Returning an Array in JavaScript

Question

I was inspired by this question to write a recursive function in JavaScript to add elements of an array and return an array accordingly.

in Pseudocode

arr = [1,2,3,4,5,6]
func (arr,2) = > [1+3+5, 2+4+6]
                 [9,12]
func (arr,3) = > [1+4,2+5,3+6]
                 [5,7,9]

So I wrote a fiddle, here.

var input = [1,2,3,4,5,6,7,8,9];

function tupleAdd(tuple,i,p,t,output){
    if(typeof t == "undefined")
        t=0;
    if(typeof p == "undefined")
        p=0;
    if(typeof output == "undefined")
        output = [];
    if(typeof output[t] =="undefined")
        output[t]=0;

    output[t]+=i[p];
    p++;
    t++;
    t>=tuple?t=0:null;
    if(p<i.length)
        tupleAdd(tuple,i,p,t,output);
    else{
        console.log(output);
        return(output);
    }
}

x = tupleAdd(3,input);
console.log(x);

My function works, as the first console.log() shows the appropriate values. What weirds me out is that when I return output (with or without parens), and try and log it again, I get undefined.

I find this deeply unsettling. Can anyone shed some light on why the return value is different than the console.log value?

What happens if you add return before the recursive call to tupleAdd()? — BenM
– BenM, Commented Nov 14, 2013 at 22:42

epascarello · Accepted Answer · 2013-11-14 22:44:16Z

3

Your recursion is broken. You are getting undefined since you do not have a return on the function call inside.

tupleAdd(tuple,i,p,t,output);

needs to be

return tupleAdd(tuple,i,p,t,output);

answered Nov 14, 2013 at 22:44

epascarello

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1 Comment

Jason Nichols Over a year ago

Thank you! Upvoted you and Ben. I just accepted his because he was first to comment. I didn't have a better criteria, since both answers were equally helpful and came in at about the same time.

BenM · Accepted Answer · 2013-11-14 22:44:20Z

You need to return the value of the recursive call:

var input = [1,2,3,4,5,6,7,8,9];

function tupleAdd(tuple,i,p,t,output){
    if(typeof t == "undefined")
        t=0;
    if(typeof p == "undefined")
        p=0;
    if(typeof output == "undefined")
        output = [];
    if(typeof output[t] =="undefined")
        output[t]=0;

    output[t]+=i[p];
    p++;
    t++;
    t>=tuple?t=0:null;
    if(p<i.length)
        return tupleAdd(tuple,i,p,t,output);
    else{
        console.log(output);
        return(output);
    }
}

x = tupleAdd(3,input);
console.log(x);

Igor Benikov · Accepted Answer · 2013-11-15 07:04:10Z

-2

function doTuple(arr, tuple){

    if(!arr.length){
        return tuple;
    }

    var el = arr.splice(0,1);

    el % 2 ? tuple.evens += +el : tuple.ods += +el;

    return doTuple(arr, tuple);
}

console.log(doTuple([1,2,3,4,5], {evens:0,ods:0}));

edited Nov 15, 2013 at 7:04

answered Nov 14, 2013 at 22:54

Igor Benikov

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