I double the source, you double the output!

Question

Your task, if you wish to accept it, is to write a program that outputs a positive integer (higher than 0). The tricky part is that if I duplicate your source code, the output must be double the original integer.

Rules

• You must build a full program. That is, your output has to be printed to STDOUT.

• The initial source must be at least 1 byte long.

• Both the integers must be in base 10 (outputting them in any other base or with scientific notation is forbidden).

• Your program must not take input (or have an unused, empty input).

• Outputting the integers with trailing / leading spaces is allowed.

• You may not assume a newline between copies of your source.

• This is code-golf, so the shortest (original) code in each language wins!

• Default Loopholes apply.

Example

Let's say your source code is ABC and its corresponding output is 4. If I write ABCABC instead and run it, the output must be 8.

Leaderboard

This uses uses @manatwork's layout.

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Answer 1

Batch, 13 bytes

@echo %~z0
:

Explanation: %~z0 expands to the length of the source file, so doubling the file simply doubles the length. The second line defines an empty label, which does nothing. When the file is doubled, it becomes a label named @echo %~z0 instead, while the third line is another empty label.

Answer 2

QBasic,  44  28 bytes

There is no newline at the end. Outputs 4 when single, 8 when doubled.

4
READ x,y
?x+y
END
DATA 4,0

Explanation

For the single version:

• 4 is a line number.
• READ x,y takes the first two values from the DATA statement and stores them in x and y. Thus, x gets 4 and y gets 0.
• ?x+y adds the two numbers and prints them.
• END exits the program.

In the doubled version, the DATA statement becomes DATA 4,04, which assigns 4 to both x and y, thus making x+y equal 8 instead.

Answer 3

Befunge-98 (PyFunge), 3 bytes

This answer draws heavy inspiration from this one, go vote them up too!

".@

Try it online! or Try it online doubled!

Explanation

When wrapping back around to the beginning of the source in quote mode, this Befunge-98 interpreter pushes at least 1 space (ASCII 32). If we print this value and exit, we get 32.

If we double the source code, we don't wrap back around, but rather end quote mode with the duplicated ". This means that the last character pushed is not a space, but a @, with (drumroll) ASCII 64! A complete coincidence that the character that ends the program in Befunge is twice that of a space.

Answer 4

Perl 5, 7 bytes

With -M5.10.0

say
+1#

Try it online!

Doubled:

say
+1#say
+1

-2 thanks to Ton Hospel

Answer 5

PHP, 30 bytes

Original

<?=strlen(file(__FILE__)[0]);#

Try it online!

PHP, 60 bytes

doubled

<?=strlen(file(__FILE__)[0]);#<?=strlen(file(__FILE__)[0]);#

Try it online!

Answer 6

Hexagony, 4 bytes

[@!)

Try it online! or Try it doubled online!

This solution was found using brute force by Martin and prints 002 regularly and 4 when doubled. This is the only 4 byte solution, but there are several hundred 5 byte solutions.

Expanded versions:

Regular:

 [ @
! ) .
 . .

Doubled:

  [ @ !
 ) [ @ !
) . . . .
 . . . .
  . . .

Hexagony has 6 instruction pointers, one starting at each corner of the hexagon and initially moving "clockwise." The [ instruction causes the interpreter to change IP one left. The first program leads us on a bit of a merry go around, executing: [![.!.)[...)!@ which results in the 002 output. The other program is a bit more tame, executing: [))[..)...[.......)!@ which results in the clean 4.

Answer 7

Python, 15 bytes

1or exit(2)
0/0

Outputs via exit code. Divides by zero and exits with code 1. Doubled:

1or exit(2)
0/01or exit(2)
0/0

Calls exit(2) and exits with code 2.

Python?, 9 bytes

';exit(2)

Throws a syntax error and exits with code 1. Doubled:

';exit(2)';exit(2)

Calls exit(2) and exits with code 2.

Answer 8

JavaScript (REPL), 2 bytes

So long as putting code in the console counts as a full program:

+1

Answer 9

J, 2 bytes

+1

+1 is 1, and +1+1 is 2.

Fun fact: since J's operator precedence is just right-to-left, this is interpreted as +(1+1)

Answer 10

Whispers v2, 23 bytes

+1
> 1
>> Output 3
>> 1

Try it online! or Try it online doubled

Outputs 1 normally and 2 when doubled.

How it works

Normal

Whispers only recognises lines that match one of a certain set of regexes. In the normal version, this reduces the program down to

> 1
>> Output 3

(i.e. the first and last lines don't match any regexes). The program then runs the last line, >> Output 3. This calls the 3rd line and outputs the result. However, as the reduced program only has 2 lines, this indexes modularly, returning the \$3 \mod 2\$th line (i.e. the first line), > 1. This is a basic nilad line which returns \$1\$, which is then outputted.

Doubled

The doubled program is

+1
> 1
>> Output 3
>> 1+1
> 1
>> Output 3
>> 1

Removing lines which don't match any regex, we get

> 1
>> Output 3
>> 1+1
> 1
>> Output 3

Again, the last line is run, telling the program to output the result of line 3. This time however, we do have a line 3 to output: >> 1+1. This does return \$2\$ but not because it evaluates \$1+1\$. Rather, it adds the value of line 1 to itself, which happens to be \$1\$.

Interestingly, this means that by changing > 1 to > n where \$n\$ is any number, the programs output \$n\$ and \$2n\$. For example

+1
> π
>> Output 3
>> 1

outputs \$\pi\$ normally or \$2\pi\$ when doubled

Answer 11

Google Sheets / Excel, 2 Bytes

The Trival Solution

+7

Or

+n

_{Such that n is an integer between 1 and 9, inclusive.}

Google Sheets and Excel both will take a leading + or - and evaluate them down to = or =-, respectively if they are the leading character of a cells formula text. As a result, -n, such that n is an integer between 1 and 9, inclusive, is a valid negative equivalent of the above

As a single formula this evaluates to a call stack that looks a little something like

+7
=7
7

However when this worksheet formula is doubled this call stack evaluates down to

+7+7
=7+7
=14

Answer 12

vJASS (Warcraft 3), 150 bytes

//! inject main
set bj_randDistCount=bj_randDistCount+1
call ClearTextMessages()
call BJDebugMsg(I2S(bj_randDistCount))
//! dovjassinit
//! endinject

prints 1

//! inject main
set bj_randDistCount=bj_randDistCount+1
call ClearTextMessages()
call BJDebugMsg(I2S(bj_randDistCount))
//! dovjassinit
//! endinject
//! inject main
set bj_randDistCount=bj_randDistCount+1
call ClearTextMessages()
call BJDebugMsg(I2S(bj_randDistCount))
//! dovjassinit
//! endinject

prints 2

Explanation:

• bj_randDistCount is a non-constant integer variable. Also, it has the shortest variable name you may find in blizzard.j.
• ClearTextMessages() clears all text messages in your in-game screen.
• I2S(int whichInt) -> string converts from integer to string
• BJDebugMsg(string str) prints text on your in-game screen.

Answer 13

Pyth, 3 bytes

s[1

sums a list

Try it!

Double-try it

Answer 14

Charcoal, 7 bytes

ＰＩＬ⊞Ｏυω

Try it online! Appends the empty string to the empty list and prints the length of the result (1) in decimal without moving the cursor. When doubled, the empty string gets appended twice so the length is now 2, which then overwrites the 1. Try it online! In verbose syntax, this is Multiprint(Cast(Length(PushOperator(u, w))));.

Answer 15

Self-modifying Brainfuck, 7 bytes

<-<201/

or doubled:

<-<201/<-<201/

Outputs 1 and 2 respectively.

The idea is simple: we use Self-modifying Brainfuck's self-modifying capability to make sure that the printing command only happens at the end of the full program.

Try it online!

Try it online, doubled!

Answer 16

Little Man Computer, 20 bytes (source)

LDA 1
ADD 5
OUT
HLT

Online Emulator (Flash)

Answer 17

bash, 16 bytes

a simple one:

wc -c<$0;exit 0;

First program: Try it online!

Second program: Try it online!

Answer 18

Python 2, 24 bytes

'+1#';exec'print 1''' ''

Abusing string syntax.

Answer 19

Powershell, 2 bytes

+1

It's a full Powershell program. The output has printed to STDOUT.

Answer 20

Brachylog, 8 bytes

w₆-₇ℕ~l"

Try it online! Try it online!Try it online!

Although Brachylog has implicit function output, full programs won't automatically print without being given arguments, making this challenge a bit more interesting.

w₆          Declaratively write a number
  -₇        which minus 7
    ℕ       is a whole number (so length won't throw an error)
     ~l     equal to the length of
       "    an (implicitly terminated) empty string.

Doubling the code turns the empty string into a string of length 7, changing the printed output from 7 to 14.

Answer 21

Python 2, 27 bytes

+7
try:_
except:_=7;print _

Try it online!

Try it doubled!

Answer 22

Vyxal, 1 byte

›

Try it Online!

Doubled:

››

Try it Online! Try it Online!

Increments the top of the stack. Since the stack is empty, 0 is incremented instead, outputting 1. When doubled, 0 is incremented twice, outputting 2.

Answer 23

K (ngn/k), 3 bytes

+/1

Try it online!

---

+/1 - sum 1. returns 1

+/1+/1 - rightmost +/1 evaluates to 1 (as above), giving +/1 1, i.e. sum the vector 1 1. returns 2

EDIT: above is incorrect (written when i had a bad understanding of how k is parsed). corrected explanation:

+/1+/1 is +/(1+/1), where the parenthesized expression is "sum 1 with a seed of 1", which evaluates to 2, which is passed to +/, returning 2

Answer 24

V, 4 bytes

øß.

Try it online!

Or Try it doubled!

Answer 25

Cubix, 6 bytes

@1u.2O

When run, this pushes 1, u-turns, Outputs as a number, and h@lts:

  @
1 u . 2
  O

When doubled, the cube is

    @ 1
    u .
2 O @ 1 u . 2 O
. . . . . . . .
    . .
    . .

which simply pushes 2, Outputs as a number, and h@lts.

Try it online!

Answer 26

R, 2 bytes

+1

prints 1 and when doubled

+1+1

prints 2

Answer 27

Fourier, 3 bytes

@^o

Try it Online!

Try it Doubled!

Explanation:

Undoubled:

@    - Clear screen
 ^   - Increment the accumulator (initialised to zero)
  o  - Output the value of the accumulator

Doubled:

@       - Clear screen
 ^      - Increment the accumulator (initialised to zero)
  o     - Output the value of the accumulator
   @    - Clear screen
    ^   - Increment the accumulator
     o  - Output the value of the accumulator

Answer 28

Ruby (REPL), 3 2 bytes

Must be run interactively for the implicit output:

+2

Outputs 2.

Doubled:

+2+2

Outputs 4.

_{2 and 4 are more interesting than 1 and 2}

Answer 29

R, 2 bytes

+T

Never expected to codegolf this much in R. Due to the +, R changes the T for a numeric, using 1 as default.

Answer 30

PHP, 26 bytes

<?=ob_clean()+ob_start()?>

Try it online!

Try it doubled!

Explanation

PHP can use an output buffer. This buffer has the facility to be cleaned without its contents ever being sent.

When the script is run just once, the output buffer has not initially been started and so cannot be cleaned, and ob_clean() returns false. The buffer is then successfully started and ob_start() returns true. false + true is really 0 + 1, and so the script echoes 1 into the buffer and ends with the buffer being outputted.

If the script is doubled then this first 1 is not output but is still in the buffer when the second half of the doubled code starts. The output buffer is now cleaned, the 1 is lost, and ob_clean() this time returns true. Another output buffer is then started (they are, in fact, stacked) and ob_start() again returns true, so this time we have true + true, which is 1 + 1, and so 2 is sent to the output buffer and then on to the actual output when the script ends.