# No A, just CAPS LOCK

What happens when the CapsLock key on your keyboard doesn't have a notch in it?

"This hPPENS."

The goal of this program is to consistently emulate keyboard misses where each A press is replaced with CapsLock. Uppercase 'A's from the source should yield the same effect. When CapsLock is enabled, capitalization is reversed.

# Test Cases

"The quick brown fox jumps over the lazy dog."
-> "The quick brown fox jumps over the lZY DOG."

"Compilation finished successfully."
-> "CompilTION FINISHED SUCCESSFULLY."

"What happens when the CapsLock key on your keyboard doesn't have a notch in it?"
-> "WhT Hppens when the CPSlOCK KEY ON YOUR KEYBOrd doesn't hVE  notch in it?"

"The end of the institution, maintenance, and administration of government, is to secure the existence of the body politic, to protect it, and to furnish the individuals who compose it with the power of enjoying in safety and tranquillity their natural rights, and the blessings of life: and whenever these great objects are not obtained, the people have a right to alter the government, and to take measures necessary for their safety, prosperity and happiness."
-> "The end of the institution, mINTENnce, ND dministrTION OF GOVERNMENT, IS TO SECURE THE EXISTENCE OF THE BODY POLITIC, TO PROTECT IT, nd to furnish the individuLS WHO COMPOSE IT WITH THE POWER OF ENJOYING IN Sfety ND TRnquillity their nTURl rights, ND THE BLESSINGS OF LIFE: nd whenever these greT OBJECTS re not obtINED, THE PEOPLE Hve  RIGHT TO lter the government, ND TO Tke meSURES NECESSry for their sFETY, PROSPERITY nd hPPINESS."

"aAaaaaAaaaAAaAa"
-> "" (Without the notch, no one can hear you scream)

"CapsLock locks cAPSlOCK"
-> "CPSlOCK LOCKS CPSlOCK"

"wHAT IF cAPSlOCK IS ALREADY ON?"
-> "wHt if CPSlOCK IS lreDY ON?"


The winning criterion is, as usual, the size of the submitted program's source code.

• Welcome to the site! This is a nice first challenge, and unfortunately very relatable for me and my fT FINGERS. – James Mar 14 '18 at 18:29
• suggested test case : teSTateSTateSTateST – Rod Mar 14 '18 at 18:44
• If only the enter key also had a notch in it so this wouldn' – 12Me21 Mar 14 '18 at 19:33
• t happen....... – 12Me21 Mar 14 '18 at 19:33
• Literally joined this site to upvote "Without the notch, no one can hear you scream" – lucasvw Mar 15 '18 at 21:10

# AutoHotKey, 7 bytes

a::vk14


// Is this valid? This really do what OP want -- replace a by CapsLock (vk14).

Run this program, and type the input from keyboard..

• "Is this valid?" OP did not specify input or output constraints so I´d consider this valid. – Nefrin Mar 15 '18 at 13:22
• Don't see too many ahk answers! – HaveSpacesuit Mar 15 '18 at 13:57
• "my code runs everywhere" – sudo rm -rf slash Mar 15 '18 at 15:02
• ThT'S GREt but how do I disBLE IT? – RobbG Mar 16 '18 at 13:52
• @RobbG just type "killLL utohotkey" ... oh wIT – Nefrin Mar 16 '18 at 14:04

# V, 9 bytes

ò/ãa
xg~$ Try it online! Hexdump: 00000000: f22f e361 0a78 677e 24 ./.a.xg~$


Explanation:

ò       " Recursively:
/ãa    "   Move forward to the next 'a' (upper or lowercase)
"   This will break the loop when there are no more 'a's
x       "   Delete the 'a'
g~$" Toggle the case of every character after the cursor's position.  # Vim, 16 bytes qq/\ca xg~$@qq@q


Assumes the input is on a single line

## Explanation

qq            Start a loop
/\ca␊         Find the first occurence of an a, end the loop if there are none left
xg~$Remove it and invert the case of the rest of the file @qq@q End the loop  • Is this case insensitive finding of 'a'? – Gnudiff Mar 16 '18 at 21:06 • @Gnudiff \c anywhere in a regex-search enables case-insensitivity – Herman L Mar 16 '18 at 21:14 • Do you need to set a specific flag for g~$ to work? Because for me it only inverts case until the end of the line, not the whole file, so this doesn't really work for multiline files for me. – Cubic Mar 19 '18 at 15:34
• @Cubic As I wrote in the answer, it "assumes the input is on a single line" – Herman L Mar 19 '18 at 15:58
• @Cubic If you wanted it to go the end of the file and support inputs on multiple lines, you could do g~vG or vG~. – James Mar 19 '18 at 17:15

# C, 72 bytes

Thanks to @Ton Hospel for helping to save 16 bytes!

t,c;f(char*s){for(t=0;c=*s++;6305%c?putchar(isalpha(c)?c^t:c):(t^=32));}


Try it online!

• You can swap the case of letters using a xor with 32 – Ton Hospel Mar 14 '18 at 19:19
• You can probably save even more by having t be 0/32 instead of even/odd (xor t with 32 for each a) and then xor letters directly with t – Ton Hospel Mar 14 '18 at 20:14
• Nice way to detect a's – Ton Hospel Mar 14 '18 at 20:17
• @TonHospel Functions have to be reusable, and I don't think it counts as reusable if you need external code to make it usable again after each call. – Steadybox Mar 15 '18 at 10:49
• 6305%c is 0 if c is 13. – Rosie F Mar 16 '18 at 11:55

# Husk, 11 bytes

Γ·§?m\:€"Aa


Try it online!

## Explanation

I'm using the somewhat obscure overloading of Γ called listNF, which constructs recursive functions that operate on lists. It corresponds to the following Haskell pattern:

listNF f = g
where g (x : xs) = f g x xs
g [] = []


The idea is that listNF takes a helper function f and returns a new function g, which takes a list. The function f takes a function, which will always be g, and the head x and tail xs of the list, and does something with them. In our application, f calls g recursively on xs. The program is interpreted like this:

Γ (· (§ (?m\) : (€"Aa")))
Γ (                     )  Create a function g that takes a list (x:xs) and applies a function on x and xs.
· (                 )   Compose g with second argument of function in parentheses.
Instead of x and xs, the function is called on x and the result of a recursive call of g on xs.
(€"Aa")    Check if x is 'A' or 'a'.
(?m\)              If it is, then swap the case of every char in g(xs).
§       :            Otherwise, prepend x to g(xs).

• Wow, good thing I refreshed before I posted my 12 byte solution: Ḟ·+m\ṁx'Ax'a. Could we get an explanation? I can't find any information on what Γ does exactly and this seems like a good chance to learn. – Sophia Lechner Mar 15 '18 at 0:02
• @SophiaLechner Done. This version of Γ is a bit hard to explain, I hope you can make sense of it. – Zgarb Mar 15 '18 at 8:16
• Wow this is slow. Is it just TIO? – FrownyFrog Mar 15 '18 at 13:00
• @FrownyFrog It's Husk. Type inference of programs containing Γ seems to be slow in general. If you're not familiar with Husk, a program is interpreted by looping through all possible structures of the program (essentially the possible placements of parentheses) and all overloadings of each built-in, and choosing the first one where the result is well-typed. The interpreter is smart enough to reject some possibilities early, but it seems that the recursive version of Γ can mess with this step and force it to loop through a lot of choices. – Zgarb Mar 15 '18 at 13:34
• @SophiaLechner I wrote a tip that explains Γ in some detail. – Zgarb Mar 15 '18 at 14:42

# Retina, 3321 17 bytes

i(TvlLLla.*
a



Try it online

Explanation:

i(              i is for case-insensitive, the paren makes it modify both stages
Tv           Transliteration, with simple overlaps (v) - 1 match at every start pos
lLLl     Replace lowercase with uppercase, and vice versa
a.*  Every 'a' will match, overlapping to the end of the string
This swaps the case on all letters after each 'a'
a               Replace all 'a's with nothing



-12 bytes thanks to Martin
-4 bytes thanks to Leo

• Amazingly this is almost as short as the current Pyth solutions – Ton Hospel Mar 14 '18 at 19:11
• I think iTaAlL__Lla[^a]*a? also works for 21 bytes. – Neil Mar 14 '18 at 20:29
• 4 bytes shorter using overlapping matches – Leo Mar 14 '18 at 21:37
• I'm interested in how this works if you have time to add the explanation. Thanks! – seshoumara Mar 15 '18 at 7:41

# C#, 121 bytes

Console.WriteLine(string.Join("",Console.ReadLine().Split(new[]{'a','A'}).Select((a,i)=>i%2==0?a:a.ToUpper()).ToList()));


** Update (thanks to @John & @aloisdg) **

# C#, 69 bytes

x=>string.Concat(x.Split('a','A').Select((a,i)=>i%2>0?a.ToUpper():a))

• Welcome to PPCG! Nice first answer! – RedClover Mar 15 '18 at 7:02
• you can save 7 bytes changing new[] { 'a', 'A' } to 'a', 'A' – John Mar 15 '18 at 8:58
• You can do it in 69 bytes with the same logic! Try it online! (use input/ouput instead of console, remove ToList, inverse the ternary and use @John comment) This is a nice first answer. Keep going! – aloisdg moving to codidact.com Mar 15 '18 at 11:02
• Both of these versions do not swap case (they transform to uppercase only) when CapsLock is enabled. This is a requirement. (See the last test case) – Broadwell Mar 15 '18 at 23:51
• @Broadwell How would you know if CapsLock is enabled? Are you sure the last test case is correct? It passes all other test cases, as far as I can see. Thanks! – Aalawlx Mar 15 '18 at 23:55

# JavaScript (ES6), 938884 82 bytes

(saved 5 bytes thanks to @Shaggy, 4 bytes thanks to @user81655, and 2 bytes thanks to @l4m2.)

a=>a.replace(A=/./g,c=>c in{a,A}?(A=!A,''):A?c:c[to${c<{}?'Low':'Upp'}erCase]())  Test cases: let f= a=>a.replace(A=/./g,c=>c in{a,A}?(A=!A,''):A?c:c[to${c<{}?'Low':'Upp'}erCase]())

console.log(f("The quick brown fox jumps over the lazy dog."));
console.log(f("Compilation finished successfully."));
console.log(f("What happens when the CapsLock key on your keyboard doesn't have a notch in it?"));
console.log(f("The end of the institution, maintenance, and administration of government, is to secure the existence of the body politic, to protect it, and to furnish the individuals who compose it with the power of enjoying in safety and tranquillity their natural rights, and the blessings of life: and whenever these great objects are not obtained, the people have a right to alter the government, and to take measures necessary for their safety, prosperity and happiness."));
console.log(f("aAaaaaAaaaAAaAa"));
console.log(f("CapsLock locks cAPSlOCK"));
console.log(f("wHAT IF cAPSlOCK IS ALREADY ON?"));

• ['to${c<'a'?'Low':'Upp'}erCase'] should save you a few bytes, replacing single quotes with backticks. – Shaggy Mar 15 '18 at 0:13 • Sure does, @Shaggy. Thanks! – Rick Hitchcock Mar 15 '18 at 15:51 • Using ^1 to have u be the parity can let you initialise it shorter: s=>s.replace(u=/./g,c=>/a/i.test(c)?(u^=1,''):+u?c[to${c<'a'?'Low':'Upp'}erCase]():c) – user81655 Mar 18 '18 at 7:45
• Also here's another tricky way to test for the letter a which is shorter: a=>a.replace(A=/./g,c=>c in{a,A}?(A^=1,''):+A?c[to${c<'a'?'Low':'Upp'}erCase]():c) – user81655 Mar 18 '18 at 7:53 • Brilliant, @user81655, especially using the in operator like that. Always more to learn! – Rick Hitchcock Mar 18 '18 at 10:45 # R, 92 bytes cat([<-(v<-el(strsplit(scan(,""),"a|A")),w<-c(F,T),chartr("a-zA-Z","A-Za-z",v)[w]),sep="")  Thank @Giuseppe for fixing the answer. Explanation # Write cat( # Replace and return, this is the function that powers # the R store at index operations, a[i]<-b [<-( # First arg - what to replace = extract first list element # of a string input after splitting at a or A v<-el(strsplit(scan(,""),"a|A")), # Second arg - index to replace = abuse vector recycling # to create infinite F, T, F, T, F, etc series w<-c(F,T), # Third arg - replacement values = replace with case toggled letters chartr("a-zA-Z","A-Za-z",v)[w]), # Write without separation sep="")  Try it online! • Perhaps I didn't make it clear, but this answer does not reverse capitalization when CapsLock is enabled (it only performs toupper), which is a requirement. – Broadwell Mar 14 '18 at 21:19 • ooohhhhhhh that's very clever with the c(F,T), although @Broadwell is right; looks like it'll be a chartr("a-zA-Z","A-Za-z",v)[w] rather than toupper – Giuseppe Mar 14 '18 at 21:22 • @Giuseppe Thanks – Vlo Mar 14 '18 at 21:35 # PowerShell Core, 105 bytes "$args"|% t*y|%{if($_-in97,65){$c=!$c}else{Write-Host -n($_,("$_"|%("*per","*wer")[$_-in65..90]))[!!$c]}}  Try it online! What with no real ternary operator and no default alias for printing to screen, it's not that short. • % t*y expands to | ForEach-Object -Method ToCharArray equiv. of "$args".ToCharArray()
• Write-Host -n is for the parameter -NoNewLine
• "$_" turns the [char] type back to [string] (chars have no upper/lower case in .Net) • |% *per does the same method call shortcut as earlier, but for .ToUpper(), same with .ToLower() • ($a,$b)[boolean test] abused as fake-ternary operator • !!$c force-casts to [bool] here it starts undefined $null so it gets forced it into existence as "caps lock:$false".
• That |% t*y is a neat trick I'll need to remember. Shorter than [char[]], which I use a lot. I'd almost say that should go onto the Tips thread. – AdmBorkBork Mar 19 '18 at 13:10
• 94 bytes: -join($args|% t*y|%{if($_-eq'a'){$c=!$c}else{(("$_"|%("*per","*wer")[$_-in65..90]),$_)[!$c]}}). thanks for a |% *ethod operator! – mazzy Jul 8 '18 at 11:55

# Perl 5-p, 3130 29 bytes

-1 byte thanks to @nwellnhof

-1 byte thanks to @ikegami

#!/usr/bin/perl -p
s/a([^a]*)a?/$1^uc$1^lc$1/egi  Try it online! • Why not simply s/a(.*?)(a|$)/uc$1/egi (22 bytes)? – nwellnhof Mar 16 '18 at 13:59 • @nwellnhof Because the capslock when active toggles the case, it doesn't just uppercase – Ton Hospel Mar 16 '18 at 15:18 • Ah, I see. Then s/a(.*?)(a|$)/$1^uc$1^lc$1/egi is one byte shorter. – nwellnhof Mar 16 '18 at 18:09 • @nwellnhof Thanks, that is very neat – Ton Hospel Mar 16 '18 at 20:51 • a([^a]*)a? is shorter than a(.*?)(a|$) – ikegami Mar 18 '18 at 8:47

# Python, 63 bytes

f=lambda s:s and[s[0]+f(s[1:]),f(s[1:]).swapcase()][s[0]in"aA"]


Another Python solution, works in Python 2 and 3. Takes a very long time for all but small inputs.

# 6502 machine code routine (C64), 51 bytes

A0 00 84 FE B1 FC F0 2A C9 41 F0 06 90 1A C9 C1 D0 08 A9 80 45 FE 85 FE B0 11
B0 06 C9 5B B0 08 90 04 C9 DB B0 02 45 FE 20 16 E7 C8 D0 D6 E6 FD D0 D2 60


Expects a pointer to a 0-terminated input string in $fc/$fd, outputs to the screen.

### Commented disassembly

 .caps:
A0 00       LDY #$00 84 FE STY$FE             ; init capslock state
.loop:
B1 FC       LDA ($FC),Y ; next char from string F0 2A BEQ .done ; NUL -> we're done C9 41 CMP #$41            ; compare to 'a'
F0 06       BEQ .isa            ; if equal, toggle capslock
90 1A       BCC .out            ; if smaller, direct output
C9 C1       CMP #$C1 ; compare to 'A' D0 08 BNE .ctog ; if not equal, check for letter .isa: A9 80 LDA #$80            ; toggle bit 7 in caps lock state
45 FE       EOR $FE 85 FE STA$FE
B0 11       BCS .next           ; and go on
.ctog:
B0 06       BCS .cZ             ; if char larger 'A', check for 'Z'
C9 5B       CMP #$5B ; compare with 'z'+1 B0 08 BCS .out ; larger or equal -> direct output 90 04 BCC .tog ; smaller -> apply capslock .cZ: C9 DB CMP #$DB            ; compare with 'Z'+1
B0 02       BCS .out            ; larger or equal -> direct output
.tog:
45 FE       EOR $FE ; toggle bit from capslock state .out: 20 16 E7 JSR$E716           ; output char
.next:
C8          INY                 ; and loop to next char
D0 D6       BNE .loop
E6 FD       INC $FD D0 D2 BNE .loop .done: 60 RTS  ### Example assembler program using the routine: Online demo Code in ca65 syntax: .import caps ; link with routine above .segment "BHDR" ; BASIC header .word$0801           ; load address
.word   $080b ; pointer next BASIC line .word 2018 ; line number .byte$9e             ; BASIC token "SYS"
.byte   "2061",$0,$0,$0 ; 2061 ($080d) and terminating 0 bytes

.bss
string:         .res    $800 .data prompt: .byte$d, "input> ", $0 .code lda #$17            ; set upper/lower mode
sta     $d018 lda #<prompt ; display prompt ldy #>prompt jsr$ab1e

lda     #<string        ; read string into buffer
sta     $fc lda #>string sta$fd

lda     #>string        ; call our caps routine on buffer
sta     $fd jmp caps ; read a line of input from keyboard, terminate it with 0 ; expects pointer to input buffer in$fc/$fd ; NO protection agains buffer overflows !!! .proc readline ldy #$0
sty     $cc ; enable cursor blinking sty$fe             ; temporary for loop variable
lda     $fd sta$2              ; initial page of string buffer
getkey:         jsr     $f142 ; get character from keyboard beq getkey sta$fb             ; save to temporary
and     #$7f cmp #$20            ; check for control character
bcs     prepout         ; no -> to normal flow
cmp     #$d ; was it enter/return? beq prepout ; -> normal flow cmp #$14            ; was it backspace/delete?
bne     getkey          ; if not, get next char
lda     $fe ; check current index bne prepout ; not zero -> ok lda$2              ; otherwise check if we're in the
cmp     $fd ; first page of the buffer beq getkey ; if yes, can't use backspace prepout: ldx$cf             ; check cursor phase
beq     output          ; invisible -> to output
sei                     ; no interrupts
ldy     $d3 ; get current screen column lda ($d1),y         ; and clear
and     #$7f ; cursor in sta ($d1),y         ;   current row
cli                     ; enable interrupts
output:         lda     $fb ; load character jsr$e716           ;   and output
ldx     $cf ; check cursor phase beq store ; invisible -> to store sei ; no interrupts ldy$d3             ; get current screen column
lda     ($d1),y ; and show ora #$80            ;   cursor in
sta     ($d1),y ; current row cli ; enable interrupts lda$fb             ; load character
store:          cmp     #$14 ; was it backspace/delete? beq backspace ; to backspace handling code ldy$fe             ; load buffer index
sta     ($fc),y ; store character in buffer cmp #$d             ; was it enter/return?
beq     done            ; then we're done.
sty     $fe bne getkey ; not zero -> ok inc$fd             ; otherwise advance buffer page
bne     getkey
done:           lda     #$0 ; terminate string in buffer with zero ldy$fe             ; get buffer index
iny
bne     termidxok       ; and advance ...
inc     $fd termidxok: sta ($fc),y         ; store terminator in buffer
inc     $cc ; disable cursor blinking rts ; return backspace: ldy$fe             ; load buffer index
bne     bsidxok         ; if zero
dec     $fd ; decrement current page bsidxok: dey ; decrement buffer index sty$fe
bcs     getkey          ; and get next key
.endproc

• I just have to say I admire you went to the effort to write in assembly. I don’t think this has much to do with the fact I used to really enjoy asm but maybe having the experience makes me more aware of what it entails. Experience or ease is besides the point to me. It brightens my day just a bit to see such enthusiasm too. – Pryftan Mar 18 '18 at 21:23
• @Pryftan thanks :) It's just a nice way to keep in practice, I'm working on some game and recently also demo code for this nice old machine :) – Felix Palmen Mar 19 '18 at 8:20
• Well it's great to see! Keep it up; I remember enjoying asm but I don't think I'd enjoy it so much nowadays (unless maybe I had an old machine like you do, that is but possibly not even then) - C is my favourite all time and it's what I primarily use. Anyway, won't let this evolve into chat - just wanted to say I appreciated the answer! – Pryftan Mar 19 '18 at 20:56

# Java 8, 119108 98 bytes

s->{int f=0,t;for(int c:s)if((t=c&95)==65)f^=1;else System.out.printf("%c",f<1|t<66|t>90?c:c^32);}


-11 bytes thanks to @OlivierGrégoire.
-10 bytes thanks to @Nevay.

Explanation:

Try it online.

s->{                           // Method with char-array parameter and no return-type
int f=0,t;                   //  Flag-integer, starting at 0
for(int c:s)                 //  Loop over the characters of the input as integers
if((t=c&95)==65)           //   If the current character is an 'A' or 'a':
f^=1;                    //    Toggle the flag (0→1 or 1→0)
else                       //   Else:
System.out.printf("%c",  //    Print integer as character
f<1|                   //     If the flag-integer is 0,
t<66|t>90?             //     or the current character isn't a letter:
c                     //      Simply output the character as is
:                      //     Else (the flag it 1 and it's a letter)
c^32);}               //      Print it with its case reversed

• Damn imperatives... they forbade me to post my answer before yours... Anyways, here's my answer, 11 bytes shorter: s->{int z=0,d;for(int c:s)if((d=c&95)==65)z^=1;else System.out.printf("%c",z<1|d<66|d>90?c:c<91?c|32:c&95);} – Olivier Grégoire Mar 15 '18 at 10:51
• @OlivierGrégoire Nice answer! And what do you mean by forbade me to post? Is your work-network that strict? – Kevin Cruijssen Mar 15 '18 at 11:16
• My answer was ready for a while: I was just polishing the test cases before posting but then suddenly endless meetings happened. – Olivier Grégoire Mar 15 '18 at 11:19
• No, it's ok, I only have to blame myself for not being quick enough before the meetings ;-) But thank you for suggesting this! – Olivier Grégoire Mar 15 '18 at 13:23
• 98 bytes: s->{int f=0,t;for(int c:s)if((t=c&95)==65)f^=1;else System.out.printf("%c",f<1|t<66|t>90?c:c^32);} – Nevay Mar 17 '18 at 17:01

# C, 167168158 131 bytes

Thanks for @Martin Ender for the code review: I've switched the stream processing for string processing to help with reusability. Also many thanks to @RiaD and @ceilingcat for their suggestions.

c,d;(*t[][2])()={{isupper,tolower},{islower,toupper}};f(char*s){for(d=1;c=*s++;)t[0][1](c)==97?d=!d:putchar(t[!t[d][0](c)][1](c));}


Try it online!

# How does it work?

/* int c is the input character,
int d is the Caps Lock flag (1=off, 0=on)  starting as "Off". */
int c, d;
/* array of comparison functions and transformation functions for each state */
(*t[][2])() = {{isupper, tolower}, {islower, toupper}};

f(char *s) {
/* Loop if we haven't hit the terminator */
for(d = 1; c = *s++;)
t[0][1](c) == 97 ?
/* If tolower(c)=='a' then flip the Caps Lock state */
d=!d:
/* Otherwise, convert character according to the following table:

Character case
Caps Lock  UPPER       LOWER
ON  tolower()   toupper()
OFF  toupper()   tolower()
*/
putchar(t[!t[d][0](c)][1](c));
}
}


# Notes

• s[][] is where the magic happens: [][0] is the comparison function and [][1] is the related transformation function for each state.
• ! is applied to the comparison function to force it into the range [0,1].
• Welcome to PPCG! Unfortunately, you can't rely on the initialisation of d like this because it means that your function is not reusable. A simple d=0; should fix it. – Martin Ender Mar 16 '18 at 10:01
• I wasn't sure if reusability or maintaining state was more important in this case. If reusability is more important, I would move the variable declarations inside the function so the beginning would read void f(){int c,d=0;[...]. In any case, the stream dies, so an edit is in order! – ErikF Mar 16 '18 at 11:05
• do you need s in your while loop? It can't became NULL unless you called with f(NULL) – RiaD Mar 16 '18 at 14:42
• d=!d for flipping – RiaD Mar 16 '18 at 14:43
• !! will be ! if you flip order of t, and start d with 1 – RiaD Mar 16 '18 at 14:54

import Data.Char
g x|x<'['=toLower x|1>0=toUpper x
f(a:b)|elem a"aA"=f$g<$>b|1>0=a:f b
f x=x


Try it online!

## Explanation

First we declare g to be the function that maps lowercase to upper case and uppercase to lowercase. This is actually the majority of our bytecount. Then we define the function f. If the input to f is of the form a:b we do

f(a:b)
|elem a"aA"=f$g<$>b
|1>0=a:f b


a and A match the first pattern and thus we apply f to the input with it's case inverted. Otherwise we move a out front and apply f to b.

# 05AB1E, 12 bytes

õ?„AaS¡Dvć?š


Try it online!

Explanation

õ?             # print an empty string (to account for the special case of only A's)
„AaS¡        # split on occurrences of "A" or "a"
D       # duplicate
v      # for each element in the top copy
ć?    # extract and print the head of the other copy
š   # switch the case of the rest of the other copy


# Wolfram Language (Mathematica), 70 bytes

#//.{x___,"a"|"A",y___}:>Join[{x},ToUpperCase@#+ToLowerCase@#-#&@{y}]&


Try it online!

Takes input and output as a list of characters. For convenience I've added code in the footer to convert this from and back to a string.

# How it works

The #//.{x___,"a"|"A",y___}:>Join[{x},...{y}]& part is standard: we find the first A (uppercase or lowercase), reverse case of that comes after the A, and repeat until there are no more A's to be found.

The interesting part is how we reverse case: the function ToUpperCase@# + ToLowerCase@# - #&. We add together the upper-cased version of the input and the lower-cased version of the input, then subtract the actual input. For example, given the list {"I","n","P","u","T"} this computes

{"I","N","P","U","T"}+{"i","n","p","u","t"}-{"I","n","P","u","T"}


{"I"+"i"-"I","N"+"n"-"n","P"+"p"-"P","U"+"u"-"u","T"+"t"-"T"}


and although Mathematica doesn't have any particular way of adding two strings, it's smart enough to simplify a+b-a to b for any values of a and b, including string values, so this simplifies to {"i","N","p","U","t"}.

# Ruby, 42 41 bytes

->s{s.sub!(/a(.*)/i){$1.swapcase}?redo:s}  Try it online! A lambda accepting a string, mutating the string in place, and returning it. The trick here is that sub returns the string (a truthy value) if a substitution was made, and returns nil otherwise. The existence of swapcase is pretty handy, too. -1 byte: Replace boolean logic with ternary operator, thanks to Asone Tuhid ->s{ s.sub!(/a(.*)/i){ # Replace "a" followed by anything with$1.swapcase         #   the case-swapped capture group
} ? redo              # If a match was found, restart the block
: s                 # Otherwise, return the modified string
}

• save 1 byte. The link was too long if I included all the test cases. – Asone Tuhid Mar 16 '18 at 6:17
• @AsoneTuhid Thanks... One of these days I'll remember to use the ternary operator straight away, so you won't have to remind me. – benj2240 Mar 16 '18 at 15:01

# PHP101 99 bytes

for($s=$argn;$i<strlen($s);$i++)lcfirst($s[$i])==a?$s=strtolower($s)^strtoupper($s)^$s:print$s[$i];  Run like this: echo '[the input]' | php -nR '[the code]'  Ungolfed: for ($s = $argn;$i < strlen($s);$i++) {
if (lcfirst($s[$i]) == 'a') {
$s = strtolower($s) ^ strtoupper($s) ^$s; // Flip the whole string's case.
} else {
print $s[$i]; // Print the current letter.
}
}


This just loops through the string with a for loop, and on each iteration it checks if the current letter is a, if so, then flip the case of the whole string (method from here), and if not, then print the current letter.

• The convention for code golf is that all code must be included. That means you have to take input as function parameter and actually declare a function (via the function keyword in php) or have a complete script (e.g. using $argn, $argv, $_GET). So at the moment this is not a correct submission. Return must be echoed or returned (only allowed for functions ofc). – Christoph Mar 15 '18 at 12:10 • Thanks for that @Christoph, I'm kind of new to golfing :). I've updated my answer now, just let me know if there's anything else that's wrong. – Ethan Mar 15 '18 at 20:14 • @Christoph Wow! 75! Very nice! You have my +1 :) – Ethan Mar 16 '18 at 16:07 # Jelly, 14 bytes Œu=”Aœp⁸ŒsJḤ$¦


Try it online!

Full program.

Explanation:

Œu=”Aœp⁸ŒsJḤ$¦ Arguments: x Œu Uppercase x =”A ^ Equals 'A' (vectorizes) œp⁸ ^ Partition ⁸ [⁸=x] ¦ Apply link A, keep results at specific indices B Œs A: Swap case$  B: Form a >=2-link monadic chain
JḤ      Arguments: y
J       Get list indices ([1, length(list)]) of y
Ḥ      Double (vectorizes) ^
This way, we only "apply" link A to even indices, so every second
element, starting from the secondd one.

• Explanation of the code? – SK19 Mar 16 '18 at 22:00
• @SK19 Added an explanation. – Erik the Outgolfer Mar 19 '18 at 10:07

# MATL, 23 20 bytes

'a A'Yb&Ybt2L)Yo2L(g


Try it online!

### Explanation:

'a A'Yb   % form a cell array containing {'a', 'A'}
&Yb       % split input into substrings, with either of those ('a' or 'A') as delimiters
t2L)      % extract out the even positioned cells from that result
Yo        % switch the case of those substrings
2L(       % place the result back in even positioned cells of the original cell array
g         % convert cell array to matrix, concatenating all substrings in the process
% implicit output


"H@'aA'm?~XHx}@w~?Yo]&h

Other methods I tried:

0w"@t'aA'm?x~}y?Yo]w]]xv!
t'aA'mXHYsot&y*XzcYowf([]H(
t'aA'mXHYsoy3Y2m*32*Z~c[]H(


# Husk, 15 bytes

ω(F·+otm\↕·≠_'a


Try it online!

### Explanation

ω(F·+(tm\)↕·≠_'a) -- example input: "Bar, baz and Foo."
ω(              ) -- apply the following, until fixpoint is reached:
↕       -- | split string with predicate
· _    -- | | the lower-cased character
≠ 'a  -- | | is not 'a'
-- | : ("B","ar, baz and Foo.")
F               -- | apply the following to the tuple
+             -- | | join the elements with..
· (   )        -- | | ..the second element: "ar, baz and Foo."
m\         -- | | | swap case: "AR, BAZ AND fOO."
t           -- | | | tail: "R, BAZ AND fOO."
-- | : "BR, BAZ AND fOO."
-- : "BR, Bz ND fOO."


# Japt v2.0a0, 16 bytes

e/a.*/i_År\l_c^H


Try it

## Explanation

e                   :Recursively replace
/a.*/i             :RegEx /a.*/gi
_            :Pass each match through a function
Å           :  Slice off the first character
r          :  Replace
\l        :  RegEx /[A-Za-z]/g
_       :  Pass each match though a function
c^     :    Bitwise XOR the character code
H    :    With 32


foreach(spliti(a,$argn)as$b)echo$a++&1?strtoupper($b)^strtolower($b)^$b:$b;  Split into substrings by A (case insensitive) then toogle every second case. Old version for(;a&$c=$argn[$i++];)trim($c,aA)?print($c^chr($f*ctype_alpha($c))):$f^=32;  walks over the string and toogles a flag if the current char is a or A else the char gets toogled depending on the flag and echoed. # SNOBOL4 (CSNOBOL4), 141 92 bytes  I =INPUT S I ANY("Aa") REM . R =REPLACE(R,&LCASE &UCASE,&UCASE &LCASE) :S(S) OUTPUT =I END  Try it online! Assumes a single line of input. A whopping 49 bytes saved by @ninjalj! Line S does all the work, explained below: I # in the subject string I match the following PATTERN: ANY("Aa") # match A or a and REM . R # match the remainder of I, assigning this to R =REPLACE( # replace the PATTERN above with R, ...) # R with swapped cases. :S(S) # and if there was a match, goto S, else goto next line  • This gives the wrong answer (as you stated in your comment, case is swapped when CapsLock is on) – mbomb007 Mar 14 '18 at 18:34 • I've edited the post to require case-swapping (rather than simply uppercase) when CapsLock is enabled because I never realized that my machine does this. – Broadwell Mar 14 '18 at 18:35 • @mbomb007 ah, I hadn't realized OP had changed it; I'm editing for an explanation now so I'll include that in the explanation. – Giuseppe Mar 14 '18 at 18:35 • I =INPUT;S I ANY("Aa") REM . R =REPLACE(R,&LCASE &UCASE,&UCASE &LCASE) :S(S); OUTPUT =I;END – ninjalj Mar 16 '18 at 1:02 • @ninjalj are you also a SNOBOL golfer or am I just terrible at golfing it?? – Giuseppe Mar 16 '18 at 13:35 # Fortran (GFortran), 307 bytes CHARACTER(999)F,G G=' ' READ(*,'(A)')F N=1 M=1 DO I=1,999 IF(F(I:I)=='a'.OR.F(I:I)=='A')THEN M=-M ELSEIF(M==1)THEN G(N:N)=F(I:I) N=N+1 ELSE J=IACHAR(F(I:I)) SELECTCASE(J) CASE(65:90) G(N:N)=ACHAR(J+32) CASE(97:122) G(N:N)=ACHAR(J-32) CASE DEFAULT G(N:N)=F(I:I) ENDSELECT N=N+1 ENDIF ENDDO PRINT*,TRIM(G) END  Try it online! Since Fortran has not "advanced" tools for dealing with strings, I came up with this little monster. Indented and commented: CHARACTER(999)F,G !Define input and output strings (up to 999 characters) G=' ' !Fill output with spaces READ(*,'(A)')F !Take input N=1 !Represent the position to be written in output string M=1 !M=-1: Change case; M=1: Do not change case DO I=1,999 IF(F(I:I)=='a'.OR.F(I:I)=='A')THEN !If the character is A... M=-M !Ah-ha - you pressed cPS-LOCK! ELSEIF(M==1)THEN !Case the character is not A, and do not need to change case... G(N:N)=F(I:I) !...only copy the character N=N+1 ELSE !Otherwise... J=IACHAR(F(I:I)) !...get ascii of current character SELECTCASE(J) CASE(65:90) !If is upper case, G(N:N)=ACHAR(J+32) !now is lower case CASE(97:122) !If is lower case, G(N:N)=ACHAR(J-32) !now is upper case CASE DEFAULT !If do not belong to alphabet, G(N:N)=F(I:I) !simply copy the character ENDSELECT N=N+1 ENDIF ENDDO PRINT*,TRIM(G) !Trim out trailing spaces END !That's all folks!  # Stax, 12 bytes ìo'½║â↨╪U?5  Run and debug it online It splits on a regex, and then alternately toggles case. Here's the same program, unpacked, ungolfed, and commented. "a|A"|s split on regex /a|A/ rE reverse and explode array to stack W repeat forever... p print top of stack with no newline :~p print top of stack, case inverted, with no newline  Run this one • I somehow cannot relate your explanation to your code. – SK19 Mar 16 '18 at 21:59 • Try stepping through the commented one, and watching the internal state of the interpreter. Does that help? – recursive Mar 16 '18 at 22:29 • @SK19: Oh, I think I see the problem. I didn't mention that stax programs have two representations. Ascii and packed. There's a lossless conversion between the two. Ascii is easy to type, but wasteful for golf, since there are only 95 symbols. The golfed program is packed, so it looks different, but it's the same program. – recursive Mar 16 '18 at 23:45 ## Javascript (ES6), 80 79 bytes (Partly based off of this answer by Rick Hitchcock. Posting as a separate answer because I don't have sufficient reputation to comment.) (Saved 1 byte thanks to @l4m2's post here.) a=>a.replace(j=/a()|./gi,(c,o=c[to${j^c>{}?'Low':'Upp'}erCase]())=>(j^=!o,o))

• Welcome to PPCG! – Laikoni Mar 19 '18 at 9:16

# Dirty, 55 bytes

⇙U◌␛⮕⇨'aA'⇗⭱∈⊭⋱2wẂ[⭱y⋱1wx⮕⭧]
\   ␛◌Ẃ!w1/      \1wX/


Try it online!

Can probably be about a third shorter.
I'll write up an explanation and golf some more when I'm on a desktop.