Equivalent for Python's list comprehension

Issue

I am playing with Go but I am having a very hard time doing things that are very simple in other languages.

I’d like to reproduce a Python comprehension:

array = [a for a in anotherArray  if (some condition)]

What is an elegant way to do it in Go? I’d really like to simplify my code, especially when using a function on array. For example:

min = min(abs(a[i], b[j]) for i in range(n)
                          for j in range(i, n))

Solution

Interestingly enough, Rob Pike just proposed (18 hours ago) the library filter which does a bit what you want:

See for instance Choose()

// Choose takes a slice of type []T and a function of type func(T) bool. (If
// the input conditions are not satisfied, Choose panics.) It returns a newly
// allocated slice containing only those elements of the input slice that
// satisfy the function.

Tested here:

func TestChoose(t *testing.T) {
    a := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
    expect := []int{2, 4, 6, 8}
    result := Choose(a, isEven)

As twotwotwo points out in the comments, the GoDoc for this library states:

Package filter contains utility functions for filtering slices through the distributed application of a filter function.

The package is an experiment to see how easy it is to write such things in Go. It is easy, but for loops are just as easy and more efficient.

You should not use this package.

This caveat is reflected in the document "Summary of Go Generics Discussions", section "Functional Code":

These are the usual higher-order functions such as map, reduce (fold), filter, zip etc.

Cases:
typesafe data transformations: map, fold, zip

Pros for using generics:
A concise way to express data transformations.

Cons for using generics:
The fastest solution needs to take into account when and in which order to apply those transformations, and how much data is generated at each step.
It is harder to read for beginners.

Alternative solutions:

use for loops and usual language constructs.


Update Q1 2022: with the first integration of generics in Go (see for instance "Tutorial: Getting started with generics"), you now have a possible mapreduce implementation with generics in Go.
See tip.playground, and the github.com/kevwan/mapreduce/v2 project.

Answered By – VonC

Answer Checked By – Mary Flores (GoLangFix Volunteer)

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