Nested For loop for initializing array of arrays for Javascript

Question

I have a very simple part of my program here that's having a problem... The elements in the array 'population' are also an array.. having 28 random numbers each 'temp' array that is being loaded to the array population. My problem is that the array 'population''s somehow have all the same arrays saved in it, it's like its being override every loop. Ive spent so much time in this very simple problem, is this some kind of bug?? The commented 'alert' is used to check the element 0 and 1 of the population. and somehow it's really being overriden every loop so every temp elements in the population array is all the same. Please help me..

var population[];
function init_population(){   
    temp = [];
    //Math.floor(Math.random()*8);
    for(i=0;i<10;i++){
        for(j=0;j<28;j++)
        temp[j] = Math.floor(Math.random()*8);
        population[i]= temp;
    //alert("population[0] = " +population[0] +" and population[1] = " +population[1]);
    }
}

init_population();

jfriend00 · Accepted Answer · 2013-01-21 07:22:09Z

1

You need to create a new temp array in the inner loop so you're not reusing the same array over and over:

var population = [];

function init_population() {   
    var temp, i, j;
    for(i=0; i<10; i++) {
        temp = [];
        for(j=0; j<28; j++) {
            temp[j] = Math.floor(Math.random()*8);
        }
        population[i] = temp;
    }
}

init_population();

Since assigning the temp array into the population array only puts a reference to the temp array in, when you keep using the same temp array over and over again, you end up with references to the same temp array at each index of the population array. If, instead, you create a new temp array in the inner loop, then each array in the population array will be different.

FYI, I also made some other corrections to your code to properly declare the variables temp, i and j as local variables so they aren't implicit global variables.

edited Jan 21, 2013 at 7:22

answered Jan 21, 2013 at 7:16

jfriend00

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3 Comments

Synectouche Over a year ago

Thank you so much for the answer, it really helped, but just to make me understand, i thought the temp array is just gonna be overridden since the j counter of the inner loop again is back to 0 once the outer loop iterates?

jfriend00 Over a year ago

@WilmorHerald - you overwrite the values of the temp array, but you put a pointer to the same temp array in every element of the population array so every element in the population array is the same. Instead, you have to create a new temp array with temp = []; inside the first loop so that each element of population contains a different array. If you still don't understand, then you need to learn about assignment "by reference" and "by value" in javascript. Google will be your friend.

Synectouche Over a year ago

Oh, now i understand clearly. Thank you so much, appreciate it. After i've understand it, i realized in a snap that my code was really idiotic. haha.. Thanks to you.

Petar Ivanov · Accepted Answer · 2013-01-21 07:17:09Z

1

This is because it's the same array. You create it with temp = [] and then assign it to each of the population[i] for i=0..9

answered Jan 21, 2013 at 7:17

Petar Ivanov

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