Underscore functional javascript

Question

I'm trying to switch in general to functional programming and want to use underscore for JavaScript.

But I'm stuck at first base. I could not create an array at all and resorted to imperative language, and I can't seem to transform them properly either: n.length is correct, but n[0].length is undefined (see fiddle)

var a = new Array(5);
for (i = 0; i < a.length; i++) {
    a[i] = new Array(6);
}
var n = _.map(a, function (row, rowIdx) {
    _.map(row, function(col, colIdx) {
        return rowIdx * colIdx
    });
});

console.log(a.length)
console.log(n.length)
console.log(a[0].length);
console.log(n[0].length);

What exactly are you trying to do? Create a multiplication matrix? — rossipedia
– rossipedia, Commented Dec 17, 2013 at 17:33
No, I plan to put other things in the matrix but I just used this as a simple way to explain what I wanted to do — Simon H
– Simon H, Commented Dec 17, 2013 at 17:56
Someone added a comment that I needed to add return before the second _.map ut they deleted it afterwards. I want to record that wisdom here — Simon H
– Simon H, Commented Dec 17, 2013 at 17:56

rossipedia · Accepted Answer · 2013-12-17 17:41:24Z

3

To "functionally" create a 5 x 6 matrix using underscore, you could do something like this:

var matrix = _.range(5).map(function (i) {
    return _.range(6).map(function (j) {
        return j * i;
    });
});

The _.range function (documentation) is a shorthand for creating arrays that are filled with sequential numbers. For example, _.range(5) returns the array [0,1,2,3,4]. Ranges start at zero.

If you didn't want to use _.range, you could do this with raw JavaScript by using array literals:

var matrix = [0,1,2,3,4].map(function (i) {
    return [0,1,2,3,4,5].map(function (j) {
        return j * i;
    });
});

answered Dec 17, 2013 at 17:41

rossipedia

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Comments

Gruff Bunny · Accepted Answer · 2013-12-17 18:07:00Z

3

Another way to create a matrix would be to use _.times:

var matrix = _.times(6, function(x){
    return _.times(7, function(y){

        // do whatever needs to be done to calculate the value at x,y

        return x + ',' + y;
    });
});

edited Dec 17, 2013 at 18:07

answered Dec 17, 2013 at 18:01

Gruff Bunny

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Comments

cstuncsik · Accepted Answer · 2016-07-13 12:19:34Z

0

In ES6/2015

var range = num => Array.apply(null, Array(num)).map((i, n) => n);

var matrix2d = (a1, a2) => a1.map(i => a2.map(j => [i, j]));

console.log(matrix2d(range(5), range(6)));

edited Jul 13, 2016 at 12:19

answered Apr 14, 2016 at 7:51

cstuncsik

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