JavaScript fundamentals confusion / convert Python

Question

What I'm trying to do is take my Python code and turn it into JavaScript, yet I cannot figure out why it runs differently in JavaScript. What am I missing?

The code is intended to take an array of values with 1's and 0's and return an array summing the 1 values as such.

[1,1,0,1] => [2,1]

[0,0,1,1,1] => [3]

[0,0,0] => []

[1,1,0,0] => [2]

Python code that works (unless I'm horribly mistaken)

def encode(arr):
    arr2=[]
    num = 0
    for i in arr:
        if i==1:
            num=num+1
        elif i==0 and num ==0:
            pass
        else:
            arr2.append(num)
            num=0
    if num>0:
        arr2.append(num)
    return arr2

JavaScript that doesn't work

function  encode(arr) {
    var arr2=[];
    var num = 0;
    for (i in arr){
        if (i==1) {
            num++;
        } else if (i == 0 && num == 0) {
            // pass 
        } else {
            arr2.push(num);
            num=0;
        }
    }

    if (num>0) {
        arr2.push(num)   
    }

    return arr2;
}

Not much, I don't know much JavaScript. Hence why I'm trying to do this project in JavaScript. — Sam Dotson, Jan 05 '18 at 18:01
Again, what doesn't work? If it's giving the wrong output, you need to say what you expect, and what it's giving. Being vague isn't helpful. — Carcigenicate, Jan 05 '18 at 18:02
Wrong **how**? Show some input and output, and explain how it's different from what you expect. — Daniel Roseman, Jan 05 '18 at 18:02
The whole `elif i==0 and num ==0:` part isn't even necessary unless I'm missing something. — Carcigenicate, Jan 05 '18 at 18:02
I would tend to agree with you, yet I haven't found a way to lose it. Without that if the first elements are 0, it will append 0 to the array. — Sam Dotson, Jan 05 '18 at 18:05
The JavaScript when given `encode([0,0,0,1])` returns '[1,0]`, it should return [1]. — Sam Dotson, Jan 05 '18 at 18:08
@SamDotson Change the `else` to `else if !(i == 0 && num == 0)`, then get rid of the condition I mentioned. — Carcigenicate, Jan 05 '18 at 18:09

le_m · Answer 1 · 2018-01-05T18:15:29.397

1

Instead of

for (i in arr) { ... }

you need to write

for (let i of arr) { ... }

This is because for ... in iterates the enumerable properties, i.e. the array indices. But you want to iterate the array values.

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in and also Why is using "for...in" with array iteration a bad idea?

edited Jan 05 '18 at 18:15

answered Jan 05 '18 at 18:04

le_m

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It works for me without the `let` as well... – Stefan Pochmann Jan 05 '18 at 18:07
2

@StefanPochmann Yes, but by leaving out the `let` declaration, `i` becomes a global variable polluting the global namespace. – le_m Jan 05 '18 at 18:08

JavaScript fundamentals confusion / convert Python

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