# 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 leave the actual characters used intact. You may chose which lines to reverse, as long as it is every other one.

You may take the text in any suitable format as long as you support zero or more lines of printable ASCII, each with zero or more characters.

### Examples:

["Here are some lines","of text for you","to make a","boustrophedon"]:

["Here are some lines","uoy rof txet fo","to make a","nodehportsuob"] or ["senil emos era ereH","of text for you","a ekam ot","boustrophedon"]

["My boustrophedon"]:

["My boustrophedon"] or ["nodehportsuob yM"]

[]:
[]

["Some text","","More text","","","Last bit of text"]:

["Some text","","More text","","","txet fo tib tsaL"] or ["txet emoS","","txet eroM","","","Last bit of text"]

• Can't understand if return and input need to be text separated lines or it can be a file or a list of lines. Dec 8, 2017 at 12:30
• @sergiol Default PPCG I/O rules apply.
Dec 8, 2017 at 12:35
• Can my code behave inconsistently, i.e. sometimes start reversing from the first line and sometimes from the second? Dec 8, 2017 at 13:02
• @EriktheOutgolfer Yes, I asked about this earlier and the wording of "You may chose which lines to reverse, as long as it is every other one." was actually changed to what it is now to make it general enough for that behaviour. Dec 8, 2017 at 13:12
• @totallyhuman Yes, as per OP.
Dec 9, 2017 at 20:07

# APL (Dyalog Classic), 4 bytes

⊢∘⌽\


The input is a vector of character vectors.

⌽ is a function that reverses a vector (when ⌽ is applied monadically).

⊢ is "dex" - a function that returns its right argument. When composed (∘) with another function f it forces the latter to be monadic as A ⊢∘f B is equivalent to A ⊢ (f B) and therefore f B.

\ is the scan operator. g\A B C ... is the vector A (A g B) (A g (B g C)) ... where g is applied dyadically (infix notation). Substituting ⊢∘⌽ for g it simplifies to:

A (A ⊢∘⌽ B) (A ⊢∘⌽ (B ⊢∘⌽ C)) ...
A (⌽B) (⌽⌽C) ....
A (⌽B) C ....


The reversals at even positions (or odd, depending on how you count) cancel out.

Try it online!

• That’s literally ]&|.&.>/\ for those who can read J. Dec 8, 2017 at 15:29
• This is really clever. Dec 8, 2017 at 18:21

zipWith($)l l=id:reverse:l  Try it online! Usage example: zipWith($)l ["abc","def","ghi"] yields ["abc","fed","ghi"].

Explanation:

l is an infinite list of functions alternating between the identity function and the reverse function.

The main function zips l and the input list with the function application $, that is for an input ["abc", "def", "ghi"] we get [id$"abc", reverse$"def", id$"ghi"].

# Husk, 4 bytes

z*İ_


Takes and returns a list of strings (the interpreter implicitly joins the result by newlines before printing). The first string is reversed. Try it online!

## Explanation

z*İ_  Implicit input.
İ_  The infinite list [-1,1,-1,1,-1,1..
z     Zip with input
*    using multiplication.


In Husk, multiplying a string with a number repeats it that many times, also reversing it if the number is negative.

• the special case of * with negatives should be added to the docs. Oct 13, 2020 at 7:35

# JavaScript (ES6), Firefox, 43 bytes

This version abuses the sort algorithm of Firefox. It generates garbage on Chrome and doesn't alter the strings at all on Edge.

a=>a.map((s,i)=>[...s].sort(_=>i&1).join)


### Test cases

let f =

a=>a.map((s,i)=>[...s].sort(_=>i&1).join)

console.log(f(["Here are some lines","of text for you","to make a","boustrophedon"]))
console.log(f(["My boustrophedon"]))
console.log(f([]))

Or Try it online! (SpiderMonkey)

# JavaScript (ES6), 45 bytes

a=>a.map(s=>(a^=1)?s:[...s].reverse().join)


### Test cases

let f =

a=>a.map(s=>(a^=1)?s:[...s].reverse().join)

console.log(f(["Here are some lines","of text for you","to make a","boustrophedon"]))
console.log(f(["My boustrophedon"]))
console.log(f([]))

# APL (Dyalog Unicode), 10 bytes

⌽¨@{2|⍳≢⍵}


Works both ways:

Try it online! with ⎕IO←1

Try it online! with ⎕IO←0

### How it works:

⌽¨@{2|⍳≢⍵} ⍝ tacit prefix fn
{   ≢⍵} ⍝ Length of the input
⍳    ⍝ generate indexes from 1 (or 0 with ⎕IO←0)
2|     ⍝ mod 2; this generates a boolean vector of 0s (falsy) and 1s (truthy)
@        ⍝ apply to the truthy indexes...
⌽¨         ⍝ reverse each element


# Perl 5, 17 + 2 (-pl) = 19 bytes

odd lines reversed

$_=reverse if$.%2


even lines reversed

$_=reverse if$|--


After @Martin's comment : input needs to have a trailing linefeed.

try it online

foldr((.map reverse).(:))[]


Try it online!

f(a:b:c)=a:reverse b:f c
f a=a


Try it online!

Recursion FTW.

# 05AB1E, 5 4 bytes

εNFR


Try it online!

Explanation

ε     # apply to each element (row)
NF   # index times do:
R  # reverse


# R, 85 bytes

for(i in seq(l<-strsplit(readLines(),"")))cat("if"(i%%2,(,rev)(l[[i]]),"\n",sep="")


Try it online!

Input from stdin and output to stdout.

Each line must be terminated by a linefeed/carriage return/CRLF, and it prints with a corresponding newline. So, inputs need to have a trailing linefeed.

• I like what you did with the function names in the if. It's less fun, but '(' could be c for -2 bytes. Also, \n can be an actual new line for -1. Sep 1, 2020 at 15:09

# K (oK), 17 14 bytes

Solution:

@[;&2!!#x;|]x:


Try it online!

Example:

@[;&2!!#x;|]x:("this is";"my example";"of the";"solution")
("this is"
"elpmaxe ym"
"of the"
"noitulos")


Explanation:

Apply reverse at odd indices of the input list:

@[;&2!!#x;|]x: / the solution
x: / store input as variable x
@[;      ; ]   / apply @[variable;indices;function] (projection)
|    / reverse
#x      / count (length) of x, e.g. 4
!        / til, !4 => 0 1 2 3
2!         / mod 2, 0 1 2 3 => 0 1 0 1
&           / where true, 0 1 0 1 => 1 3


Notes:

• switched out &(#x)#0 1 for &2!!#x to save 3 bytes

# APL (Dyalog), 12 bytes

⍳∘≢{⌽⍣⍺⊢⍵}¨⊢


Try it online!

# Python 2, 40 36 bytes

-4 bytes thanks to @Mr.Xcoder

def f(k):k[::2]=map(reversed,k[::2])


Try it online!

Outputs by modifying the input list

# Python 2, 43 bytes

f=lambda k,d=1:k and[k[0][::d]]+f(k[1:],-d)


Try it online!

# Jelly, 5 4 bytes

U¹ƭ€


Try it online!

Thanks HyperNeutrino for -1 bytes! (actually because I never knew how ƭ works due to lack of documentation, this time I got lucky)

• Tried ¦ with m (7 bytes). s2U2¦€;/ is also 7 bytes. Dec 8, 2017 at 13:30

# Alumin, 66 bytes

hdqqkiddzhceyhhhhhdaeuzepkrlhcwdqkkrhzpkzerlhcwqopshhhhhdaosyhapzw


Try it online!

FLAG: h
hq
CONSUME A LINE
qk
iddzhceyhhhhhdaeuze
pk
rlhcw
REVERSE STACK CONDITIONALLY
dqkkrhzpkzerlhcwqops

OUTPUT A NEWLINE
hhhhhdao
syhapzw

• @totallyhuman This is actually my language. Dec 8, 2017 at 16:18

# J, 9 bytes

(,|.&.>)/


Reduce from right to left, reversing all strings in the result and prepending the next string as is.

Try it online!

We can do 6 using ngn’s approach, but there will be extra spaces:

]&|./\


Try it online!

# T-SQL, 65 bytes

Our standard input rules allow SQL to input values from a pre-existing table, and since SQL is inherently unordered, the table must have row numbers to preserve the original text order.

I've defined the table with an identity column so we can simply insert lines of text sequentially (not counted toward byte total):

CREATE TABLE t
(i int identity(1,1)
,a varchar(999))


So to select and reverse alternating rows:

SELECT CASE WHEN i%2=0THEN a
ELSE reverse(a)END
FROM t
ORDER BY i


Note that I can save 11 bytes by excluding the ORDER BY i, and that is likely to return the list in the original order for any reasonable length (it certainly does for the 4-line example). But SQL only guarantees it if you include the ORDER BY, so if we had, say, 10,000 rows, we would definitely need this.

# V, 4 bytes

òjæ$ Try it online! ò ' <M-r>ecursively (Until breaking) j ' Move down (breaks when we can't move down any more) æ$   ' <M-f>lip the line to the end$ # Perl 6, 44 bytes lines.map: ->\a,$b?{a.put;.flip.put with $b}  Try it lines # get the input as a list of lines .map: -> \a,$b? {        # $b is optional (needed if there is an odd number of lines) a.put; # just print with trailing newline .flip.put with$b # if $b is defined, flip it and print with trailing newline }  Comma delimited: # Stax, 12 bytes ü«äì╠▒╕█╬pεû  Run and debug it input: ABC,def,GHI,jkl,MNO,pqr,STU Newline delimited: # Stax, 8 bytes Çε÷┘)¼M@  Run and debug it input: ABC def GHI jkl MNO pqr STU  output for both: CBA def IHG jkl ONM pqr UTS  # Pyth, 6 bytes .e@_Bb  Try it here! • .e ~ Enumerated map. k is the index, b is the current element. • _Bb ~ Bifurcate b with its reverse. • @ ~ Modular index in ^ with k. # Actually, 7 bytes ;r'R*♀ƒ  Explanation: ;r'R*♀ƒ ;r range(len(input)) 'R* repeat "R" n times for n in range ♀ƒ call each string as Actually code with the corresponding input element as input (reverse each input string a number of times equal to its index)  Try it online! ## Alice, 13 bytes M%/RM\ d&\tO/  Try it online! Input via separate command-line arguments. Reverses the first line (and every other line after that). ### Explanation  At the beginning of each loop iteration there will always be zero on top of the stack (potentially as a string, but it will be converted to an integer implicitly once we need it). M Push the number of remaining command-line arguments, M. % Take the zero on top of the stack modulo M. This just gives zero as long as there are arguments left, otherwise this terminates the program due to the division by zero. / Switch to Ordinal mode. t Tail. Implicitly converts the zero to a string and splits off the last character. The purpose of this is to put an empty string below the zero, which increases the stack depth by one. M Retrieve the next command-line argument and push it as a string. / Switch back to Cardinal mode. d Push the stack depth, D. &\R Switch back to Ordinal mode and reverse the current line D times. O Print the (possibly reversed) line with a trailing linefeed. \ Switch back to Cardinal mode. The instruction pointer loops around and the program starts over from the beginning.  # Tcl, 61 bytes proc B L {lmap e$L {expr [incr i]%2?"$e":"[string rev$e]"}}


Try it online!

# Standard ML (MLton), 51 bytes

fun$(a::b::r)=a::implode(rev(explode b))::$r| $e=e  Try it online! Usage example: $ ["abc","def","ghi"] yields ["abc","fed","ghi"].

Explanation:

$ is a function recursing over a list of strings. It takes two strings a and b from the list, keeps the first unchanged and reverses the second by transforming the string into a list of characters (explode), reversing the list (rev), and turning it back into a string (implode). • +1, not enough ML solutions imo – jfh Dec 8, 2017 at 22:36 # Stacked, 18 bytes [{x i:x$revi*}map]


Try it online!

This simply reverses each string in the array according to its position.

[:#':>#,tr[...$rev*]map]  Same approach, but generates indices manually. # Python 2, 45 bytes lambda k:[r[::i%-2|1]for i,r in enumerate(k)]  Try it online! # Python 2, 46 bytes f=lambda k:k and[k[0][::len(k)%-2|1]]+f(k[1:])  Try it online! # Jelly, 5 bytes UJḤ$¦


*Jelly outputs nested lists joined by spaces, and there is no native string type. Strings in Jelly are lists of characters, and that's why they're displayed that way. If you want them to be merged by spaces, Try this.

# Retina, 18 bytes

{O\$^\G.

*2G
2A


Try it online! Explanation: The first stage reverse the first line, then the second stage prints the first two lines, after which the third stage deletes them. The whole program then repeats until there is nothing left. One trailing newline could be removed at a cost of a leading ;.

# Wolfram Language (Mathematica), 33 bytes

Fold[StringReverse@*Append,{},#]&


Try it online!

## How it works

StringReverse@*Append, when given a list of strings and another string as input, adds the string to the end of the list and then reverses all of the strings.

Folding the input with respect to the above means we:

• Reverse the first line.
• Add the second line to the end and reverse both of them.
• Add the third line to the end and reverse all three.
• Add the fourth line to the end and reverse all four.
• And so on, until we run out of lines.

Each line gets reversed one time fewer than the previous line, so the lines alternate direction.

# CJam, 11 bytes

{2/Waf.%:~}


Try it online! (CJam array literals use spaces to separate elements)

Explanation:

{              Begin block, stack: ["Here are some lines" "of text for you" "to make a" "boustrophedon"]
2/            Group by 2:         [["Here are some lines" "of text for you"] ["to make a" "boustrophedon"]]
W           Push -1:            [["Here are some lines" "of text for you"] ["to make a" "boustrophedon"]] -1
a          Wrap in array:      [["Here are some lines" "of text for you"] ["to make a" "boustrophedon"]] [-1]
f.%       Vectorized zipped array reverse (black magic):
[["senil emos era ereH" "of text for you"] ["a ekam ot" "boustrophedon"]]
:~     Flatten:            ["senil emos era ereH" "of text for you" "a ekam ot" "boustrophedon"]
}


Explanation for the Waf.% "black magic" part:

• W is a variable preinitialized to -1. a wraps an element in an array, so Wa is [-1].
• % pops a number n and an array a and takes every nth element of the array. When n is negative, it also reverses it, meaning that W% reverses an array.
• . followed by a binary operation applies that operation to corresponding elements of an array, so [1 2 3] [4 5 6] .+ is [5 7 9]. If one array is longer than the other, the elements are kept without modification, meaning that Wa.% reverses the first element of an array.
• f followed by a binary operation will take an element from the stack and then act like {<that element> <that operation>}%, that is, go through each element in the array, push its element, push the element first popped from the stack, run the operation, and then collect the results back into an array. This means that Wa.f%` reverses the first element of every element in the array.