Javascript loop error: Uncaught TypeError

Question

I am practicing Javascript and have the following code which returns an Uncaught TypeError: Cannot set property '0' of undefined error on the sets[i][j] = initial_sets [i][j]; line. The idea seems to be correct, but I can't figure out why I am getting the error.

var sets = [[],[]]; //Declared this 2D array based on solution in https://stackoverflow.com/questions/16512182/how-to-create-empty-2d-array-in-javascript
var initial_sets=[[a,b],[c,d],[e,f]]; //2D array
var i,j;

//2D array sets is being filled by 2D array initial_sets
for (i=0; i<initial_sets.length; i++) {
    for (j=0; j<initial_sets[i].length; j++) {
            sets[i][j] = initial_sets[i][j];
    }
}

I tried the solution posted in Uncaught TypeError: Cannot set property '0' of undefined " by creating a 1D array and then assigning another dimension to it, but I still get the same error.

You have 3 arrays in initial_sets but only 2 in sets so set[2] is undefined — ibrahim mahrir
– ibrahim mahrir, Commented Jan 15, 2017 at 16:49
[[],[]]; here size of array is 2, so how can you fit anything at 3rd index — A.T.
– A.T., Commented Jan 15, 2017 at 16:57
based on this solution stackoverflow.com/questions/16512182/… it is a declaration for a 2D array. not an array with only 2 indices — user2883071
– user2883071, Commented Jan 15, 2017 at 17:11

ibrahim mahrir · Accepted Answer · 2017-01-15 16:53:36Z

2

var sets = [];
var initial_sets=[[a,b],[c,d],[e,f]];
var i,j;

for (i=0; i<initial_sets.length; i++) {
    // check if sets[i] exist. if not push a new emty array to it.
    if(!sets[i]) sets.push([]);
    for (j=0; j<initial_sets[i].length; j++) {
            sets[i][j] = initial_sets[i][j];
    }
}

then you won't have to initialize sets with empty arrays, it will push them when needed.

answered Jan 15, 2017 at 16:53

ibrahim mahrir

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Comments

Ebrahim Pasbani · Accepted Answer · 2017-01-15 16:56:31Z

0

Dimensions of the arrays are not the same.

var sets = []; 
var initial_sets=[["a","b"],["c","d"],["e","f"]];
var i,j;

for (i=0; i<initial_sets.length; i++) { 
    var tmp = [];
    for (j=0; j<initial_sets[i].length; j++) {
            tmp[j] = initial_sets[i][j];
    }
    sets[i] = tmp;
}

OR

var sets = []; 
var initial_sets=[["a","b"],["c","d"],["e","f"]];
var i,j;

for (i=0; i<initial_sets.length; i++) { 
    sets[i] = []; 
    for (j=0; j<initial_sets[i].length; j++) {
            sets[i][j] = initial_sets[i][j];
    }    
}

edited Jan 15, 2017 at 16:56

answered Jan 15, 2017 at 16:54

Ebrahim Pasbani

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5 Comments

user2883071 Over a year ago

yes, they are.. they are both 2D. And sets will dynamically increase to the size of initial_sets.

Ebrahim Pasbani Over a year ago

@user2883071 When i==2, sets[i] is undefined. It's true dynamically increases but the value of sets[2] is undefined. So sets[2][j] is undefined and can not be set

user2883071 Over a year ago

Apologies, I see what you are saying. I believe my confusion arose when I tried to declare a 2D array. I tried initializing a 2D array as such: var sets = [][]; but that gave me an error: Uncaught SyntaxError: Unexpected token ], which is why I used var sets = [[],[]];

user2883071 Over a year ago

It was based on the following solution: stackoverflow.com/questions/16512182/…

Ebrahim Pasbani Over a year ago

@user2883071 You defined [[], []] correctly. And you know access array with indices causes the array length to be increased. But the point is when you want to access to sets[i][j] and i==2, the sets[i] does not get valued yet and is undefined. So before accessing to sets[2][j] you need to set value for sets[2]

Mayank Shukla · Accepted Answer · 2017-01-15 17:16:09Z

0

Try this:

var sets = [];
var initial_sets=[['a','b'],['c','d'],['e','f']];
var i,j;

for (i=0; i<initial_sets.length; i++){ 
  sets[i]=[];
  for (j=0; j<initial_sets[i].length; j++){
    sets[i].push(initial_sets[i][j]);
  }
}
console.log(sets);

answered Jan 15, 2017 at 17:16

Mayank Shukla

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