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which is better?

utilae · 2005-06-12T19:04:04

Which is better? This: if(X=="A" && Y==1) ;//do stuff else if(X=="A" && Y==2) ;//do stuff else if(X=="B" && Y==1) ;//do stuff else if(X=="B" && Y==2) ;//do stuff Or this: if(X=="A") { if(Y==1) ;//do stuff else if(Y==2) ;//do stuff } else if(X=="B") { if(Y==1) ;//do stuff else if(Y==2) ;//do stuff }

General and Gameplay Programming Programming

Started by utilae June 11, 2005 10:55 PM

18 comments, last by GameDev.net 18 years, 10 months ago

Fred304

382

June 12, 2005 11:19 AM

Assuming X is a char and there are no other possible values than you postet, I'd go for:

switch (((X&1) << 1)|(Y&1)){    case 3:        // do stuff (X=='A', Y==1)        break;    case 2:        // do stuff (X=='A', Y==2)        break;    case 1:        // do stuff (X=='B', Y==1)        break;    case 0:        // do stuff (X=='B', Y==2)        break;}

You could also use an array of function pointers and call the proper function:

void doStuff3(){    // do stuff (X=='A', Y==1)}void doStuff2(){    // do stuff (X=='A', Y==2)}void doStuff1(){    // do stuff (X=='B', Y==1)}void doStuff0(){    // do stuff (X=='B', Y==2)}typedef void (*pfv)();pfv doStuff[] = {doStuff0, doStuff1, doStuff2, doStuff3};void test(char X, int Y){    doStuff[((X&1) << 1)|(Y&1)];}

Maybe the compiler is smart enough to transfer the switch solution into something similar to the array of function pointers solution... dunno.

[Edited by - Fred304 on June 12, 2005 1:19:31 PM]

Conner McCloud

1,135

June 12, 2005 12:12 PM

Quote:Original post by Fred304
Maybe the compiler is smart enough to transfer the switch solution into something similar to the array of function pointers solution... dunno.

If so, I strongly suggest you get a new compiler.

CM

Fred304

382

June 12, 2005 01:10 PM

Quote:Original post by Conner McCloud
Quote:Original post by Fred304
Maybe the compiler is smart enough to transfer the switch solution into something similar to the array of function pointers solution... dunno.

If so, I strongly suggest you get a new compiler.

I meant "similar" in a way that the compiled code does not check against each of the four values and conditionally jumps but rather uses a LUT to jump to the right label. Why would that do any harm?

Conner McCloud

1,135

June 12, 2005 01:20 PM

Quote:Original post by Fred304
I meant "similar" in a way that the compiled code does not check against each of the four values and conditionally jumps but rather uses a LUT to jump to the right label. Why would that do any harm?

Ahh...yeah, making a jump table wouldn't be outrageous. Moving them into proper functions would not be a good move. Either way, you've completely destroyed readability with your solutions, so they're quite sub-optimal whatever the case.

CM

Omaha

100

June 12, 2005 01:20 PM

For small groups of comparisons like this, using an if-elseif-else structure should be more efficient than a switch. If you're working with groups of values that are relatively tightly grouped (I.e. if(x==4 || x==5 || x==6 || ...)) use a switch because then the compiler can probably just generate a jump table. And as the number of potential comparisons (I.e. if you're comparing X and Y against 20 different values) definitely use a switch.

As for which of the original two examples you should use, I concur with the others who say the second. Because if X!="A", precluding optimization, in the first case you get two failed branches and then possibly one more depending on the value of Y. In the second calse, if X!="A", you get one failed branch and then possibly one more, again, dependent on Y. But then again what is it with the SPARC you can actually tell it if the branch is expected to fail and thus that will make it more efficient if it fails... branch delay slots muck it up because then I think it actually becomes GOOD to fall through a branch or something; I can never remember exactly which architecture does what but in this case I'd go with the latter but this is really "Trying to get blood from the rock" optimization...

Genjix

100

June 12, 2005 02:13 PM

Quote:Original post by stroma
second one. but if it grows up i would use switch function for them, if, elif... doesnt looks good

can't use switches on std::string's [smile]

supercoder74

154

June 12, 2005 03:32 PM

Quote:Original post by Genjix
Quote:Original post by stroma
second one. but if it grows up i would use switch function for them, if, elif... doesnt looks good

can't use switches on std::string's [smile]

use the .c_str() function to convert it to a char*.
string s;
s.c_str();//returns char* version of s

I program in my sleep,but when I sleep I use the partition in my head that doesnt have g++ or the .net library, so im kinda screwed.

Anonymous

June 12, 2005 03:50 PM

Quote:Original post by supercoder74
Quote:Original post by Genjix
Quote:Original post by stroma
second one. but if it grows up i would use switch function for them, if, elif... doesnt looks good

can't use switches on std::string's [smile]

use the .c_str() function to convert it to a char*.
string s;
s.c_str();//returns char* version of s

Switching on char *'s is a bad idea because it probably won't do what you want. Actually, I'm not positive it even works. If it does work, then:

#include <stdio.h>int main(void) {  char str[] = "A";  switch (str) {    case "A": puts("A"); break;    default: puts("Not A"); break;  }  return 0;}

Will print "Not A".

If you want to do this with strings, you really need an if/else ladder, a map, or something of that sort.

Omaha

100

June 12, 2005 05:54 PM

Quote:Switching on char *'s is a bad idea because it probably won't do what you want. Actually, I'm not positive it even works. If it does work, then:
*** Source Snippet Removed ***
Will print "Not A".

If you want to do this with strings, you really need an if/else ladder, a map, or something of that sort.

Actually, the compiler, seeing that all three are the same and knowing that that data will be stored in static memory may very well in that instance have "A"="A";

Everyone loves the unpredictability of compilation!

Seriously though, just avoid the whole thing and use strcmp or one of your own functions or something depending on the usage.

Anonymous

June 12, 2005 07:04 PM

Quote:Original post by Omaha
Quote:Switching on char *'s is a bad idea because it probably won't do what you want. Actually, I'm not positive it even works. If it does work, then:
*** Source Snippet Removed ***
Will print "Not A".

If you want to do this with strings, you really need an if/else ladder, a map, or something of that sort.

Actually, the compiler, seeing that all three are the same and knowing that that data will be stored in static memory may very well in that instance have "A"="A";

Everyone loves the unpredictability of compilation!

Seriously though, just avoid the whole thing and use strcmp or one of your own functions or something depending on the usage.

Actually, I specifically made str its own array so that it would print "Not A" even in the presence of a clever compiler. If str had been a pointer, it may have printed "A" or "Not A" depending on how "clever" the compiler is.

As you say, though, the point is that strcmp() is the proper way to compare C-strings.

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