using a string as the for loop expressions and condition

Question

The following loop works:

<html>
<body>
<script type="text/javascript">
var i=0;
for (i=0;i<=5;i++)
{
document.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.

is something preventing you using normal syntax as in the first version? why do you want to use version 2? how did you even arrive at the conclusion that such syntax is acceptable? — SilentGhost, Jan 28 '11 at 16:23

score 5 · Answer 1 · answered Jan 28 '11 at 16:27

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 />");
}

Ha ha. Just realised the link I posted is actually where this code block has come from! :) — El Ronnoco, Jan 28 '11 at 16:34

score 4 · Answer 2 · answered Jan 28 '11 at 16:30

4

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)

answered Jan 28 '11 at 16:30

I think your statements about the differences between *code* and *data* are very insightful. Although Lisp comes to mind. – ChaosPandion Jan 28 '11 at 16:32
Side note: I'm waiting for the lisp users coming along and explaining how in lisp, the two are the same (which is of course only half the truth: Many perfectly useful programs won't use it outside of macros). Please don't disappoint me :) – Jan 28 '11 at 16:40

score 0 · Answer 3 · answered Jan 28 '11 at 16:28

0

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.

answered Jan 28 '11 at 16:28

Claudio

1

using a string as the for loop expressions and condition

3 Answers3