We had a question once which only failed on Sundays

Question

Inspired by We had a unit test once which only failed on Sundays, write a program or function that does nothing but throw an error when it is Sunday, and exit gracefully on any other day.

Rules:

• No using input or showing output through the usual IO methods, except to print to STDERR or your language's equivalent. You are allowed to print to STDOUT if it's a by-product of your error.
• A function may return a value on non-Sundays as long as it doesn't print anything
• Your program may use a Sunday from any timezone, or the local timezone, as long as it is consistent.
• An error is a something that makes the program terminate abnormally, such as a divide by zero error or using an uninitialised variable. This means that if any code were to be added after the part that errors, it would not be executed on Sunday.
• You can also use statements that manually create an error, equivalent to Python’s raise.
• This includes runtime errors, syntax errors and errors while compiling (good luck with that!)
• On an error there must be some sign that distinguishes it from having no error
• This is code-golf, so the shortest bytecount in each language wins!

I'll have to wait til Sunday to check the answers ;)

Answer 1

Bash + coreutils, 15 14 bytes

`date|grep Su`

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Answer 2

PHP 7, 12 bytes

1%date("w");

On PHP 7 it throws an exception of type DivisionByZero on Sundays. The same happens if it is interpreted using HHVM.

On PHP 5 it displays a warning (on stderr) on Sundays:

PHP Warning:  Division by zero in Command line code on line 1

On any PHP version, it doesn't display anything on the other days of the week.

Run using the CLI:

php -r '1%date("w");'

or try it online!

Two more bytes can be squeezed by stripping the quotes (1%date(w);) but this triggers a notice (that can be suppressed by properly set error_reporting = E_ALL & ~E_NOTICE in php.ini).

Answer 3

Java 8, 69 43 34 bytes

v->1/new java.util.Date().getDay()

-26 bytes thanks to @OlivierGrégoire.
-9 bytes thanks to @Neil.

Explanation:

Try it here.

• v->{...} (unused Void null parameter) is one byte shorter than ()->{...} (no parameter).
• new java.util.Date().getDay() will return 0-6 for Sunday-Saturday, so 1/... will give an java.lang.ArithmeticException: / by zero error if the value is 0, which only happens on Sundays.

Answer 4

PHP, 15 bytes

<?@date(w)?:\n;

Assumes default settings.

Output on Sundays

Fatal error: Undefined constant 'n' on line 1

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Answer 5

Python 3, 33 bytes

import time
"Su"in time.ctime()>q

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Python 3, 50 bytes

from datetime import*
datetime.now().weekday()>5>q

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Saved ~3 bytes thanks to Rod.

Answer 6

Pyth, 8 7 bytes

 l-6.d9

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Explanation

    .d9 # Get the current day of week (0 = Monday, 6 = Sunday)
  -6    # Subtract 6 from the day
 l      # Try to calculate the log base 2 of the result of the previous operation raising a "ValueError: math domain error" on sundays
        # there is an extra space at the start, to supress the output on the other days

Answer 7

05AB1E, 45 44 bytes

As 05AB1E doesn't have a built in for getting the day of the week, I've used Zeller's Rule to calculate it.

Prints a newline to stderr in case of a Sunday (observable in the debug view on TIO)

žežf11+14%Ì13*5÷žgžf3‹-т%D4÷žgт÷©4÷®·(O7%i.ǝ

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Explanation

The general formula used is
DoW = d + [(13*(m+1))/5] + y + [y/4] + [c/4] - 2*c
Where DoW=day of week, d=day, m=month, y=last 2 digits of year, c=century and and expression in brackets ([]) is rounded down.

Each month used in the formula correspond to a number, where Jan=13,Feb=14,Mar=3,...,Dec=12
As we have the current month in the more common format Jan=1,...,Dec=12 we convert the month using the formula
m = (m0 + 11) % 14 + 1

As a biproduct of March being the first month, January and February belong to the previous year, so the calculation for determining y becomes
y = (year - (m0 < 3)) % 100

The final value for DoW we get is an int where 0=Sat,1=Sun,...,6=Fri.
Now we can explicitly throw an error if the result is true.

Answer 8

JavaScript, 23 Bytes

Date().slice(1)>'um'&&k

Full program.

The variable k must not be defined.

JavaScript, 20 bytes by Rick Hitchcock

/Su/.test(Date())&&k

JavaScript, 19 bytes by apsillers

Date().match`Su`&&k

Answer 9

Ruby, 15 bytes

1/Time.now.wday

wday will return 0 on Sunday causing a ZeroDivisionError: divided by 0 error. For example: 1/Time.new(2018,1,7).wday.

Answer 10

Haskell + Data.Dates, 55 bytes

import Data.Dates
succ.dateWeekDay<$>getCurrentDateTime

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This uses the fact that Sunday is the last day of the week. dateWeekDay returns the day of the week as a WeekDay type, which is simply defined as

data WeekDay = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday

WeekDay is an instance of Enum, thus we can use succ and pred to get the successor or predecessor of a weekday, e.g. succ Monday yields Tuesday.

However, Sunday is the last enum entry, so calling succ Sunday results in the following error:

fail_on_sunday.hs: succ{WeekDay}: tried to take `succ' of last tag in enumeration
CallStack (from HasCallStack):
  error, called at .\Data\Dates.hs:56:34 in dates-0.2.2.1-6YwCvjmBci55IfacFLnAPe:Data.Dates

Edit 1: Thanks to nimi for -3 bytes!
Edit 2: -11 bytes now that functions are allowed.

---

Full program: 88 81 74 69 66 bytes

import Data.Dates
main=pure$!succ.dateWeekDay<$>getCurrentDateTime

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pure is needed to lift the resulting WeekDay back into the IO Monad. However, Haskell sees that the value is not output in any way by the program, so lazy as it is, the expression is not evaluated, so even on Sundays the program would not fail. This is why $! is needed, which forces the evaluation even if Haskell would normally not evaluate the expression.

---

Previous approach with Data.Time: 127 124 bytes

import Data.Time.Clock
import Data.Time.Calendar.WeekDate
c(_,_,d)|d<7=d
main=getCurrentTime>>=(pure$!).c.toWeekDate.utctDay

Try it online! These are some impressive imports. Change d<7 to e.g. d/=5 to test failure on a Friday. Fails with the following exception: Non-exhaustive patterns in function c.

Answer 11

Perl 5, 13 bytes

1/(gmtime)[6]

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Ported @biketire's answerj

removed 3 bytes with @mik's reminder

Answer 12

VBA / VBScript, 22 20 bytes

^{_{Saved 2 bytes thanks to Taylor Scott.}}

a=1/(Weekday(Now)-1)

This should be run in the Immediate Window. Weekday() returns 1 (Sunday) through 7 (Saturday) so this creates a divide by zero error on Sunday. Otherwise, no output.

Answer 13

TI-Basic 84+, 23 bytes

getDate
0/(1-dayOfWk(Ans(1),Ans(2),Ans(3

Needs date & time commands, which are 84+ and higher only.

Answer 14

C, 35, 34 27 bytes

f(n){n/=time(0)/86400%7^3;}

-7 bytes with thanks to @MartinEnder and @Dennis

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Answer 15

Vyxal O, 25 bytes

kðtD4/‟₀/N‟:400/kτṠ7%2<[←

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Vyxal doesn't have a weekday function, so I made one myself.

Answer 16

R, 31 bytes 30 bytes

if(format(Sys.Date(),'%u')>6)a

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No output on non-Sundays, Error: object 'a' not found on Sundays.

format(Sys.Date(),'%u') was the shortest way I could find to get weekday, it outputs a character-class number for day of week, with 7 for Sundays. We can compare to a numeric 7, and if true attempt to use an undefined object.

Saved a byte thanks to Giuseppe!

Answer 17

Zsh -G, 14 bytes

>`date`
dd Su*

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• date: get the date in the format Sun 28 Mar 07:55:54 BST 2021
• >: create a file named according to each of the words in the output
• dd: "copy and convert" - here it basically does nothing
  • this command expects no arguments
• Su*: search for a file starting with Su
  • if a file matching that exists (which happens when it is Sunday), then it is passed to dd
    • since dd expects no arguments, this produces an error
  • with the -G option: if no file matches (i.e. it is not Sunday), don't error as usual

This produces a bit of extra junk output to STDERR (not an error though) which may or may not be allowed; the question isn't really clear. Here's an alternative answer which doesn't:

Zsh -G, 18 bytes

>`date`
mv <-> Su*

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<-> matches any file whose name is a number, of which there are two: the day of the month, and the year.

When there is no Su* file, this is renames the day of the month to the year (overwriting the year), which works fine.

When Sun exists, it tries to move the day of the month and the year into the directory Sun, which fails because Sun is a file, not a directory.

Answer 18

jq, 42 characters

(39 characters code + 3 characters command line option)

now|strftime("%w")|strptime("%d")|empty

Just trying a different approach here: parse week day number (0..6) as month day number (1..31).

Sample run:

bash-4.4$ TZ=UTC faketime 2018-01-06 jq -n 'now|strftime("%w")|strptime("%d")|empty'

bash-4.4$ TZ=UTC faketime 2018-01-07 jq -n 'now|strftime("%w")|strptime("%d")|empty'
jq: error (at <unknown>): date "0" does not match format "%d"

Note that jq only handles UTC dates.

Try it online!

Answer 19

Q, 20 Bytes

if[1=.z.d mod 7;'e]

.z.d returns the current date. mod does the modulo of the current date, which returns an int. If the date is a sunday, .z.d mod 7 returns 1. If 1=1, (on sunday), and error is raised using the ' operator For brevity the error is just the e character.

Answer 20

Julia 0.6, 32 bytes

Thanks to @Dennis for pointing out < saves a byte over !=.

@assert Dates.dayofweek(now())<7

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Answer 21

VBA 18 bytes

This relies on the inbuilt function date() returning a day number that remainders 1 if divided by 7, so may be OS and/or CPU specific.

a=1/(date mod 7-1)

It runs in the VBA project Immediate window.

Answer 22

C,  68  55 bytes

Thanks to @Ken Y-N for saving 13 bytes!

#import<time.h>
f(n){time(&n);n/=gmtime(&n)->tm_wday;;}

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Answer 23

MATL, 12 bytes

vZ'8XOs309>)

The error produced on Sundays is:

• Interpreter running on Octave:

  MATL run-time error: The following Octave error refers to statement number 9:  )
  ---
  array(1): out of bound 0
  

• Interpreter running on Matlab:

  MATL run-time error: The following MATLAB error refers to statement number 9:  )
  ---
  Index exceeds matrix dimensions
  

To invert behaviour (error on any day except on Sundays), add ~ after >.

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Explanation

This exploits the fact that

• indexing into an empty array with the logical index false is valid (and the result is an empty array, which produces no output); whereas

• indexing with true causes an error because the array lacks a first entry.

Commented code:

v       % Concatenate stack. Gives empty array
Z'      % Push current date and time as a number
8XO     % Convert to date string with format 8: gives 'Mon', 'Tue' etc
s       % Sum of ASCII codes. Gives 310 for 'Sun', and less for others
309>    % Greater than 309? Gives true for 'Sun', false for others
)       % Index into the empty array
        % Implicit display. Empty arrays are not displayed (not even newline)

Answer 24

Excel, 24

Closing quote and paren not counted toward final score, as Excel will autocorrect both of those. Tested in Excel 2016.

=IF(MOD(TODAY()-1,7)^0,"")

I think this is different from the other submission enough to warrant another answer.

How it works:

• I've rolled the error into the conditional check. As it turns out, TODAY() mod 7 also gives the weekday. If it's a Sunday, this means that MOD(TODAY()-1,7) is 0, and 0^0 is an error in Excel.
  • Of course, I could also have divided 1 by the MOD() value or used +6 instead of -1.
• If it's not Sunday, the MOD(...,7) will be non-zero, which when raised to 0, returns 1, a truthy value. This makes the IF return an empty string (our "nothing").
• The if statement, therefore, cannot evaluate to FALSE, because it errors or returns nothing.

Alternative

Here's one that works just as well, but uses a Name error instead:

=IF(MOD(TODAY(),7)=1,A,"")

Answer 25

TI-Basic, 22 21 bytes

getDate
log(log(dayOfWk(Ans(1),Ans(2),Ans(3

-1 byte by using MarcMush's method but replacing ln( with log(.

Only works on TI-84+/SE. Assumes that the date is set correctly before the program is run. There is a newline at the end.

Answer 26

APL (Dyalog), 23 bytes

⎕CY'dfns'
o←÷7|⌊days⎕TS

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Answer 27

Ocaml, 46 bytes

open Unix
let()=1/(gmtime(time())).tm_wday;()

and in the ocaml REPL, we can achieve better by removing the let and the final :():

$ open Unix;;1/(gmtime(time())).tm_wday;;<CR>

which is 41 bytes (incuding 1 byte for the carriage return).

Answer 28

SAS, 36 bytes

%put %eval(1/(1-%index(&sysday,Su)))

Answer 29

Julia 0.5, 28 bytes

Dates.dayofweek(now())<7||~-

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This does not work with 0.6, but it does with 0.4.

Answer 30

C# (.NET Core), 55 54 48 bytes

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Saved 1 byte thanks to Shaggy

Saved 5 byte thanks to Emigna

Saved 1 byte thanks to Kevin Cruijssen

_=>{var k=1/(int)System.DateTime.Now.DayOfWeek;}

Lucky that Sunday is indexed 0 in enum or else it would've needed to be (System.DayOfWeek)7