JS failing to populate arrays from array

Question

I have an array of exam result, it contains the date the exam was sat and the grade achieved. I want to put all the grades in one array and the dates in another so I can plot them in a chart.js application. How can I separte them?

Results Array example layout:

Results{

[0] {examDate: "2017-10-16T10:30:00", <ResultEntry>grade: A}

[1] {examDate: "2017-15-16T11:00:00", <ResultEntry>grade: C}

[2] {examDate: "2017-16-16T11:30:00", <ResultEntry>grade: B}

[3]{examDate: "2017-20-16T12:00:00", <ResultEntry>grade: B}

}

But what I try doesn't populate the two new arrays

var dateArray;
var gradeArray;
var counter = 0;

for (members in results) {
    dateArray[counter] = members[0];
    gradeArray[counter] = members[1];
    counter++;//increment storage index
}

I have Jquery installed on my project can the .each functionality be used to achieve this?

Try dateArray[counter] = members.examDate and dateArray[counter] = members.grade — Walk
– Walk, Commented Oct 24, 2017 at 9:03

Mihai Alexandru-Ionut · Accepted Answer · 2017-10-24 09:02:57Z

3

You can use map method by passing a callback function.

The map() method creates a new array with the results of calling a provided function(callback) on every element in the calling array.

let results=[{examDate: "2017-10-16T10:30:00", grade: 'A'},{examDate: "2017-15-16T11:00:00", grade: 'C'},{examDate: "2017-16-16T11:30:00", grade: 'B'},{examDate: "2017-20-16T12:00:00", grade: 'B'}];
let dates=results.map(function(a){ 
    return a.examDate;
});
let grades=results.map(function(a){
    return a.grade;
});
console.log(dates);
console.log(grades);

You can use also arrow functions.

 let grades=results.map(a => a.grade);

answered Oct 24, 2017 at 9:02

Mihai Alexandru-Ionut

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Comments

MysterX · Accepted Answer · 2017-10-24 09:03:59Z

0

You need to get values of fields from array elements. Check this:

var dateArray = [];
var gradeArray = [];
var counter = 0;

for (var member in results) {
    dateArray.push(member.examDate);
    gradeArray.push(member.grade);
}

answered Oct 24, 2017 at 9:03

MysterX

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Comments

gurvinder372 · Accepted Answer · 2017-10-24 09:05:39Z

0

If results is an array, then in this line

for (members in results) {

members is basically the index value 0,1,..

so members[0] will be undefined

I have Jquery installed on my project can the .each functionality be used to achieve this?

Yes, but why not use the native forEach instead

var dateArray = [], gradeArray = [];
results.forEach( function( result ){
  dateArray.push( result.examDate );
  gradeArray.push( result.grade );
});

answered Oct 24, 2017 at 9:05

gurvinder372

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Comments

Gorka Hernandez · Accepted Answer · 2017-10-24 09:16:48Z

Considering that the results array is has the following shape:

let results = [
    { examDate: "2017-10-16T10:30:00", grade: 'A' },
    { examDate: "2017-15-16T11:00:00", grade: 'C' },
    { examDate: "2017-16-16T11:30:00", grade: 'B' },
    { examDate: "2017-20-16T12:00:00", grade: 'B' }
];

Array.prototype.map is perfect for this purpose:

let dateArray = results.map(result => result.examDate);
let gradeArray = results.map(result => result.grade);

You can also loop over the elements in a few different ways:

let dateArray = [],
    gradeArray = [];

for (let result of results) {
    dateArray.push(result.examDate);
    gradeArray.push(result.grade);
}

And a solution similar to yours:

var dateArray = [],
    gradeArray = [];

for (var i = 0; i < results.length; i++) {
    dateArray[i] = results[i].examDate;
    gradeArray[i] = results[i].grade;
}

You can also push them into the array instead of setting a position to the value:

var dateArray = [],
    gradeArray = [];

for (var i = 0; i < results.length; i++) {
    dateArray.push(results[i].examDate);
    gradeArray.push(results[i].grade);
}

And using jQuery each as you mention:

var dateArray = [],
    gradeArray = [];

$.each(results , function(i, value) {
    dateArray.push(value.examDate);
    gradeArray.push(value.grade);
});

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