OracleOfNJ OracleOfNJ - 1 month ago 5
Javascript Question

Javascript equivalent to Clojure's "reductions" or python's itertools.accumulate

Is there a JavaScript equivalent to Clojure's "reductions" function or Python's

itertools.accumulate
? In other words, given an array
[x_0, x_1, x_2 ... x_n-1]
and a function
f(prev, next)
, it would return an array of length
n
with values:

[x_0, f(x_0, x_1), f(f(x_0, x_1), x_2)... f(f(f(...)), x_n)]


I'm simulating the desired behavior below:

function accumsum(prev, next) {
last = prev[prev.length - 1] || 0;
prev.push(last + next);
return prev;
}

var x = [1, 1, 1, 1];
var y = x.reduce(accumsum, []);
var z = y.reduce(accumsum, []);

console.log(x);
console.log(y);
console.log(z);


which displays:

[ 1, 1, 1, 1 ]
[ 1, 2, 3, 4 ]
[ 1, 3, 6, 10 ]


But I'm wondering if there is a way to write something simpler like

[1, 1, 1, 1].reductions(function(prev, next) {return prev + next;});


If not, is there a more idiomatic way to do this in JavaScript than what I wrote?

Answer
var a = [1, 1, 1, 1];
var c = 0;
a.map(function(x) { return c += x; })
// => [1, 2, 3, 4]

a.reduce(function(c, a) {
  c.push(c[c.length - 1] + a);
  return c;
}, [0]).slice(1);
// => [1, 2, 3, 4]

I'd use the first one, personally.

EDIT:

Is there a way of doing your first suggestion that doesn't require me to have a random global variable (c in this case) floating around? If I forgot to re-initialize c back to 0, the second time I wrote a.map(...) it would give the wrong answer.

Sure - you can encapsulate it.

function cumulativeReduce(fn, start, array) {
  var c = start;
  return array.map(function(x) {
    return (c = fn(c, x));
  });
}
cumulativeReduce(function(c, a) { return c + a; }, 0, [1, 1, 1, 1]);
// => [1, 2, 3, 4]
c
// => ReferenceError - no dangling global variables