Understanding for loops

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#23 opened on Mar 28, 2017 3 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 2/5
Estimated time: 1-3 hours
Newbie friendliness: 35/100

Issue type: Documentation
Clarity: Needs clarification
Activity status: Stale
Tech stack: javascript
Domain: documentation

Research direction

No repository file or test is named in the issue. Start with the supplied multiplyAll example and review the surrounding JavaScript learning material; done means the material clearly answers what i and j represent, why the loops increment them, and how the nested arrays are traversed.

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Description

question

@sophielevens asked me this question about understanding for loops. The question is from Free Code Camp.

Question
If you have a multi-dimensional array, you can use the same logic as the prior waypoint to loop through both the array and any sub-arrays. Here is an example:

var arr = [
  [1,2], [3,4], [5,6]
];
for (var i=0; i < arr.length; i++) {
  for (var j=0; j < arr[i].length; j++) {
    console.log(arr[i][j]);
  }
}

This outputs each sub-element in arr one at a time. Note that for the inner loop, we are checking the .length of arr[i], since arr[i] is itself an array.
Instructions
Modify function multiplyAll so that it multiplies the product variable by each number in the sub-arrays of arrand the answer is this

function multiplyAll(arr) {
var product = 1;
// Only change code below this line
for (var i=0; i < arr.length; i++) {
for (var j=0; j < arr[i].length; j++) {
product *= arr[i][j];
}
}
// Only change code above this line
return product;
}
// Modify values below to test your code
multiplyAll([[1,2],[3,4],[5,6,7]]);

Her questions were:

Where on earth did [i] and [j] come from? Have googled it and can't find out what they mean
Also, (i++) is apparently the same as (i = i + 1) why do we want to add 1 to i?

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