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

[Hywel]

Re: Using equality in 'for' loop - why not?

In article <c97mqu$ihk$1$8 300dec7@news.de mon.co.uk>, Mark Anderson
says...[color=blue]
> 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);
> }[/color]

This fails because after the first loop "y" becomes 12. Try
for (y=1; y!=5; y++)

[color=blue]
> Why do the first two fail? If i is value 5 then i==5 is true, as is
> i===5.[/color]

The loop executes while the condition is true. In your example you
assign 1 to y, so the condition y==5 is false.

[color=blue]
> Can anyone explain what I'm missing here?[/color]

You're missing the documentation:

http://tinyurl.com/jbie

--
Hywel I do not eat quiche

http://kibo.org.uk/

http://kibo.org.uk/mfaq.php

[Erwin Moller]

Re: Using equality in 'for' loop - why not?

Mark Anderson wrote:

Hi Mark
[color=blue]
> 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);
> }[/color]

yes, I think it works.
Just not as expected.
Your 'test' fails the first time because y=1 and not 5.

Think of for-loop as follows, maybe that helps:

for (initialize; test; increment){
statements;
}

is equal to:

initialize;
while(test) {
statements;
increment;
}


Now you can explain the rest too. :-)
[color=blue]
> ..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[/color]


Regards,
Erwin Moller

[Lee]

Re: Using equality in 'for' loop - why not?

Mark Anderson said:[color=blue]
>
>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?[/color]

You're setting x=5 and y=1 and seem to be surprised that y!=5.
Without knowing you, it's hard to guess whether this is oversight
or if you really are missing something.

[Stephen Chalmers]

Re: Using equality in 'for' loop - why not?


"Hywel" <hyweljenkins@h otmail.com> wrote in message
news:MPG.1b2172 c03e6af95e98972 7@news.individu al.net...[color=blue]
> In article <c97mqu$ihk$1$8 300dec7@news.de mon.co.uk>, Mark Anderson
> says...[color=green]
> > 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);
> > }[/color]
>
> This fails because after the first loop "y" becomes 12.[/color]

Why is the first iteration performed and how does 'y' become 12?

--
S.C.

[Mick White]

Re: Using equality in 'for' loop - why not?

Stephen Chalmers wrote:

[color=blue]
> Why is the first iteration performed and how does 'y' become 12?
>[/color]
x = 5;
for (y=1; y==5; y+=1) {
alert(x * y);
}[color=blue]
> S.C.[/color]
It doesn't.
The loop never starts, since y doesn't equal 5.

Mick

[Mark Anderson]

Re: Using equality in 'for' loop - why not?

Lee

"Lee" <REM0VElbspamtr ap@cox.net> wrote in message
news:c97pch02pu 7@drn.newsguy.c om...[color=blue]
> Mark Anderson said:[color=green]
> >
> >A 'for' loop takes 3 arguments (initialize; test; increment). The[/color][/color]
'test'[color=blue][color=green]
> >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?[/color]
>[/color]
[color=blue]
> Without knowing you, it's hard to guess whether this is oversight
> or if you really are missing something.
>[/color]
[color=blue]
> You're setting x=5 and y=1 and seem to be surprised that y!=5.[/color]

And so I should be! I'm saying "start with y=1 ad while y is not 5
(ergo less than or equal to 5) do the loop and increment by five.

Your explanation is implying I'm testing in loop #1...

x = 5;
for (y=5; (y===5); etc...

I feel oppressed by all the right-brain thinking here.

Why didn't the blokes (perhaps ladies?) who wrote the spec just say
explicitly don't use ==/!=/===/!== in for loop test if they don't work.
I've looked at the NS docs - and there's no indication that these aren't
allowed.

Please folks I'm not an engineer or a logic PhD. Could posters please
*explain* their put-downs rather than just say you're wrong. I asked the
question both because I don't understand and I wish to understand why.
So far all that I've read is "I'm cleverer than you" answers telling me
I'm wrong. I know! Just please explain why...

Regards

Mark

[Klaus Johannes Rusch]

Re: Using equality in 'for' loop - why not?

Mark Anderson wrote:[color=blue]
> And so I should be! I'm saying "start with y=1 ad while y is not 5
> (ergo less than or equal to 5) do the loop and increment by five.[/color]

You set y to 1 and then loop while y is equal to 5. Since y is not equal
to 5 before the first iteration, the loop body is never executed.
[color=blue]
> Why didn't the blokes (perhaps ladies?) who wrote the spec just say
> explicitly don't use ==/!=/===/!== in for loop test if they don't work.
> I've looked at the NS docs - and there's no indication that these aren't
> allowed.[/color]

They are allowed, and there are cases where they make sense, for example

for (counter = 0; stillrunning == 1; counter++) {
if (something) stillrunning = 0;
}

The condition can be more complex, too -- in any case the way the loop
is processed is such that the condition is checked *before* executing
the loop, so if your condition is not true to start with, the loop is
executed zero times.

Hope that helps

--
Klaus Johannes Rusch
KlausRusch@atme dia.net

http://www.atmedia.net/KlausRusch/

[Evertjan.]

Re: Using equality in 'for' loop - why not?

Mark Anderson wrote on 28 mei 2004 in comp.lang.javas cript:[color=blue]
> This doesn't work...
> x = 5;
> for (y=1; (y==5); y+=1) {
> alert(x * y);
>}[/color]

Yes it works

start with y=1
then do something WHILE y==5

WHICH boolean is not true from the start,
so no alert output !!!
[color=blue]
> ..nor does...
> x = 5;
> for (y=1; (y===5); y+=1) {
> alert(x * y);
>}[/color]

Yes it works

start with y=1
then do something WHILE y===5

WHICH boolean is NOT true from the start,
so no alert output !!!
[color=blue]
> ...but this does..
> x = 5;
> for (y=1; (y<6); y+=1) {
> alert(x * y);
>}[/color]

Yes it surely works

start with y=1
then do something WHILE y<6

WHICH boolean is true when y=1, 2, 3, 4, 5

So 5 times an alert output

=============== =====

btw:
1 the () around y<6 are not needed
2 y+=1 is the same as y++
3 alerts in a loop can drive you crazy
4 using var can sometimes save you trouble when in a function,
because it leaves global values alone

Commonly this is used:

var x = 5;
for (var y=1; y<6; y++) {
document.write( x*y + "<br>");
}


--
Evertjan.
The Netherlands.
(Please change the x'es to dots in my emailaddress)

[Evertjan.]

Re: Using equality in 'for' loop - why not?

Klaus Johannes Rusch wrote on 28 mei 2004 in comp.lang.javas cript:
[color=blue]
> They are allowed, and there are cases where they make sense, for example
>
> for (counter = 0; stillrunning == 1; counter++) {
> if (something) stillrunning = 0;
>}[/color]

No, no sense, but this does:

var stillrunning = 17
for (counter = 0; stillrunning == 1; counter++) {
if (counter==2300 or somethingelse) stillrunning = 1;
}

though I would prefer:

var stillrunning = true
for (counter = 0; stillrunning ; counter++) {
if (counter==2301 or somethingelse) stillrunning = false;
}


--
Evertjan.
The Netherlands.
(Please change the x'es to dots in my emailaddress)

[Mark Anderson]

Re: Using equality in 'for' loop - why not?

Stephen,

"Stephen Chalmers" <me@here.com> wrote in message
news:40b78067_2 @mk-nntp-2.news.uk.tisca li.com...[color=blue]
>
> "Hywel" <hyweljenkins@h otmail.com> wrote in message
> news:MPG.1b2172 c03e6af95e98972 7@news.individu al.net...[color=green]
> > In article <c97mqu$ihk$1$8 300dec7@news.de mon.co.uk>, Mark Anderson
> > says...[color=darkred]
> > > A 'for' loop takes 3 arguments (initialize; test; increment). The[/color][/color][/color]
'test'[color=blue][color=green][color=darkred]
> > > must equate as true or false
> > >
> > > This doesn't work...
> > > x = 5;
> > > for (y=1; (y==5); y+=1) {
> > > alert(x * y);
> > > }[/color]
> >
> > This fails because after the first loop "y" becomes 12.[/color]
>
> Why is the first iteration performed and how does 'y' become 12?
>
> --
> S.C.[/color]

Wait a minute, a 'for' loop excutes *until* test it true. Ergo, on
iteration #1, i=1 and test is 'false' and lop #1 executes - except it
doesn't. Can you please prove y=12 rather than state such a provactive
aasertion without explanation. I posted here to learn not be told I'm
too dumb to understand.

I don't mind rules as long as they can be understood. It seems the logic
on the for loop rests on some abstruse logic that - self-evidently -
isn't obvious.

Even so I'm a pragmatist and I'll go with the flow but it's a shame no
JS book or online tutorial I can find says not to use == or === (or
their negative equivalents) in a for loop as the test.

Regards

Mark

[Mark Anderson]

Re: Using equality in 'for' loop - why not?

Evertjan,

"Evertjan." <exjxw.hannivoo rt@interxnl.net > wrote in message
news:Xns94F7DE8 466340eejj99@19 4.109.133.29...[color=blue]
> Mark Anderson wrote on 28 mei 2004 in comp.lang.javas cript:[color=green]
> > This doesn't work...
> > x = 5;
> > for (y=1; (y==5); y+=1) {
> > alert(x * y);
> >}[/color]
>
> Yes it works
>
> start with y=1
> then do something WHILE y==5
>
> WHICH boolean is not true from the start,
> so no alert output !!!
>[color=green]
> > ..nor does...
> > x = 5;
> > for (y=1; (y===5); y+=1) {
> > alert(x * y);
> >}[/color]
>
> Yes it works
>
> start with y=1
> then do something WHILE y===5
>
> WHICH boolean is NOT true from the start,
> so no alert output !!!
>[color=green]
> > ...but this does..
> > x = 5;
> > for (y=1; (y<6); y+=1) {
> > alert(x * y);
> >}[/color]
>
> Yes it surely works
>
> start with y=1
> then do something WHILE y<6
>
> WHICH boolean is true when y=1, 2, 3, 4, 5
>
> So 5 times an alert output
>
> =============== =====
>
> btw:
> 1 the () around y<6 are not needed
> 2 y+=1 is the same as y++
> 3 alerts in a loop can drive you crazy
> 4 using var can sometimes save you trouble when in a function,
> because it leaves global values alone
>
> Commonly this is used:
>
> var x = 5;
> for (var y=1; y<6; y++) {
> document.write( x*y + "<br>");
> }
>
>
> --
> Evertjan.
> The Netherlands.
> (Please change the x'es to dots in my emailaddress)[/color]

And the prize goes to EvertJan. Now it is explained I understand.
Shame no books tutorials stoop so low as to explain this simple fact.

I now know why I won't use ++ etc., except in the exceptional cases such
as Klaus just described.

Thanks. My faith in Usenet is restored!

Regards

Mark

[Lee]

Re: Using equality in 'for' loop - why not?

Mark Anderson said:[color=blue]
>
>Lee
>
>"Lee" <REM0VElbspamtr ap@cox.net> wrote in message
>news:c97pch02p u7@drn.newsguy. com...[color=green]
>> Mark Anderson said:[color=darkred]
>> >
>> >A 'for' loop takes 3 arguments (initialize; test; increment). The[/color][/color]
>'test'[color=green][color=darkred]
>> >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?[/color]
>>[/color]
>[color=green]
>> Without knowing you, it's hard to guess whether this is oversight
>> or if you really are missing something.
>>[/color]
>[color=green]
>> You're setting x=5 and y=1 and seem to be surprised that y!=5.[/color]
>
>And so I should be! I'm saying "start with y=1 ad while y is not 5
>(ergo less than or equal to 5) do the loop and increment by five.[/color]

No, you've got the sense of the test wrong, and possibly
misunderstand the increment (did you mean to say "increment by one"?).

You're saying "start with y=1 and while y *is* 5 do the loop
and increment by 1".

The loop is performed *while* the condition is true,
not *until* it is true.

If you want to start with y=1 and increment by 1 while y is
not 5, you should use:

for (y=1; y<5; y++)

[Hywel]

Re: Using equality in 'for' loop - why not?

In article <40b78067_2@m k-nntp-2.news.uk.tisca li.com>, Stephen Chalmers
says...[color=blue]
>
> "Hywel" <hyweljenkins@h otmail.com> wrote in message
> news:MPG.1b2172 c03e6af95e98972 7@news.individu al.net...[color=green]
> > In article <c97mqu$ihk$1$8 300dec7@news.de mon.co.uk>, Mark Anderson
> > says...[color=darkred]
> > > 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);
> > > }[/color]
> >
> > This fails because after the first loop "y" becomes 12.[/color]
>
> Why is the first iteration performed and how does 'y' become 12?[/color]

My mistake - I used document.write( y) in a simpler test but forgot to
use something to separate the values.

Mark - the reason your code is wrong is simple: you're testing to see it
y is equal to 5. You're trying you get a loop to work while y is 5. As
you start your loop with y=1, it is never 5. How many people have told
you that?

If you read the JavaScript documentation at
http://developer.netscape.com/ you'll see that the syntax of a for loop
is quite simple:

for ([initial-expression]; [condition]; [increment-expression]) {
statements
}

The fact that the tutorials you're used do not explicitly tell you not
to use == or === is simple: the test will never evaluate in the manner
that you need for your code to continue. The logic is quite clear. For
example, this code
for (x=0; x<5; x++)
says assign the value zero to x. While x is less than 5 increment it.
When x is equal to 5, stop.

Your code
for (y=1; (y==5); y+=1)
says assign the value one to y. While y is equal to 5, increment it.
When, and how, will y ever become 5 in your code?

I appreciate that there is a learning process involved and that it may
not be always clear, but I'm sure you'll find it becomes more obvious
the more you use JavaScript.

--
Hywel I do not eat quiche

http://kibo.org.uk/

http://kibo.org.uk/mfaq.php

[Thomas 'PointedEars' Lahn]

Re: Using equality in 'for' loop - why not?

Mark Anderson wrote:
[color=blue]
> "Stephen Chalmers" <me@here.com> wrote in message
> news:40b78067_2 @mk-nntp-2.news.uk.tisca li.com...[/color]

Please do not write attribution novels, see
<http://netmeister.org/news/learn2quote.htm l>
[color=blue][color=green]
>> "Hywel" [...] wrote [...][color=darkred]
>> > [...] Mark Anderson [...] says...
>> > > This doesn't work...
>> > > x = 5;
>> > > for (y=1; (y==5); y+=1) {
>> > > alert(x * y);
>> > > }
>> >
>> > This fails because after the first loop "y" becomes 12.[/color]
>>
>> Why is the first iteration performed and how does 'y' become 12?
>> [...][/color]
>
> Wait a minute, a 'for' loop excutes *until* test it true. [...][/color]

If you think of `test' as in

for (init; test; loop_stmt)
{
...
}

that is wrong. A "for" loop executes *as long as* `test'
evaluates to `true'. Such a loop is head-controlled,
terminated and thus equivalent to

init;
while (test)
{
...
loop_stmt;
}

That is not equivalent to

init;
do
{
...
loop_stmt;
}
while (test)

as you apparently think.
[color=blue]
> Ergo, on iteration #1, i=1 and test is 'false' and lop #1 executes[/color]

No, it does not, unless `test' is true, i.e. "y" evaluates to 5. If we
assume that this is only a snippet, it may execute as previous statements
may have assigned 5 to y. If we assume that this is the whole code, it
never executes, since no value has been assigned to "y" and thus its
value is `undefined' which is even with type casting not equal to 5.


PointedEars