efficiency of && vs. array

[christopher]

hi,

i'm working with data for a clinical trial. i write data edit-checks
using a program called DataFax, which has its own built-in programming
language which is loosely based on C. as such, the code example might
not have the exact same syntax as C, but the principle is the same.

i'm working on a edit check that will test a series of values, like
so:

if (a == 1 && b == 1 && c == 1 && d == 1)
then....

it occured to me that i could store a, b, c, and d in an array and use
an iterator perform the same function:

number i = 1;
group TEST a, b, c, d; /* create an array called TEST and store a,b,c,d
*/
if (condition) {
while (i <= 4) {
if (TEST == 1) {...;}
i = i + 1;}
then....

my question is: which one of these methods is the best practice? is one
inherently more efficient than the other? am i making sense?

thanks,

chris

 

[pete]

hi,

i'm working with data for a clinical trial. i write data edit-checks
using a program called DataFax, which has its own built-in programming
language which is loosely based on C. as such, the code example might
not have the exact same syntax as C, but the principle is the same.

i'm working on a edit check that will test a series of values, like
so:

if (a == 1 && b == 1 && c == 1 && d == 1)
then....

it occured to me that i could store a, b, c, and d in an array and use
an iterator perform the same function:

number i = 1;
group TEST a, b, c, d; /* create an array called TEST and store a,b,c,d
*/
if (condition) {
while (i <= 4) {
if (TEST == 1) {...;}
i = i + 1;}
then....

my question is: which one of these methods is the best practice?
is one
inherently more efficient than the other? am i making sense?

The first way is computationally less intensive.

 

[August Karlstrom]

i'm working with data for a clinical trial. i write data edit-checks
using a program called DataFax, which has its own built-in programming
language which is loosely based on C. as such, the code example might
not have the exact same syntax as C, but the principle is the same.

i'm working on a edit check that will test a series of values, like
so:

if (a == 1 && b == 1 && c == 1 && d == 1)
then....

it occured to me that i could store a, b, c, and d in an array and use
an iterator perform the same function:

number i = 1;
group TEST a, b, c, d; /* create an array called TEST and store a,b,c,d
*/
if (condition) {
while (i <= 4) {
if (TEST == 1) {...;}
i = i + 1;}
then....

my question is: which one of these methods is the best practice? is one
inherently more efficient than the other? am i making sense?

My recommendation is to program for maximum clarity. The difference in
efficiency is probably negligible (you know the three rules of
optimization, right?). What representation to use depends on where the
variables a, b, c and d come from. Are they related? How are they related?


August

 

[christopher]

no - i don't know the three rules of optimization. what are they?

 

[pete]

no - i don't know the three rules of optimization. what are they?

I can only remember the first two:

First rule of optimization: "Don't do it."
Second rule (for experts only): "Don't do it yet."

http://groups.google.com/group/comp.lang.c/msg/ae12fa3f503ee66f

 

[Richard Heathfield]

christopher said:

no - i don't know the three rules of optimization. what are they?

I thought there were only two. They are:

Rule 1: Don't do it.

Rule 2 (for experts only): Don't do it /yet/.

My candidate for Rule 3 would be: If you must do this damn silly thing,
don't do it in this damn silly way.

 

[Chris Dollin]

christopher said:


I thought there were only two. They are:

Rule 1: Don't do it.

Rule 2 (for experts only): Don't do it /yet/.

My candidate for Rule 3 would be: If you must do this damn silly thing,
don't do it in this damn silly way.

Mine would be: find out where to optimise by measurement, not guesswork.

 

[Christian Bau]

hi,

i'm working with data for a clinical trial. i write data edit-checks
using a program called DataFax, which has its own built-in programming
language which is loosely based on C. as such, the code example might
not have the exact same syntax as C, but the principle is the same.

i'm working on a edit check that will test a series of values, like
so:

if (a == 1 && b == 1 && c == 1 && d == 1)
then....

it occured to me that i could store a, b, c, and d in an array and use
an iterator perform the same function:

number i = 1;
group TEST a, b, c, d; /* create an array called TEST and store a,b,c,d
*/
if (condition) {
while (i <= 4) {
if (TEST == 1) {...;}
i = i + 1;}
then....

my question is: which one of these methods is the best practice? is one
inherently more efficient than the other? am i making sense?

It occured to me that you could write a, b, c, and d to a file and read
them back from that file everytime you need them. You could then use
some encryption to make the process of reading from the file more
secure.

You are not by any chance working on any IT project paid for by the
British government?

 

[Christian Bau]

no - i don't know the three rules of optimization. what are they?

Don't do it.
Don't do it yet.
Don't do it unless you measure the effect.

 

[tmp123]

hi,

i'm working with data for a clinical trial. i write data edit-checks
using a program called DataFax, which has its own built-in programming
language which is loosely based on C. as such, the code example might
not have the exact same syntax as C, but the principle is the same.

i'm working on a edit check that will test a series of values, like
so:

if (a == 1 && b == 1 && c == 1 && d == 1)
then....

it occured to me that i could store a, b, c, and d in an array and use
an iterator perform the same function:

number i = 1;
group TEST a, b, c, d; /* create an array called TEST and store a,b,c,d
*/
if (condition) {
while (i <= 4) {
if (TEST == 1) {...;}
i = i + 1;}
then....

my question is: which one of these methods is the best practice? is one
inherently more efficient than the other? am i making sense?

thanks,

chris

Assuming that the language you use is like C, and the "..." doesn't
suppress any important details of the variant, then:

1) The expresssion handling of C fix that if, by example, if a==1 is
false, the remainder expression is not evaluated. However, in the loop
version, it seems that all the comparations will be done before to
execute or not the "then" clause.

2) Use TEST instead of a named variable goes, in most part of
compilers, to more CPU expensive evaluation:
* "a" could be translated with: 1) read memory at address of a.
* "TEST" could be evaluated with: 1) add i to TEST address; 2) read
memory"

3) And, of course, there are also the time to init "i", ...

Kind regards.

 

[Richard Heathfield]

Chris Dollin said:

Mine would be: find out where to optimise by measurement, not guesswork.

That's another way of saying the same thing.

Actually, please forgive me for saying so, Chris, but I think mine is more
generally useful.

 

[Laurent Deniau]

hi,

i'm working with data for a clinical trial. i write data edit-checks
using a program called DataFax, which has its own built-in programming
language which is loosely based on C. as such, the code example might
not have the exact same syntax as C, but the principle is the same.

i'm working on a edit check that will test a series of values, like
so:

if (a == 1 && b == 1 && c == 1 && d == 1)
then....

it occured to me that i could store a, b, c, and d in an array and use
an iterator perform the same function:

number i = 1;
group TEST a, b, c, d; /* create an array called TEST and store a,b,c,d
*/
if (condition) {
while (i <= 4) {
if (TEST == 1) {...;}
i = i + 1;}
then....

my question is: which one of these methods is the best practice? is one
inherently more efficient than the other? am i making sense?

Assuming that you have considered the rules from previous post on
premature optimisation and decided that they do not apply to your case,
and assuming that a, b, c, d are of integer type then:

if (1 & a & b & c & d) {

}

will be probably slightly faster since it generates only one test which
should be better on processors with branch prediction.

a+, ld.

 

[Chris Dollin]

Chris Dollin said:


That's another way of saying the same thing.

Well ... certainly mine is an instance of yours, but yours applies to
more situations than optimisation [1] ...

Actually, please forgive me for saying so, Chris, but I think mine is more
generally useful.

[2]


.... but gives less specific guidance; which works best is likely
target-dependent.

How about a revised your-3:

3. Don't be stupid about it.

[1] EG swimming from London to Paris by way of Seattle.

[2] Forgive? Moi? My grudges are held for eternity! No second of my time
will be wasted in avoiding plotting your downfall!! My arcane influence
is unavoidable!!!

<fx:knocking/>

Why, there's Alec Guinness and his friends, come for a pint!!!!

 

[Richard Heathfield]

Laurent Deniau said:

Assuming that you have considered the rules from previous post on
premature optimisation and decided that they do not apply to your case,
and assuming that a, b, c, d are of integer type then:

if (1 & a & b & c & d) {

}

will be probably slightly faster since it generates only one test which
should be better on processors with branch prediction.

But it doesn't test a, b, c and d against 1. Rather, it checks whether they
have the least significant bit set. Not the same thing.

 

[Richard Heathfield]

Chris Dollin said:

How about a revised your-3:

3. Don't be stupid about it.

That's even more widely applicable, as it applies to absolutely everything.

 

[Laurent Deniau]

Laurent Deniau said:




But it doesn't test a, b, c and d against 1. Rather, it checks whether they
have the least significant bit set. Not the same thing.

Are you sure?

a+, ld.

 

[tmp123]

Are you sure?

a+, ld.

a=3;

if (a==1) /* jumps to else */
if(1&a) /* jumps to then */

(not tested).

 

[Laurent Deniau]

Laurent Deniau said:




But it doesn't test a, b, c and d against 1. Rather, it checks whether they
have the least significant bit set. Not the same thing.

Sorry you are right (I had bool in mind):

if ((1 & a & b & c & d) == (1 | a | b | c | d)) {
}

should be better.

a+, ld.

 

[Chris Dollin]

Chris Dollin said:


That's even more widely applicable, as it applies to absolutely
everything.

In context, `it` is bound to `optimisation` ...

I'm stopping now. Buy you a pint sometime?

 

[Richard Heathfield]

Chris Dollin said:

In context, `it` is bound to `optimisation` ...

....and optimisation is bound t... oh, skip it. It wasn't all that funny
anyway.

I'm stopping now.

Good plan.

Buy you a pint sometime?

Better plan - although finding the time could be an insoluble opportunity!