how can i add a value from an array to a value from an another one in javascript?

Question

So, I have two arrays: grade[] and grade2[] that contains elements I give from windows.prompt . I want to add a value from grade with a value from grade2 so i can calculate "media". But the code that i wrote just concats those two values, doesn't add them actually. I am a beginner and I hope you can help me. Thank you!

function display_student() 
{ 
 var n=window.prompt("Number of students: "+"");
 var name= new Array(n);
 var grade=new Array(n); 
 var grade2=new Array(n); 
 var y=0; 
 for (y=0; y<n; y++) 
 { 
name[y]=window.prompt("Student name:","");
grade[y]=window.prompt("Grade 1: ","5");
grade2[y]=window.prompt("Grade 2: ","5")

document.write("</br>"+"Student name: "+name[y]+"</br>"+"Grade 1: "+grade[y]+"    </br>"+"Grade 2: "+grade2[y]+"</br>");
var media=(grade[y]+grade2[y])/2;

document.write("Media: "+media+"</br>");
if(media<5)
document.write("Failed");
else
document.write("Promoted");
  } 
}

this line grade2[y]=window.prompt("Grade 2: ","5") is missing a semi-colon — Manse
– Manse, Commented Feb 16, 2012 at 14:49

Pulkit Goyal · Accepted Answer · 2012-02-16 14:48:10Z

2

Use the parseInt function before adding.

var media=(parseInt(grade[y]) + parseInt(grade2[y]))/2;

answered Feb 16, 2012 at 14:48

Pulkit Goyal

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mas-designs · Accepted Answer · 2012-02-16 14:47:09Z

1

Because they are strings and no numbers.
Take a look at parseInt(string, radix)

http://jibbering.com/faq/notes/type-conversion/

answered Feb 16, 2012 at 14:47

mas-designs

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Comments

bfavaretto · Accepted Answer · 2012-02-16 14:48:03Z

0

Use:

var media=(parseInt(grade[y], 10)+parseInt(grade2[y], 10))/2;

Assuming your grades are integers. Otherwise, use parseFloat (in that case, you won't need the second parameter).

answered Feb 16, 2012 at 14:48

bfavaretto

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mWillis · Accepted Answer · 2012-02-16 14:50:35Z

0

Use parseFloat:

var media=(parseFloat(grade[y])+parseFloat(grade2[y]))/2;

answered Feb 16, 2012 at 14:50

mWillis

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Comments

Manse · Accepted Answer · 2012-02-16 14:52:20Z

The values in the array are of type String (as returned by the window.prompt function) you need to convert them to numbers to add them together :

function display_student() {
    var n = window.prompt("Number of students: " + "");
    var name = new Array(n);
    var grade = new Array(n);
    var grade2 = new Array(n);
    var y = 0;
    for (y = 0; y < n; y++) {
        name[y] = window.prompt("Student name:", "");
        grade[y] = window.prompt("Grade 1: ", "5");
        grade2[y] = window.prompt("Grade 2: ", "5"); // added semi-colon here too

        document.write("</br>" + "Student name: " + name[y] + "</br>" + "Grade 1: " + grade[y] + "    </br>" + "Grade 2: " + grade2[y] + "</br>");
        var media = (parseFloat(grade[y]) + parseFloat(grade2[y])) / 2;

        document.write("Media: " + media + "</br>");
        if (media < 5) {
            document.write("Failed");
        } else {
            document.write("Promoted");
        }
    }
}

Working example : http://jsfiddle.net/z5cUw/

Mark Schultheiss · Accepted Answer · 2012-02-16 15:13:44Z

This is an indirect answer because this is tagged homework.

Because window.prompt returns a string by default, you need to convert this string to a number. There are several methods to do this.

You can use parseInt() as several people have suggested. Returns an integer value.
You can use parseFloat() as suggested.
You can force it using a simple number division on the prompt: window.prompt("Grade 1: ","5")/1; or during the media calculation: var media = (grade[y]/1 + grade2[y]/1) / 2;
You can use the Number() function: Number(window.prompt("Grade 1: ", "5"));

It is up to you to determine which is the appropriate method for your needs. What if the value entered is NOT a number? Study each of these methods to determine which best suites your needs.

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