Personally, I wouldn't bother optimising my code at this level. You might
save a few micro seconds, but consider the bigger picture. There are lots of
other things your program can do that are potentially more time and resource
consuming, eg. db access, disk access, network access, algorithms.
Run a profiler and find out where your program is inefficient. Consider
using worker threads to run lengthy operations.
As for the answer to your original question of which is more efficient ...
should you really care? Perhaps what is more important is code readability.
IMHO, something like
a in [1..3, 5]
is more readable (and fewer keys to type) than
((a >= 3) and (a <= 5)) or (a = 5)
BUT
type
TSomeKind = (
skSomeKind = 1,
skAnotherKind = 2,
skYetAnotherKind = 3,
skYouGuessedIt = 4,
skFinally = 5
);
a in [skSomeKind, skAnotherKind, skYetanotherKind, skFinally]
perhaps better conveys the meaning of the code, assuming the integers 1..5
have special meaning in your program.
Then, if you only had those enumerations to care about, your condition
reduces to
a <> skYouGuessedIt
which is much more readable, and I am sure more efficient than the other
constructs above, if efficiency at this level is that important to you.
Regards,
Dennis.
From: Ross Levis
Sent: Thursday, June 9, 2016 12:00 AM
To: 'NZ Borland Developers Group - Delphi List'
Subject: Re: [DUG] In [set] efficiency
So it’s best to reduce the set size as much as possible. Looking at my
code, I have some variables declared as Word (16-bit) with 65535 possible
values but most are Byte.
From: [email protected]
[mailto:[email protected]] On Behalf Of Peter Ingham
Sent: Wednesday, 8 June 2016 8:15 p.m.
To: [email protected]
Subject: Re: [DUG] In [set] efficiency
(From memory & without firing up a compiler to verify) ...
Sets are implemented as bitmaps with one bit allocated to each possible
value of the set type (hence the limitations on the number of possible
elements in the set).
So if you have
type bytesset = [0..7]; // 8 possible values
var a: bytesset ;
begin
if a in [1,4] then
The set literal [1,4] ends up as a byte with the value $0A.
The test "a in [1,4]" is equivalent to "(a AND $0A) <> 0".
The generated code (for a set of this size) is an AND followed by a
conditional jump. Very efficient.
It gets a little nastier when the possible values gets larger as the size of
the set grows (set of AnsiChar is 32 bytes; Set of Widechar exceeds the
permissible size of a set).
The precise code generated by case varies between jump tables and linear
comparisons.
Regards
On 8/06/2016 5:27 p.m., Steve Peacocke wrote:
If I remember correctly, the compiler changes both to array anyway so you
come out with exactly the same compiled code.
Perhaps someone can confirm this or tell me how wrong I am?
Steve Peacocke
+64 220 612-611
On 8/06/2016, at 4:56 PM, Ross Levis <[email protected]> wrote:
I’m wondering which is more efficient to process...
if (a=1) or (a=2) then ...
or
if a in [1,2] then ...
If the answer is the first method, does it make a difference if more numbers
are checked, eg. if a in [1..3,5] then
Cheers.
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