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Thread: [RESOLVED] Some for loop help please.

  1. #1
    Join Date
    Oct 2009
    Posts
    4

    resolved [RESOLVED] Some for loop help please.

    Hi folks. Here's the code I have so far:

    Code:
    <script language=javascript type=text/javascript>
    var i, sum=0;
    
    for (i=1;i<=5;i++)
    {
    sum = sum+i;
    document.write("The total is: " +sum);
    document.write("<br>");
    }
    </script>
    What I now need to do is modify the above to allow the user to input a value. So basically that would be 1+2+3+4+5+n?

    I made an attempt at it, but I just don't know where to put n in the loop. I am very new to javascript.

    Attempt:
    Code:
    <html>
    <head>
    <title>For Loop</title>
    <script language=javascript type=text/javascript>
    var i, sum=0;
    var n =eval(document.myform.number.value);
    
    for (i=1;i<=5;i++)
    {
    sum = sum+i;
    document.write("The total is: " +sum);
    document.write("<br>");
    }
    </script>
    
    </head>
    <body>
    <form name="myform" onSubmit="calculate();">
    <h2>Enter a number</h2>
    <p><label for="number">Input number here: </label><input type="text" size="10" name="number" id="number"/> </p>
    <input type="submit" value="Calculate"/>
    </form>
    
    </body>
    </html>
    Anyway thanks.

  2. #2
    Join Date
    Aug 2007
    Location
    London
    Posts
    410
    First things first, I would definitely NOT use that eval() statement to get the number out since there is unvalidated user input and they could put anything (e.g. "alert(\"hello\");" or something annoying).

    What you should probably do is create a function which does your calculation for you and then writes it to a specific element. So, exemplia gratia:

    Code:
    //Define the function
    function doMySum() {
    	//First get the number from the user input
    	//I assume this should be an integer, rather than a float?
    	var n = document.getElementById("number").value.parseInt();
    	
    	//Create an empty string to hold your "The total is: ..." stuff
    	var sum_string = "";
    	
    	//Create iterator and sum variables
    	var i;
    	var sum = 0;
    	
    	//Now run the for loop
    	for(i=0; i < n; i++) {
    		//Add the sum
    		sum = sum + (i+1);
    		
    		//Add a <br/> before this line if it's not the first line
    		if(i>0) { sum_string += "<br/>"; }
    		
    		//Write the sum string
    		sum_string += "The total is: " + sum;
    	}
    	
    	//Now write the output to a div called "output".
    	//This will over-write anything currently in there.
    	document.getElementById("output").innerHTML = sum_string;
    	
    	//Return false to stop any links being followed or forms being submitted.
    	return false;
    }
    Then you can just add
    HTML Code:
    onsubmit="return doMySum();"
    to the form tag.
    Don't forget to create the "output" element (a <div>) should do the trick.

    I hope that helps.
    Last edited by blue-eye-labs; 10-27-2009 at 08:19 PM. Reason: HTML entities were messed around with.

  3. #3
    Join Date
    Aug 2007
    Location
    London
    Posts
    410
    Sorry, that should be
    Code:
    var n = parseInt(document.getElementById("number").value);
    Instead of
    Code:
    var n =document.getElementById("number").value. parseInt();
    Apologies.

  4. #4
    Join Date
    Dec 2003
    Location
    Bucharest, ROMANIA
    Posts
    15,428
    Quote Originally Posted by blue-eye-labs View Post
    Sorry, that should be
    Code:
    var n = parseInt(document.getElementById("number").value);
    Better
    Code:
    var n = parseInt(document.getElementById("number").value, 10);
    If no base radix specified, parseInt() might consider some decimal numbers as octal, because, in fact, parseInt(number,base) method was designed to parse a number from a certain base to decimal. It is safer to specify which is that certain base, 10 in your case.

    See:
    Code:
    var x='0112';
    alert(parseInt(x)); //returns 74
    alert(parseInt(x,10)); //returns 112
    Last edited by Kor; 10-28-2009 at 08:08 AM.

  5. #5
    Join Date
    Aug 2007
    Location
    London
    Posts
    410
    @Kor:

    Thanks for that, I hadn't realised.

  6. #6
    Join Date
    Oct 2009
    Posts
    4
    Thank-you both for your replies that works perfectly

    I assume that if I want to limit the number input by the user I would use an if statement and have a message show telling them to input a number between 0 and 20?

  7. #7
    Join Date
    Aug 2007
    Location
    London
    Posts
    410
    I assume that if I want to limit the number input by the user I would use an if statement and have a message show telling them to input a number between 0 and 20?
    Yup, just check the value of n and if it's outside of your range then have some sort of popup, alert, red-div or whatever else you can think of

  8. #8
    Join Date
    Oct 2009
    Posts
    4
    Ok, thanks for the help dude

  9. #9
    Join Date
    Aug 2007
    Location
    London
    Posts
    410
    You're welcome. Mark the thread as resolved.

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