Array comprehension in JavaScript? [duplicate]

Question

I was wondering what the neatest way would be to convert (from Python) a list comprehension into Javascript. Is there anything which will make this readable and not a mess?

    non_zero_in_square = [ grid[row][col]
                           for row in range(start_row, start_row+3)
                           for col in range(start_col, start_col+3)
                           if grid[row][col] is not 0
                         ]

This is quite a good example of a list comprehension, as it has multiple fors and and an if.

I should add that the range bit is covered here (I can't live without range).

As a JavaScript developer who wouldn't know Python if it came up for a chat at the bus stop, that's some funky ass syntax you Pythonistas have got there. — Mitya
– Mitya, Commented Jul 13, 2012 at 23:03
Notice that JavaScript 1.7 has Array Comprehension, unfortunately are implemented only in Firefox at the moment. I hope we will get soon in other browsers as well (see harmony — ZER0
– ZER0, Commented Jul 13, 2012 at 23:35

Pointy · Accepted Answer · 2012-07-13 23:17:42Z

3

Well it would be somewhat messy to do this with the .map() method, because the outer calls really need to return arrays. Thus you're probably best with the pedestrian:

var nonZero = [];
for (var row = startRow; row < startRow + 3; ++row)
  for (var col = startCol; col < startCol + 3; ++col)
    if (grid[row][col] !== 0) nonZero.push(grid[row][col];

edited Jul 13, 2012 at 23:17

answered Jul 13, 2012 at 23:10

Pointy

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2 Comments

elclanrs Over a year ago

Right. Plus I would enclose it in a self-executing function assigned to the variable to protect nonZero

Pointy Over a year ago

@elclanrs yes I agree; this assumes some sort of scope akin to the OP, but if it were some sort of tool then nonZero should be a local variable.

dm03514 · Accepted Answer · 2012-07-13 23:09:33Z

2

Coffee script support list comprehension syntax and is probably the neatest as it follows syntax exactly. Unfortunately it is an intermediary and would be compiled to multi line javascript

http://coffeescript.org/#loops

They show you how it coverts to vanilla javascript.

answered Jul 13, 2012 at 23:09

dm03514

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Comments

lucabelluccini · Accepted Answer · 2013-11-22 16:34:11Z

0

Mozilla JS Documentation, ES 1.7 supports them natively.

Example:

var numbers = [1, 2, 3, 4];
var doubled = [i * 2 for (i of numbers)];

answered Nov 22, 2013 at 16:34

lucabelluccini

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1 Comment

Michael Scott Asato Cuthbert Over a year ago

You won't find any support today in popular browsers except Firefox: kangax.github.io/compat-table/es6 .

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