using a string as the for loop expressions and condition
The following loop works:
<html>
<body>
<script type="text/javascript">
var i=0;
for (i=0;i<=5;i++)
{
docume开发者_StackOverflow社区nt.write("The number is " + i);
document.write("<br />");
}
</script>
</body>
</html>
But the following doesn't:
<html>
<body>
<script type="text/javascript">
var i=0;
var x="i=0;i<=5;i++"
for (x)
{
document.write("The number is " + i);
document.write("<br />");
}
</script>
</body>
</html>
I'd just like to create a simple variable. Please bear with me as I'm a newbie in JavaScript and let me know what I'm missing.
Let me provide my sample Google gadget:
<?xml version="1.0" encoding="UTF-8" ?>
<Module>
<ModulePrefs title="Sample Gadget" />
<UserPref name="order"
display_name="Results Order"
default_value="i = 0; i <= 5; i++" datatype="enum">
<EnumValue value="i = 0; i <= 5; i++" display_value="Ascending"/>
<EnumValue value="i = 5; i >= 0; i--" display_value="Descending"/>
</UserPref>
<Content type="html"><![CDATA[
<script type="text/javascript">
var i=0;
for (__UP_order__)
{
document.write("The number is " + i);
document.write("<br />");
}
</script>
]]></Content>
</Module>
It doesn't work because of the tags <> (they're not supported), and that's why I tried to define a variable for the EnumValue value.
When you say var x="i=0;i<=5;i++" you are creating a text string. This is not interpreted by JavaScript as you are expecting.
There is a definite difference between statements and text strings. Even though it looks to the eye like the same thing, it looks to the interpreter like a text string, like "hello" or "sdflkjsdflkjsdflj". JavaScript is not expecting a text string as loop parameters, it is expecting the three loop control parameters/statements. If you want to have a loop which starts and ends at different points, do something like this...
var i=0;
var start=0; //you can change the start position by changing this
var end=5; //and you can change the end also
for (i=start;i<=end;i++)
{
document.write("The number is " + i);
document.write("<br />");
}
In short: You're confusing code with data. "i=0;i<=5;i++" is data (a piece of text, a string). But when writing a for-loop you have to write initialization, condition and step as code - you cannot pass text that happens to look like the code you'd write there. (In fact, you don't want to - what should happen when the data isn't like valid code? Not to mention it's not needed - see El Ronnoco's)
Because x is a string and you cannot use for statement with a string inside. If you need to change the upper bound of a for statement you can use a variable instead the fix number 5.
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