Using equality in 'for' loop - why not?

[Mark Anderson]

A 'for' loop takes 3 arguments (initialize; test; increment). The 'test'
must equate as true or false

This doesn't work...
x = 5;
for (y=1; (y==5); y+=1) {
alert(x * y);
}
...nor does...
x = 5;
for (y=1; (y===5); y+=1) {
alert(x * y);
}
....but this does..
x = 5;
for (y=1; (y<6); y+=1) {
alert(x * y);
}

Why do the first two fail? If i is value 5 then i==5 is true, as is
i===5.

Can anyone explain what I'm missing here?

Regards

[Thomas 'PointedEars' Lahn]

Stephen Chalmers wrote:

"Thomas 'PointedEars' Lahn" <Po*********@nu rfuerspam.de> wrote in message
news:40******** ******@PointedE ars.de...

Please do not write attribution novels, see
<http://netmeister.org/news/learn2quote.htm l>

`for' is not different in ECMAScript and its
implementations from any other language of the C family.

Hmmm. In ECMAScript, the scope of variables declared in the initialisation
statement, is not the same as it is in C++.

ACK. I was referring to the test expression, this is a C-style `for'.
PointedEars

[Dr John Stockton]

JRS: In article <c9************ *******@news.de mon.co.uk>, seen in
news:comp.lang. javascript, Mark Anderson <ma**@notmeyear dley.demon.co.u k

posted at Fri, 28 May 2004 16:45:50 : A 'for' loop takes 3 arguments (initialize; test; increment). The 'test'
must equate as true or false

ISTM that this is not a matter for the FAQ, but that it is a matter for
tutorials.

In different languages, loops are expressed in many different ways; but
ISTM that in the relevant respect there are exactly two different
classes.

For any loop, there is an initialisation, there is a change-between-
loops, and there is a condition. The condition is always equivalent to
a Boolean - that is inevitable, though Booleanisation may be implicit.

One class of loop has a termination condition; false to run.
One class of loop has a continuation condition; true to run.

The first class is exemplified by
Algol's for J := 3 step 6 until 9 // J=9
Pascal's for J := 8 down to 4 // J=4
repeat Inc(J) ; ... until J=9
(the first two could be interpreted with <=, and are probably
implemented that way; but the mind is likely to consider the condition
as "finish after J equals given value");
and the second by
Javascript's for (J=3 ; J<=9 ; J+=6)
Pascal's while J<9 do begin Inc(J) ; ... end ;

In the second, the Boolean is usually, but not always, an ordered
comparison - > >= < <=, explicit or implicit. However :
for (J=new Date();J.getMon th()==4;J=new Date()) {}
for (J=new Date();J.getMon th()!=5;J=new Date()) {}
should, ISTM, each loop until June.
The OP has been giving a termination condition where a continuation
condition is required.
There is another dichotomy, that of whether the finalisation condition
is evaluated before or after the loop body
while (new Date()<12345678 90000) {}
do {} while (new Date()<12345678 90000)
each of which should loop until 2009-02-13 Fri 23:31:30 GMT. IIRC, a
FORTRAN IV loop tested at the end, so the loop was always executed once;
but an Algol 60 loop tested at the beginning.

--
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<URL:http://jibbering.com/faq/> Jim Ley's FAQ for news:comp.lang. javascript
<URL:http://www.merlyn.demo n.co.uk/js-index.htm> jscr maths, dates, sources.
<URL:http://www.merlyn.demo n.co.uk/> TP/BP/Delphi/jscr/&c, FAQ items, links.

[Lasse Reichstein Nielsen]

"Mark Anderson" <ma**@notmeyear dley.demon.co.u k> writes:

Shame no books tutorials stoop so low as to explain this simple fact.
What about:
<URL:http://devedge.netscap e.com/library/manuals/2000/javascript/1.5/reference/stmt.html#10048 04>
<URL:http://msdn.microsoft. com/library/en-us/script56/html/js56jsstmfor.as p?frame=false>
I now know why I won't use ++ etc., except in the exceptional cases such
as Klaus just described.

I always use ++, and can see no problem with that. It's just the comparison
that should usually not be an "equals-to" test.

/L
--
Lasse Reichstein Nielsen - lr*@hotpop.com
DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleD OM.html>
'Faith without judgement merely degrades the spirit divine.'

[rh]

Lasse Reichstein Nielsen wrote:

"Mark Anderson" <ma**@notmeyear dley.demon.co.u k> writes:

Shame no books tutorials stoop so low as to explain this simple fact.

What about:
<URL:http://devedge.netscap e.com/library/manuals/2000/javascript/1.5/reference/stmt.html#10048 04>
<URL:http://msdn.microsoft. com/library/en-us/script56/html/js56jsstmfor.as p?frame=false>

Both references appear to be satisfactory for the description of the
for "condition" .

However, the latter reference describes the "initialization ",
"condition" , and "increment" as being required. The former describes
the "condition" as optional, with the potential mistaken implication
that the other two are required.

In fact, the form

for (;;); // Infinite loop, by the way

is compliant with ECMA-262/3. Later browsers (don't know about earlier
ones) accept the above form, including MS IE 6, so it appears it may
be just the documentation that is not entirely "with the program". :)

../rh