How to make an average of an array in JavaScript

Question

This is for my GCSE Computer Science. Here's the code:

<html>
<p id="demo"></p>
<script>
var middaytemp = []; midnighttemp = [];
for (count = 0; count <= 29; count++) {
middaytemp[count] = prompt("What was the temperature at midday");
midnighttemp[count] = prompt("what was the temperature at midnight?");
document.getElementById("demo").innerHTML += middaytemp[count] + "&deg; Celcius<br>";
document.getElementById("demo").innerHTML += midnighttemp[count] + "&deg; Celcius<br>";
}
</script>
</html>

As you can see it asks these two questions 29 times each. I've got them in an array but I need to find the average of the array. so when the temperatures are put in at the end of the code it shows the average for the night and average for the day.

Those questions are asked thirty times each. But where are you stuck? What part(s) do you not understand how to implement? — David Thomas
– David Thomas, Commented Jan 14, 2016 at 19:43
Yeah I know. That was a typo. I need to find the average and output it. Thanks again. Luke — L. Davies
– L. Davies, Commented Jan 14, 2016 at 19:48
Sum the values inside the for loop with something like middaytotal and midnighttotal. Then after the loop you calc the average, no aditional loops or functions are needed. — DontVoteMeDown
– DontVoteMeDown, Commented Jan 14, 2016 at 19:49
Which average? Arithmetic? Geometric? Harmonic? Power? A composition of them, e.g.: arithmetic-geometric (AGM)? Median? Mode? — deamentiaemundi
– deamentiaemundi, Commented Jan 14, 2016 at 19:50

rlemon · Accepted Answer · 2016-01-14 20:33:09Z

2

Use a reduce! This will return the sum of midday into sum.

let sum = midday.reduce((x,y) => x+y);
sum = sum/midday.length; //average

I used a an ES6 feature here called a lambda expression or "fat arrow function". It is the lexical equivalent of an anonymous function, which is used as the callback parameter for the array reduce function.

var sum = midday.reduce(function(x,y) {
    return x+y;
});

With a lambda expression, the return is implied, which is why you only need => x+y. Multiple lines would require { ... }. Here is the documentation for arrow function syntax.

edited Jan 14, 2016 at 20:33

rlemon

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answered Jan 14, 2016 at 19:45

Sterling Archer

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4 Comments

David Thomas Over a year ago

Look at the OP's code; you need to explain what Array.prototype.reduce() does, and what arrow functions are, and how they work.

Retsam Over a year ago

Slight point of clarification "With a lambda, the return is implied", should probably say something like "With a single expression lambda".

user400654 Over a year ago

or just remove all of that from the answer, leaving the last snippet since the rest is irrelevant, evil grin

Sterling Archer Over a year ago

@KevinB you get out of here! Go on, get!

Richard Dickins · Accepted Answer · 2016-01-14 21:07:47Z

1

Just iterate through the array, since your values are strings, they have to be converted to an integer first. And average is just the sum of values divided by the number of values.

var sum = 0; for( var i = 0; i < elmt.length; i++ ){ sum += parseInt( elmt[i], 10 ); //don't forget to add the base } var avg = sum/elmt.length; document.write( "The sum of all the elements is: " + sum + " The average is: " + avg );

edited Jan 14, 2016 at 21:07

answered Jan 14, 2016 at 19:44

Richard Dickins

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5 Comments

L. Davies Over a year ago

How do you iterate through the array? Could you give an example? Thanks again. Luke

Richard Dickins Over a year ago

var sum = 0; for( var i = 0; i < elmt.length; i++ ){ sum += parseInt( elmt[i], 10 ); //don't forget to add the base } var avg = sum/elmt.length; document.write( "The sum of all the elements is: " + sum + " The average is: " + avg );

Richard Dickins Over a year ago

^sorry - not used to using the correct formatting. I hope it still makes sense.

Sterling Archer Over a year ago

@RichardDickins you should edit that into your answer, not as a comment so that it's easier to read.

Richard Dickins Over a year ago

@SterlingArcher Cheers. Is there any way to send a direct message to one user?

Nikhil Aggarwal · Accepted Answer · 2016-01-14 19:43:00Z

0

You can achieve the results via following

var sumTempDay = 0;

for (var i = 0 ; i < middaytemp.length; i++) {
   sumTempDay += middaytemp[i];

}

var avgTempDay = sumTempDay/middaytemp .length;

var sumTempNight = 0;

for (var i = 0 ; i < midnighttemp.length; i++) {
   sumTempNight += midnighttemp[i];

}

var avgTempNight = sumTempNight/midnighttemp.length;

answered Jan 14, 2016 at 19:43

Nikhil Aggarwal

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2 Comments

L. Davies Over a year ago

Thanks a lot. Really helped. I'll do that. Where do I put the prompt("whats the temp at midday"). Sorry. I'm new to this. Thanks. Luke

L. Davies Over a year ago

where do i put the prompt where i ask the temp? sorry i'm new to this. Thanks. Luke

Conta · Accepted Answer · 2016-01-14 19:57:38Z

0

You need to first convert the string into float. Then calculate the average.

var sum = 0
middaytemp.forEach(function(val){
    sum += parseFloat(val);
})
var average = sum/middaytemp.length;

answered Jan 14, 2016 at 19:57

Conta

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