(Essentially the inverse of Find the needle in the haystack)

Given two tuples, (w, h) and (x, y), generate a haystack composed of a single random printable ASCII character of w width and h height with a needle made of a different random printable ASCII character at (x, y) when measured from the upper-left.

For example, when given (5,4) for the width and height, and (3,1) (zero-indexed) for the location of the needle, a possible haystack could be the following:


Another possibility could be


among hundreds of others.

Rules and Clarifications

  • Input and output can be given by any convenient method. This means you can take input as a list of list of integers, as a single string, as two integers via command-line and a tuple via function argument, etc.
  • You can print the result to STDOUT or return it as a function result.
  • Either a full program or a function are acceptable.
  • You can choose whether the (x, y) tuple is zero-indexed or one-indexed, but please specify in your solution which you're using.
  • You do not get to pick which characters to use. That's part of the challenge - randomly selecting the characters.
  • Every possible output for a given input must have a non-zero chance of appearing, but the randomness doesn't need to be uniform.
  • The haystack is guaranteed to be at least 2x2 in size, so it's unambiguous which is the needle and which is the hay.
  • There is only ever one needle, and it's only ever one character in size, and it's guaranteed to be within the boundaries of the haystack.
  • Standard loopholes are forbidden.
  • This is so all usual golfing rules apply, and the shortest code (in bytes) wins.
  • \$\begingroup\$ Can we return a flat list of characters? \$\endgroup\$ – TFeld Oct 16 at 15:02
  • 4
    \$\begingroup\$ Obligatory xkcd about the choice of the characters. Applies to a few early answers... :p \$\endgroup\$ – Arnauld Oct 16 at 15:34
  • 2
    \$\begingroup\$ @Arnauld I don't know what was the magic here, but I completely read the challenge as I can use any ASCII printable characters "that I like", instead of "random", and it is funny another person also did same exact thing! I didn't mean to implement the random in such a beautiful way :P \$\endgroup\$ – Night2 Oct 16 at 16:17
  • 1
    \$\begingroup\$ @UnrelatedString Yes and yes? Those are standard I/O rules. \$\endgroup\$ – AdmBorkBork Oct 17 at 12:23
  • 1
    \$\begingroup\$ @TaylorScott Yes, that's covered in the opening paragraph. The two characters need to be different. \$\endgroup\$ – AdmBorkBork Nov 18 at 13:34

33 Answers 33


Japt -R,34 30 29 24 bytes

;E=öx)ÎpU*V hW*U+XEg1)òU

Try it

5 Bytes saved thanks to @Shaggy.

Inputs are U,V => size , W,X => needle coords 0 indexed

 ;E=   // set of printable characters 
     öx) // random permutation 
 Î // => g0  pU*V    repeats U*V times first element of set
           hW*U+XEg1) // overwrite at W,X with 2nd element 
                      òU // split
  • \$\begingroup\$ @Shaggy my first try, the best I can do, any suggestions are implicitly welcome! \$\endgroup\$ – AZTECCO Oct 20 at 20:49
  • 1
    \$\begingroup\$ A few quick savings for you here. \$\endgroup\$ – Shaggy Oct 22 at 9:11
  • \$\begingroup\$ Thanks @Shaggy! The good thing is that I 've understood your improvements, though I'm still far from a good usage of this language, maybe due to my poor experience in JS \$\endgroup\$ – AZTECCO Oct 22 at 13:34
  • \$\begingroup\$ you don't really need a JS background to learn Japt. \$\endgroup\$ – Shaggy Oct 22 at 17:56

Zsh, 108 bytes

Needle location is 1-indexed

repeat $2 : $[h=r++?h:RANDOM%95]&&(repeat $1 s+=${(#)$((n=r-$4||++c-$3?h:(n=RANDOM%94)+(n>=h),32+n))};<<<$s)

Try it online!

repeat $2
  # set hay when row is 0 (shorter than setting it before due to the cost of "(( ))")
  # ": $[  ]" always exits true. "((  ))" would exit false if hay == 0
  : $[hay = row++ ? hay : RANDOM % 95] && (
    # inner loop in (subshell), $str and $col will be empty every time
    repeat $1
      # ${(#)$((x))} expands to the Unicode character at codepoint $x
      str+=${(#)$((offset =
        row - $4 || ++col - $3
        ? hay
        : (offset = RANDOM % 94) + (offset >= hay),
        32 + offset
      <<< $str

Proof of correctness by exhaustion, for all combinations of RANDOM%95 and RANDOM%94.


C (clang), 130 117 116 109 106 bytes


Try it online!

Saved 13 thanks to @ceilingcat Didn't thought using just only time() was enough And also didn't thought printf accepted a int pointer as a format string!

Needle is 0 indexed

c[2]; // needle and hay stored in array 
 x+=y*w; // sets counter for needle (x no more needed)
     h*=++w; // while incrementing w for newline sets counter for haystack 
     y++<h  // more than 2 iterations to fill array
     |*c==c[1]; // until they're different 
     c[y%2]=time(0)%94+33; // assign value to array (I've excluded spaces )
 for( ;h--; ) printf(h%w?c+!x--:"\n")
 // output: on h%w =0 => newline 
 //              if  x != 0 => hay
 //              when x=0 => needle 
 //              and continue to finish haystack 

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