For in Loop to calculate array values

@goose2013Jan 22.2013

Anybody able to offer any help…

[code=php] function enterBook() { var myBooks = new Array() var myPrices = new Array()

for (i = 1; i <= 5; i++) { myBooks[i] = prompt(“Enter title of book”,””); myPrices[i] = parseFloat(prompt(“Enter price of book”,””)); document.getElementById(“output”).innerHTML += i + “. ” + myBooks[i] + “. Price: £” + myPrices[i] + “<br />”; }

for (var i in myPrices) { document.getElementById(“total”).innerHTML =+ i; } } [/code]

I need to calculate the total of the myPrices array values using the ‘for in’ loop and display them to my page, i think im pretty close ?

Thanks!

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JavaScript

6 Comments(s) _↴

@pitikoJan 22.2013 — #You can try this code:

<i>
 </i>function enterBook() 
 { 
 var myBooks = new Array();
 var myPrices = new Array(); 
 var total = 0;
 for (i = 1; i &lt;= 5; i++) 
 { 
 myBooks[i] = prompt("Enter title of book",""); 
 myPrices[i] = parseFloat(prompt("Enter price of book","")); 
 document.getElementById("output").innerHTML += i + ". " + myBooks[i] + ". Price: £" + myPrices[i] + "&lt;br /&gt;"; 
 } 
 for (var i in myPrices) 
 { 
 total += myPrices[i];
 document.getElementById("total").innerHTML = total; 
 } 
 }

@goose2013authorJan 22.2013 — #Superb much appreciated!

@toicontienJan 22.2013 — #You do not want to use a for-in loop to iterate over an Array object. A for-in loop serves a very different purpose: It iterates over all the properties in an object. You want to use a regular loop when iterating over an Array.

<i>
 </i>var obj = {
 a: "foo",
 b: 123
 };
 
 for (var key in obj) {
 if (obj.hasOwnProperty(key)) { // filter out properties that do not exist on "obj" but exist in its prototype chain
 alert(key + "=" + obj[key]);
 }
 }
 
 var arr = ["foo", 123];
 
 for (var i = 0, length = arr.length; i &lt; length; i++) {
 alert("arr[" + i + "]=" + arr[i]);
 }

@ReFreezedJan 22.2013 — #You're missing a [FONT=courier new]var[/FONT] keyword before the [I]i[/I] in the for loop. Without it [I]i[/I] will become a global variable. Also, using the array literal notation ([FONT=courier new][][/FONT]) is shorter/faster/cleaner than writing [FONT=courier new]new Array()[/FONT], like so: [FONT=courier new]myArray = [];[/FONT] Just a general tip... Also note that [FONT=courier new]for (var ... in ...)[/FONT] is used to iterate through the members of an object and generally not to loop through an array.

Finally, if you're not using the values in the [I]myBooks[/I] and [I]myPrices[/I] arrays later in your script then you don't need any arrays here - you can just combine your two loops and save the values returned from the prompts in temporary variables inside the loop:

<i>
 </i>function enterBook()
 {
 var total = 0;
 for (var i = 1; i &lt;= 5; i++)
 { 
 var title = prompt("Enter title of book","");
 var price = parseFloat(prompt("Enter price of book","")); 
 document.getElementById("output").innerHTML += i + ". " + title + ". Price: £" + price + "&lt;br /&gt;";
 total += price;
 }
 document.getElementById("total").innerHTML = total;
 }

(Edit: woops, missed toicontien's reply ^{^^{^)}}

@TcobbJan 23.2013 — #Just to add to the previous comments:

(1) For-In loops are slower than the "normal" for loops

(2) A javascript array is an object, and it has various properties that are functions. If you have:

[CODE]var a = [];  //create empty array
 var i;
 for(i in a){  //display all properties
 alert(i);
 }[/CODE]

You will see a lot of stuff that aren't numerical values at all. If you try adding them to

"total" you will get a very bizarre result that you don't want at all.

@JMRKERJan 23.2013 — #Just to add to the previous comments:

(1) For-In loops are slower than the "normal" for loops

(2) A javascript array is an object, and it has various properties that are functions. If you have:

[CODE]var a = [];  //create empty array
 var i;
 for(i in a){  //display all properties
 alert(i);
 }[/CODE]

You will see a lot of stuff that aren't numerical values at all. If you try adding them to

"total" you will get a very bizarre result that you don't want at all.[/QUOTE]

Executing the above code ONLY will display nothing.

Modifying to show both key and value assignments will at least show something.

<i>
 </i>&lt;script type="text/javascript"&gt;
 var a = [];  //create empty array
 a['a'] = 1;
 a['b'] = 2;
 a['c'] = 3;
 
 var i;
 var sum = 0;
 for(i in a){  //display all properties
 alert(i+' '+a[i]);
 sum += a[i];
 }
 alert('Sum: '+sum);
 &lt;/script&gt;

Note: If you have any methods attached to the 'a' array, they would show as well,

which would not be what you might expect for the summation portion of your question.

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