javascript recursive parameter behavior

Question

I want to list all the possible parameter combinations. I wrote a recursive algorithm that should solve the problem. The issue is that the variables I use as the recurive parameters does not keep their old values before the recursive calls.

The problem

Let's say I have 2 parameters with the following possible values:

P1 = A or B P2 = C or D or E

I want to find the possible combinations:

AC AD AE BC BD BE

The recursive "solution"

var params = []; var param1 = ['A', 'B', 'C']; var param2 = ['E', 'F']; var param3 = ['G', 'H', 'I']; params.push(param1); params.push(param2); params.push(param3); function r(ps, s) { console.log(ps.length + " " + s.length); if (ps.length == 0) { console.log(s); return; } var p = ps[0]; for (idx in p) { s.push(p[idx]); console.log("Before call " + s); r(ps.slice(1), s); } } var sol = []; r(params, sol);

JS Fiddle

The runtime issue

When running the program, the s variable keeps growing. The first 3 iterations gives:

Before call A Before call A, E Before call A, E, G

After the first call to return, I expect the s variable to contain AE because it was its value before the call.

Note: what you want is the Cartesian Product of your values. I have a question about this with excellent JavaScript answers here: stackoverflow.com/questions/9422386/… See also the links in the opening sentence for other non-lazy JavaScript versions. — Phrogz
– Phrogz, Commented Dec 14, 2014 at 16:09

cyon · Accepted Answer · 2014-12-14 19:51:40Z

To fix your algorithm you need to pop the element from the array when you return from the recursive call:

for (idx in p) { s.push(p[idx]); r(ps.slice(1), s); s.pop(); // remove the element once we are done with the recursion }

Note however, that console.log(s) will show the reference to the object which means that in the console you will see the s empty because the algorithm will complete quickly and after it's done s becomes empty because of the popping. To force console.log to print the contents and not the reference you can use console.log(s.toString()):

if (ps.length == 0) { console.log(s.toString()); return; }

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