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Adding One To A Variable

Grabeorama

By Grabeorama
in JavaScript

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Grabeorama Member

Grabeorama

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Here is my code:in the <head>

<script>var number;function myfunction()	{		number+=1;	}</script>[code]in the <body>[code]<input type="button" value="Add 1 to aidan" onclick="myfunction()">	<script>		document.write(number);	</script>

What I want, is that when I click the button, it adds one to the variable "number" is there something wrong with this code?

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jeffman w3schools Guru

jeffman

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In some languages, if you add 1 to an undefined variable, it casts the variable into a number, with the value 0, and does the add. Javascript is so loosely typed you'd think it would allow that, but it doesn't. Change your first line to this:var number = 0;Hint:Start writing things like number++ instead of number += 1. It's not better--just more idiomatic. You'll look more like one of the gang. :)

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Synook w3schools Guru

Synook

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It's not better--just more idiomatic.
Well, theoretically, it should be slightly faster as the interpreter doesn't have to interpret the "1". Benchmark anyone?
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Grabeorama Member

Grabeorama

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It still doesn't change the variable "number"The part where I have document.write(number); still shows "0" after I have pushed the button.This is the code I'm using now:

<head><title>New Page 1</title>	<script>	var number = 0;	function myfunction()		{			number++;		}	</script></head><body>	<input type="button" value="Add 1 to number" onclick="myfunction()">	<script>	document.write(number);	</script></body>

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jeffman w3schools Guru

jeffman

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That's because your second script executes as the page loads. It is not associated in any way with your button, and it never runs again, no matter how many times you push the button. (If it did, you'd be in trouble, because it would erase your page!) myfunction() is associated with your button because you explicitly told it to be, in your onclick attribute.If you want to check the value of number, add a line like this to myfunction():alert(number);And please, please, please stop using document.write().

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Grabeorama Member

Grabeorama

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o.k.Is there anyway of checking it without an alert box?I want to make a sort of slideshow for pictures, so when you click the left arrow it takes 1 from number and when you click the right arrow it adds one to number. Then depending on the value of number I would display the appropriate image.and what could I use instead of document.write() ?Thanks :)

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boen_robot w3schools Guru

boen_robot

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I don't quite get what you mean, where do I put this code?
The second part - ideally, at the end of your myfunction() function, replacing "Any string you like" with the actual string of course.The first part - anywhere within the body of your HTML document.
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Grabeorama Member

Grabeorama

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Thanks :)It still doesn't work though :)All it shows is the button and underneath it has the number "0" that doesn't change when the button is clicked.Now I have this code:

<html><head><title>New Page 1</title>	<script type="text/javascript">		var number = 0;		function myfunction()			{				number++;				el = document.getElementById("message");				el.InnerHTML = "Hello There";			}	</script></head><body>	<input type="button" value="click me!" onclick="myfunction()">	<div>		<p id="message"></p>		<script>			document.write(number);		</script>	</div></body></html>

Thanks to those who have helped! :)

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Grabeorama Member

Grabeorama

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Sorry about the double post but I was wondering, is there a more efficient way of doing this:

<html><head>	<title>Black - Home</title>	<link type="text/css" rel="stylesheet" href="black_css.css">	<script type="text/javascript">		var image = 0;		var number = 0;		function picture_add()			{				if (image<4)					{		   			image++;		   		}		   	switch (image)		   		{		   			case 0:		   				number="This is zero";		   				break;		   			case 1:		   				number="This is one";		   				break;		   			case 2:		   				number="This is two";		   				break;		   			case 3:		   				number="This is three";		   				break;		   			case 4:		   				number="This is four";		   				break;		   		}			  el = document.getElementById("message");			  el.innerHTML = number;		  }		function picture_sub()		  {				  if (image>0)					  {					  image--;				  }			  switch (image)		   		{		   			case 0:		   				number="<font style='color: red;'>This is zero </font>";		   				break;		   			case 1:		   				number="This is one";		   				break;		   			case 2:		   				number="This is two";		   				break;		   			case 3:		   				number="This is three";		   				break;		   			case 4:		   				number="This is four";		   				break;		   		}			  el = document.getElementById("message");			  el.innerHTML = number;		  }	</script></head><body>	<div id="heading">		<h1>BLACK</h1>	</div>		<div>		<font class="arrow" onclick="picture_sub()"><</font>	   <font id="message"></font>	   <font class="arrow" onclick="picture_add()">></font>	</div></body></html>

Thanks in advance :)

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jeffman w3schools Guru

jeffman

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Yup. Very similar functions can be combined if you pass a flag that tells the function where to make changes. In this case, I used the value of the flag as well. You'll notice I changed your <font> tags to <span> tags since <font> is deprecated. I also took out the inline style and created a class. You may be puzzled by the ternary operators (the one with the ?). Try reading up on them if you don't understand, or ask and I'll explain. When you get used to them, they can be more natural to read than if/else statements, and they take up less space.

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"   "http://www.w3.org/TR/html4/strict.dtd"><html>	<head>		<meta http-equiv="content-type" content="text/html;charset=iso-8859-1">		<title></title>		<style type="text/css">			.red {				color: #ee0000;			}		</style>		<script type="text/javascript">			var image = 0;			var number = "";			function change_picture(i) {				image += i;				i == 1  && image > 4 ? image = 0 : null;				i == -1 && image < 0 ? image = 4 : null;				switch (image) {					case 0:						number = (i == -1) ? "<span class='red'>This is zero</span>" : "This is zero";						break;					case 1:						number="This is one";						break;					case 2:						number="This is two";						break;					case 3:						number="This is three";						break;					case 4:						number="This is four";						break;				}				el = document.getElementById("message");				el.innerHTML = number;			}		</script>	</head>	<body>		<div id="heading">			<h1>BLACK</h1>		</div>		<div>			<span class="arrow" onclick="change_picture(-1)">< </span>			<span id="message"></span>			<span class="arrow" onclick="change_picture(1)"> ></span>		</div>	</body></html>

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