RE: [PHP] IF statement madness
Comparing a float with an integer can have problems. You could try something
like:
if(abs($i - $target) .1) {
//then they are essentially equal
}
Kirk
-Original Message-
From: James E Hicks III [mailto:[EMAIL PROTECTED]
Sent: Friday, March 14, 2003 11:22 AM
To: [EMAIL PROTECTED]
Subject: [PHP] IF statement madness
Help save my sanity! What can I do to the IF statement in the
following code to
make it print the line that says By God they are equal in
value.? I have tried
the following changes;
1. using === instead of ==
2. placing (float) in front of the $i and $target
inside and before the IF
statement.
?
$start = 215;
$end = 217;
$target = 216;
for ($i=$start; $i=$end; $i+=.1){
if ( $i == $target ){
echo (BR$i - $target, By God, the are
equal in value.);
} else {
echo (BR$i - $target, Eternal Damnation,
they aren't
equal!);
}
}
?
James E Hicks III
Noland Company
2700 Warwick Blvd
Newport News, VA 23607
757-928-9000 ext 435
[EMAIL PROTECTED]
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Re: [PHP] IF statement madness
At 19:21 14.03.2003, James E Hicks III said:
[snip]
Help save my sanity! What can I do to the IF statement in the following
code to
make it print the line that says By God they are equal in value.? I have
tried
the following changes;
1. using === instead of ==
2. placing (float) in front of the $i and $target inside and before
the IF
statement.
?
$start = 215;
$end = 217;
$target = 216;
for ($i=$start; $i=$end; $i+=.1){
if ( $i == $target ){
echo (BR$i - $target, By God, the are equal in value.);
} else {
echo (BR$i - $target, Eternal Damnation, they aren't
equal!);
}
}
?
[snip]
Before the if statement in your loop place this line:
echo \$i is now: $ibr /;
You'll notice that most probably your $i variable will miss $target by some
hunderths or thousandths, since you're working with it as a float. This is
due to the way fp numbers are stored and calculated (using exponents and
mantissa).
Workaround: don't use floats if you want to hit a certain value at the spot:
?
$start = 215;
$end = 217;
$target = 216;
for ($i=($start*10); $i=($end*10); $i++){
if ( $i == ($target*10) ){
echo (BR$i - $target, By God, the are equal in value.);
} else {
echo (BR$i - $target, Eternal Damnation, they aren't
equal!);
}
}
?
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RE: [PHP] IF statement madness
What happens if you change $target to 216.0?
-- jon
-Original Message-
From: James E Hicks III [mailto:[EMAIL PROTECTED]
Sent: Friday, March 14, 2003 1:22 PM
To: [EMAIL PROTECTED]
Subject: [PHP] IF statement madness
Help save my sanity! What can I do to the IF statement in the following code
to
make it print the line that says By God they are equal in value.? I have
tried
the following changes;
1. using === instead of ==
2. placing (float) in front of the $i and $target inside and before the IF
statement.
?
$start = 215;
$end = 217;
$target = 216;
for ($i=$start; $i=$end; $i+=.1){
if ( $i == $target ){
echo (BR$i - $target, By God, the are equal in value.);
} else {
echo (BR$i - $target, Eternal Damnation, they aren't
equal!);
}
}
?
James E Hicks III
Noland Company
2700 Warwick Blvd
Newport News, VA 23607
757-928-9000 ext 435
[EMAIL PROTECTED]
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RE: [PHP] IF statement madness
-Original Message-
From: James E Hicks III [mailto:[EMAIL PROTECTED]
Sent: 14 March 2003 18:22
Help save my sanity! What can I do to the IF statement in the
following code to
make it print the line that says By God they are equal in
value.? I have tried
the following changes;
1. using === instead of ==
2. placing (float) in front of the $i and $target
inside and before the IF
statement.
?
$start = 215;
$end = 217;
$target = 216;
for ($i=$start; $i=$end; $i+=.1){
if ( $i == $target ){
echo (BR$i - $target, By God, the are
equal in value.);
} else {
echo (BR$i - $target, Eternal Damnation,
they aren't
equal!);
}
}
?
Rule 1: DON'T USE FLOATS AS LOOP CONTROL VARIABLES.
Rule 2: DON'T COMPAR|E CALCULATED FLOATING POINT VALUES FOR EQUALITY.
By the vary nature of how floating-point numbers are stored on computers,
there is a tiny inaccuracy in all floating point operations. In many cases
this dosn't matter -- the inaccuracy is beyond the level of significance.
However, if you compare two calculated values to each other, you must always
allow for the possiblity of that tiny inaccuracy -- for example, don't do:
if ((10.0/3.0)*3.0 == 10.0) // *never* true!
but instead something like:
if (abs((10.0/3.0)*3.0 - 10.0) 1e-30) // adjust 1e-30 to suit
In loops like the one you've written, you compound the inaccuracy by doing
many floating-point additions to the same variable -- by the time you reach
your endpoint, you're probably significantly off. The only way to truly
compensate for this is to use integers to control your loop, and divide down
by the appropriate power of 10 at the start of each iteration, so:
$start = 215;
$end = 217;
$target = 216;
for ($ii=$start*10; $ii=$end*10; $ii++){
$i = $ii/10.0;
// etc.
}
Cheers!
Mike
-
Mike Ford, Electronic Information Services Adviser,
Learning Support Services, Learning Information Services,
JG125, James Graham Building, Leeds Metropolitan University,
Beckett Park, LEEDS, LS6 3QS, United Kingdom
Email: [EMAIL PROTECTED]
Tel: +44 113 283 2600 extn 4730 Fax: +44 113 283 3211
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RE: [PHP] IF statement madness
Thank you (all who resonded)!!! It makes sense now.
Now I can wait until 5:00pm, quitting time, to go crazy!!
James
-Original Message-
From: Johnson, Kirk [mailto:[EMAIL PROTECTED]
Sent: Friday, March 14, 2003 1:23 PM
To: [EMAIL PROTECTED]
Subject: RE: [PHP] IF statement madness
Comparing a float with an integer can have problems. You could try something
like:
if(abs($i - $target) .1) {
//then they are essentially equal
}
Kirk
-Original Message-
From: James E Hicks III [mailto:[EMAIL PROTECTED]
Sent: Friday, March 14, 2003 11:22 AM
To: [EMAIL PROTECTED]
Subject: [PHP] IF statement madness
Help save my sanity! What can I do to the IF statement in the
following code to
make it print the line that says By God they are equal in
value.? I have tried
the following changes;
1. using === instead of ==
2. placing (float) in front of the $i and $target
inside and before the IF
statement.
?
$start = 215;
$end = 217;
$target = 216;
for ($i=$start; $i=$end; $i+=.1){
if ( $i == $target ){
echo (BR$i - $target, By God, the are
equal in value.);
} else {
echo (BR$i - $target, Eternal Damnation,
they aren't
equal!);
}
}
?
James E Hicks III
Noland Company
2700 Warwick Blvd
Newport News, VA 23607
757-928-9000 ext 435
[EMAIL PROTECTED]
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