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Over is a higher-order function in multiple languages such as APL (). It takes 2 functions and 2 values as arguments, applies the first function to both values, then applies the second to their result. For example, using to represent Over:

1 ²⍥+ 2

We would first calculate ² of each argument: 1² = 1 and 2² = 4. We then apply + to these, yielding 5.

You are to take as input:

  • A black box function, \$f\$, which takes an integer as input and returns an integer
  • A black box function, \$g\$, which takes 2 integers as input and returns a single integer
  • 2 integers, \$a\$ and \$b\$.

You should then return the result of \$g(f(a), f(b))\$.

If you have a builtin specifically for this (e.g. APL's , Husk's ¤ etc.), consider including a non-builtin answer as well. It might even get you an upvote :)

You may input and output in the most convenient format for your language, and in any convenient method, including taking \$a\$ and \$b\$ as a pair/list/tuple [a, b]

For the sake of simplicity, you can assume that the black-box function will always input and output integers within your language's integer domain, and that \$a\$, \$b\$ and the output will be with your language's integer domain.

This is , so the shortest code in bytes wins

Test cases

f
g
a, b -> out

f(x) = x²
g(x,y) = x - y
-2, 2 -> 0

f(x) = φ(x)     (Euler totient function)
g(x,y) = 2x + y
5, 9 -> 14

f(x) = x³-x²-x-1
g(x,y) = y⁴-x³-y²-x
-1, -1 -> 22

f(x) = x
g(x,y) = x / y   (Integer division)
-25, 5 -> -5
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  • 1
    \$\begingroup\$ Can we take a & b as an array [a,b]? \$\endgroup\$
    – Shaggy
    Apr 20, 2021 at 7:12
  • \$\begingroup\$ @Shaggy Yes you can \$\endgroup\$ Apr 20, 2021 at 11:31

45 Answers 45

1
2
2
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Racket, 30 bytes

(λ(f g x)(apply g (map f x)))

Try it online! Takes a and b as a list i.e. '(a b).

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Java (JDK), 30 bytes

(f,g,x,y)->g.a(f.a(x),f.a(y));

Try it online!

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1
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Python 2, 37 bytes

lambda f,g,a,b:reduce(g,map(f,[a,b]))

Try it online!

Some approach different than dingledooper

Python 2, 27 bytes

lambda f,g,a,b:g(f(a),f(b))

Try it online!

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1
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Zsh -P, 16 bytes

eval g '`f '$@\`

Try it online!

Expects the functions to be predefined as f and g.

The -P option makes the $@ array expand to wrap the word around every element, e.g. x$@y with A B makes xAy xBy instead of xA By. This allows it to generalise implicitly for more than 2 inputs.


For 1 byte less, you can have an uninteresting answer that only works with 2 inputs:

Zsh, 15 bytes

g `f $1` `f $2`

Try it online!

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Stacked, 11 bytes

[@:g"!...g]

Try it online!

Simple anonymous function. Takes input on stack as (a b) f g.

Works by storing g as a temporary function variable. Then, applies the " transformation on function f, which makes it apply on each element its called on. ! calls this function f" on the tuple (a b). Then, we simply put these two elements on the stack and call g.

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CJam, 4 bytes

q~%*

Inputs are: function g; array of numbers; function f. Functions are defined as code blocks.

Try it online! Or verify all test cases: 1, 2, 3, 4.

Explanation

q   e# Read all input as an unevaluated string
~   e# Evaluate. Pushes a code block, an array, and a code block to the stack
%   e# Map (second function over the array)
*   e# Reduce (the array using the first function)
    e# Print (implicitly)
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Raku, 17 bytes

{&^g(|@^x».&^f)}

Try it online!

&^f and &^g are the function arguments to the over function, and @^x is the list of arguments to f. @^x».&^f maps the arguments to f over f, and | flattens that list into the arguments to g.

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Lua, 41 bytes

function k(f,g,x,y)return g(f(x),f(y))end

Try it online!

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1
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MMIX, 32 bytes (8 instrs)

Takes arguments f,g,x,y.

(jxd)

00000000: fe040004 c1070200 bf060000 c1080300  “¥¡¥Ḋ¬£¡Ḃ©¡¡Ḋ®¤¡
00000010: bf070000 bf050100 f6040004 f8060000  Ḃ¬¡¡Ḃ¦¢¡ẇ¥¡¥ẏ©¡¡

Disassembly:

on  GET    $4,rJ
    SET    $7,$2
    PUSHGO $6,$0    // $6 = f(x)
    SET    $8,$3
    PUSHGO $7,$0    // $7 = f(y)
    PUSHGO $5,$1    // $5 = g($6,$7)
    PUT    rJ,$4
    POP    6,0      // return $5,f,g,x,y,$4
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1
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Clojure, 17 16 bytes

#(%(%2%3)(%2%4))

Try it online!

Takes inputs in the order of \$g, f, a, b\$.

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PPL v1.0.11, 31 bytes

fnv(f,g,x,y){
returng(f(x,y))
}

This must be run on PPL version 1.0.11 (or later) because the ability to pass functions as parameters was only added in v1.0.11. Fairly simple, if you can get past the unreadability of the "mashing tokens together".

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Python, 27 bytes

lambda f,g,a,b:g(f(a),f(b))
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1
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Knight, 22 bytes

O E++P" "+=f+P" "+P+fP

Try it online!

Since knight doesn't have the concept of functions, this program instead takes in the "functions" as strings from standard in and outputs to standard out. Specifically, in f, g, x and y on separate lines in input, and outputs g(f(x),f(y)). It does this by combining the inputs into a single string, then evaluating (adding appropriate white space).

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  • \$\begingroup\$ It does have a concept of functions. BLOCK (B). I guess it doesn't work though, because they don't take arguments. \$\endgroup\$
    – Steffan
    Aug 10 at 21:14
0
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dc, 16 bytes

4Rdsfxsrlfxlr3Rx

Try it online!

Takes input as the four preceding items on the stack, and pushes the result onto the stack.

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0
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Fig, \$2\log_{256}(96)\approx\$ 1.646 bytes

RM

See the README to see how to run this

The perfect job for a functional language. Here is an example program with \$a = 1\$, \$b = 2\$, \$f(x) = -x\$, \$g(x, y) = x - y\$, producing the correct result of 1:

RMw1 2'Nx'-

Explanation:

RM # Takes input as [a, b], f, g
 M # Map the input with the first function
R  # Reduce the result by the second function
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