n]

Try it online!

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7
\$\begingroup\$

Starry, 27 22 bytes

5 bytes saved thanks to @miles!

, + +  **       +   *.

Try it online!

Explanation

,             Read number (n) from STDIN and push it to the stack
 +            Duplicate top of the stack
 +            Duplicate top of the stack
  *           Pop two numbers and push their product (n*n)
*             Pop two numbers and push their sum (n+n*n)
       +      Push 2
   *          Pop two numbers and push their division ((n+n*n)/2)
.             Pop a number and print it to STDOUT
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  • \$\begingroup\$ 22 bytes. \$\endgroup\$ – miles Jul 18 '17 at 23:20
  • \$\begingroup\$ @miles Thanks! Very good idea! \$\endgroup\$ – Luis Mendo Jul 18 '17 at 23:53
7
\$\begingroup\$

05AB1E, 2 bytes

LO

Try it online!

How it works

     #input enters stack implicitly
L    #pop a, push list [1 .. a]
 O   #sum of the list
     #implicit output 

Gauss sum, 4 bytes

>¹*;

Try it online!

How it works

>       #input + 1
 ¹*     #get original input & multiply
   ;    #divide by two 
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  • 3
    \$\begingroup\$ ÝO also works and means hello. \$\endgroup\$ – Magic Octopus Urn Jul 19 '17 at 15:01
  • 1
    \$\begingroup\$ L0 and behold.. \$\endgroup\$ – dylnan Dec 11 '17 at 16:16
7
\$\begingroup\$

Java (OpenJDK 8), 10 bytes

a->a++*a/2

Try it online!

Took a moment to golf down from n->n*(n+1)/2 because I'm slow.

But this isn't a real Java answer. It's definitely not verbose enough.

import java.util.stream.*;
a->IntStream.range(1,a+1).sum()

Not bad, but we can do better.

import java.util.stream.*;
(Integer a)->Stream.iterate(1,(Integer b)->Math.incrementExact(b)).limit(a).reduce(0,Integer::sum)

I love Java.

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  • 1
    \$\begingroup\$ If you want it to be even more verbose why use a lambda!? :P \$\endgroup\$ – TheLethalCoder Jul 19 '17 at 12:36
  • 2
    \$\begingroup\$ I was aiming for verbose lambdas, I could write a full program if I wanted to be particularly eloquent :P \$\endgroup\$ – Xanderhall Jul 19 '17 at 12:38
  • 1
    \$\begingroup\$ The exact same solution was already posted \$\endgroup\$ – Winter Jul 19 '17 at 18:56
  • 1
    \$\begingroup\$ I must have missed it, but in any case, I tend to not look at the contents of other answers. I prefer to write my own golf. \$\endgroup\$ – Xanderhall Jul 20 '17 at 11:21
7
\$\begingroup\$

Check, 5 bytes

:)*$p

Check isn't even a golfing language, yet it beats CJam!

Try it online!

Explanation:

The input number is placed on the stack. : duplicates it to give n, n. It is then incremented with ), giving n, n+1. * multiplies the two together, and then $ divides the result by 2. p prints the result and the program terminates.

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6
\$\begingroup\$

MATL, 2 bytes

:s

Try it online!

Not happy smiley.

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  • 2
    \$\begingroup\$ Damn it, finally a challenge I could easily answer in MATL, but you beat me to it :( \$\endgroup\$ – Lui Jul 19 '17 at 6:27
6
\$\begingroup\$

Taxi, 687 bytes

Go to Post Office:w 1 l 1 r 1 l.Pickup a passenger going to The Babelfishery.Go to The Babelfishery:s 1 l 1 r.Pickup a passenger going to Cyclone.Go to Cyclone:n 1 l 1 l 2 r.[a]Pickup a passenger going to Addition Alley.Pickup a passenger going to The Underground.Go to Zoom Zoom:n.Go to Addition Alley:w 1 l 1 r.Pickup a passenger going to Addition Alley.Go to The Underground:n 1 r 1 r.Switch to plan "z" if no one is waiting.Pickup a passenger going to Cyclone.Go to Cyclone:n 3 l 2 l.Switch to plan "a".[z]Go to Addition Alley:n 3 l 1 l.Pickup a passenger going to The Babelfishery.Go to The Babelfishery:n 1 r 1 r.Pickup a passenger going to Post Office.Go to Post Office:n 1 l 1 r.

Try it online!

Un-golfed with comments:

[ n = STDIN ]
Go to Post Office: west 1st left 1st right 1st left.
Pickup a passenger going to The Babelfishery.
Go to The Babelfishery: south 1st left 1st right.
Pickup a passenger going to Cyclone.
Go to Cyclone: north 1st left 1st left 2nd right.

[ for (i=n;i>1;i--) { T+=i } ]
[a]
Pickup a passenger going to Addition Alley.
Pickup a passenger going to The Underground.
Go to Zoom Zoom: north.
Go to Addition Alley: west 1st left 1st right.
Pickup a passenger going to Addition Alley.
Go to The Underground: north 1st right 1st right.
Switch to plan "z" if no one is waiting.
Pickup a passenger going to Cyclone.
Go to Cyclone: north 3rd left 2nd left.
Switch to plan "a".

[ print(T) ]
[z]
Go to Addition Alley: north 3rd left 1st left.
Pickup a passenger going to The Babelfishery.
Go to The Babelfishery: north 1st right 1st right.
Pickup a passenger going to Post Office.
Go to Post Office: north 1st left 1st right.

It's 22.6% less bytes to loop than it is to use x*(x+1)/2

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5
\$\begingroup\$

Julia, 10 bytes

n->n*-~n/2

Try it online!

11 bytes (works also on Julia 0.4)

n->sum(1:n)

Try it online!

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5
\$\begingroup\$

Brainfuck, 24 Bytes.

I/O is handled as bytes.

,[[->+>+<<]>[-<+>]<-]>>.

Explained

,[[->+>+<<]>[-<+>]<-]>>.
,                           # Read a byte from STDIN
 [                  ]       # Main loop, counting down all values from n to 1
  [->+>+<<]                 # Copy the i value to *i+1 and *i+2
           >[-<+>]          # Move *i+1 back to i
                  <-        # Move back to i, lower it by one. Because *i+2 is never reset, each iteration adds the value of i to it.
                     >>.    # Output the value of *i+2
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  • 2
    \$\begingroup\$ It's pretty cool that Brainfuck is able to beat some higher-level languages in this challenge. \$\endgroup\$ – GarethPW Jul 19 '17 at 10:49
  • \$\begingroup\$ Is that legit for me to add an answer in Lenguage (just for fun) using your code? @ATaco \$\endgroup\$ – V. Courtois Jul 26 '17 at 12:44
  • \$\begingroup\$ I don't think so, as it would be the same code, just encoded different. @V.Courtois \$\endgroup\$ – ATaco Jul 26 '17 at 13:07
  • \$\begingroup\$ @ATaco Ahh you're right. \$\endgroup\$ – V. Courtois Jul 26 '17 at 13:19
5
\$\begingroup\$

,,,, 6 bytes

:1+×2÷

Explanation

:1+×2÷

:       ### duplicate
 1+     ### add 1
   ×    ### multiply
    2÷  ### divide by 2

If I implement range any time soon...

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4
\$\begingroup\$

Retina, 13 bytes

.+
$*
1
$`1
1

Try it online! Explanation: The first and last stages are just unary ⇔ decimal conversion. The middle stage replaces each 1 with the number of 1s to its left plus another 1 for the 1 itself, thus counting from 1 to n, summing the values implicitly.

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4
\$\begingroup\$

><>, 7+3 = 10 bytes

Calculates n(n+1)/2.
3 bytes added for the -v flag

:1+2,*n

Try it online!

Or if input can be taken as a character code:

><>, 9 bytes

i:1+2,*n;

Try it online!

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  • 2
    \$\begingroup\$ Using the other math-approach ((n^2+n)/2) is also 7 bytes: ::*+2,n \$\endgroup\$ – steenbergh Jul 19 '17 at 8:18
4
\$\begingroup\$

dc, 7 bytes

d1+*2/p

OR

d2^+2/p

OR

dd*+2/p

Try it online!

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4
\$\begingroup\$

Cubix, 12 10 bytes

*,)2I://O@

Initial version

....I:)*2,O@

Try it online!

Explanation

Expanded onto a cube, the code looks like this:

    * ,
    ) 2
I : / / O @ . .
. . . . . . . .
    . .
    . .

The instruction pointer (IP) starts at the I, moving east. It continues moving east until it comes across the / mirror, which reflects it north. When the IP reaches the top of the code, it wraps around to the last . on the third line, moving south. Then it wraps to the penultimate . on the last line, moving north. Then it reaches the / mirror again, which reflects it east, only for the next / to reflect it north again. This time, the IP wraps to the penultimate . on the third line, and then the last . on the last line.

The instructions are executed in the following order.

I:)*2,O@ # Explanation
I        # Take input as an integer and push it to the stack
 :       # Duplicate the input
  )      # Increment one of the inputs
   *     # Multiply the input by input+1
    2    # Push 2 to the stack
     ,   # Integer devide the multiplication result by 2
      O  # Output the result
       @ # End program
\$\endgroup\$
4
\$\begingroup\$

x86-64 Machine Code, 7 bytes

31 C0
01 C8
E2 FC
C3  

The above bytes define a function that accepts a single parameter, n, and returns a value containing the sum of all integers from 1 to n.

It is written to the Microsoft x64 calling convention, which passes the parameter in the ECX register. The return value is left in EAX, like all x86/x86-64 calling conventions.

Ungolfed assembly mnemonics:

       xor  eax, eax    ; zero out EAX
Next:  add  eax, ecx    ; add ECX to EAX
       loop Next        ; decrement ECX by 1, and loop as long as ECX != 0
       ret              ; return, with result in EAX

Try it online!
(The C function call there is annotated with an attribute that causes GCC to call it using the Microsoft calling convention that my assembly code uses. If TIO had provided MSVC, this wouldn't be necessary.)


By the unusual standards of code golf, you see that this iterative looping approach is preferable to approaches that use the more sane mathematical formula (n(n+1) / 2), even though it is obviously vastly less efficient in terms of run-time speed.

Using number theory, ceilingcat's implementation can still be beat by one byte. Each of these instructions are essential, but there is a slightly shorter encoding for IMUL that uses EAX implicitly as a destination operand (actually, it uses EDX:EAX, but we can just ignore the upper 32 bits of the result). This is only 2 bytes to encode, down from 3.

LEA takes three bytes as well, but there's really no way around that because we need to increment while preserving the original value. If we did a MOV to make a copy, then INC, we'd be at 4 bytes. (In x86-32, where INC is only 1 byte, we'd be at the same 3 bytes as LEA.)

The final right-shift is necessary to divide the result in half, and is certainly more compact (and more efficient) than a multiplication. However, the code should really be using shr instead of sar, since it's assuming that the input value, n, is an unsigned integer. (That assumption is valid according to the rules, of course, but if you know that the input is unsigned, then you shouldn't be doing a signed arithmetic shift, as the upper bit being set in a large unsigned value will cause the result to be incorrect.)

8D 41 01                lea    eax, [rcx+1]
F7 E9                   imul   ecx
D1 E8                   shr    eax, 1
C3                      ret

Now only 8 bytes (thanks to Peter Cordes). Still, 8 > 7.

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  • 1
    \$\begingroup\$ Actually, one-operand imul ecx or mul ecx would work and save a byte in the closed-form implementation. I didn't spot that right away; I was about to comment that it was optimal for both performance and code-size before realizing that an implicit eax operand was fine. \$\endgroup\$ – Peter Cordes Jul 22 '17 at 8:42
  • \$\begingroup\$ I wondered if add+loop would be shorter than imul while looking at the other answer. Handy that there's a standard calling convention that passes the first arg in ecx \$\endgroup\$ – Peter Cordes Jul 22 '17 at 8:45
  • 1
    \$\begingroup\$ Wow, I can't believe I missed the one-operand form! I should really know by now not to say things like "cannot be beat". When will I learn?! Thanks, @Peter. \$\endgroup\$ – Cody Gray Jul 22 '17 at 11:08
4
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8th, 21 12 bytes

Saved 9 bytes thanks to FryAmTheEggman

dup 1+ * 2 /

Usage and output

ok> : sum dup n:1+ * 2 / ;

ok> 5 sum .
15
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  • 3
    \$\begingroup\$ Actually, I think you saved 9 bytes thanks to Gauss ;) But thanks for the credit! \$\endgroup\$ – FryAmTheEggman Jul 18 '17 at 21:10

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