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[bloodline]

I have a list that I want to compare a value against to determine the value's "rank", the problem is that once I have determined the rank I wish to end the search... In ASM this is easy (please bare with my half remembered 68k):

_start  move.l _number,d0
          cmpi.l #4185,d0
          blt.s _case4184
          cmpi.l #4507,d0
          blt.s _case4507
          cmpi.l #4883,d0
          blt.s _case4883
          cmpi.l #5327,d0
          blt.s _case5327
          move.l 0,d0
_break

           rts

_case4185
           move.l #4,d0
           jmp _break

_case4507
           move.l #3,d0
           jmp _break

_case4883
           move.l #2,d0
           jmp _break

_case5327
           move.l #1,d0
           jmp _break

But I want to write this in C... Suffice to say the actual list is much larger and CPU time is at a premium, is there any way I can write it without using Goto?

[jorkany]

Try using a switch statement. It should produce code very similar to what you have posted, usually using a jump table but check the asm output to make sure it's acceptable.



Also, you might want to consider examining your algorythmic boundaries! (j/k)

[TheBilgeRat]

Originally Posted by jorkany

Try using a switch statement. It should produce code very similar to what you have posted, usually using a jump table but check the asm output to make sure it's acceptable.



Also, you might want to consider examining your algorythmic boundaries! (j/k)

I see what you did there

[bloodline]

Originally Posted by jorkany

Try using a switch statement. It should produce code very similar to what you have posted, usually using a jump table but check the asm output to make sure it's acceptable.



Also, you might want to consider examining your algorythmic boundaries! (j/k)

Can't use a switch, notice that I score the number based on it being lower than a threshold value... The problem is that the threshold boundaries are not linear (and for the purpose of this task can't be calculated... I'm probably going to have a loop that compares against an array of threshold values. I was just hoping it could be done in a nice simple table for maximum performance

[Thorham]

Originally Posted by bloodline

The problem is that the threshold boundaries are not linear (and for the purpose of this task can't be calculated...

What are those values exactly?

Originally Posted by bloodline

I'm probably going to have a loop that compares against an array of threshold values. I was just hoping it could be done in a nice simple table for maximum performance

If you can have an array of threshold values, then you might still have your table. How big is the largest value?

[bloodline]

Originally Posted by Thorham

What are those values exactly?

If you can have an array of threshold values, then you might still have your table. How big is the largest value?

The values are rotational time values and I need to set an index dependant on a specific time. They scale somewhat geometrically, but not exactly, and I don't have enough CPU time to calculate that anyway :-/

The largest value is 60,000 (a lookup table is totally out of the question)

[Thorham]

Originally Posted by bloodline

The values are rotational time values and I need to set an index dependant on a specific time. They scale somewhat geometrically, but not exactly, and I don't have enough CPU time to calculate that anyway :-/

I was going to say that you don't have to calculate anything if you know the thresholds beforehand, and can just use a table, but ...

Originally Posted by bloodline

The largest value is 60,000 (a lookup table is totally out of the question)

... if a 60000 entry long array is out of the question, then what's the target system (memory constraints)? Is such a table too large? Or is the problem that the thresholds are floating point? If it's floating point, and you don't need more than three decimals of precision (which 60,000 leads me to believe), then a table may still possible (fixed point).

Please provide us with some more information here, because you've left out crucial details.

[Thorham]

How long is that list exactly? If it's not too large you can use a table, which would certainly be the fastest way.

[SamuraiCrow]

Originally Posted by bloodline

I have a list that I want to compare a value against to determine the value's "rank", the problem is that once I have determined the rank I wish to end the search... In ASM this is easy (please bare with my half remembered 68k):

_start  move.l _number,d0
          cmpi.l #4185,d0
          blt.s _case4184
          cmpi.l #4507,d0
          blt.s _case4507
          cmpi.l #4883,d0
          blt.s _case4883
          cmpi.l #5327,d0
          blt.s _case5327
          move.l 0,d0
_break

           rts

_case4185
           move.l #4,d0
           jmp _break

_case4507
           move.l #3,d0
           jmp _break

_case4883
           move.l #2,d0
           jmp _break

_case5327
           move.l #1,d0
           jmp _break

But I want to write this in C... Suffice to say the actual list is much larger and CPU time is at a premium, is there any way I can write it without using Goto?

int test(num)
{
  if (num<4185)return 4;
  if (num<4507)return 3;
  if (num<4883)return 2;
  if (num<5327)return 1;
  return 0;
}

This is an exact conversion of the Assembly code you gave and it doesn't use any goto statements.

[bloodline]

Originally Posted by SamuraiCrow

int test(num)
{
  if (num<4185)return 4;
  if (num<4507)return 3;
  if (num<4883)return 2;
  if (num<5327)return 1;
  return 0;
}

This is an exact conversion of the Assembly code you gave and it doesn't use any goto statements.

Damn, I couldn't see for looking! Cheers hahahah

[Fats]

Originally Posted by SamuraiCrow

int test(num)
{
  if (num<4185)return 4;
  if (num<4507)return 3;
  if (num<4883)return 2;
  if (num<5327)return 1;
  return 0;
}

If you don't want to do it in a function you could use ? : operator like:

  rank =
    (num<4185) ? 4 :
    (num<4507) ? 3 :
    (num<4883) ? 2 :
    (num<5327) ? 1 :
    0;

greets,
Staf.

[bloodline]

Ahhh! Good call, I'm quite comfortable with usig a function, but I won't forget this either!! Many thanks for your help

[Duce]

Threads like this are what makes A.org great. People helping people out.

[bloodline]

Originally Posted by Duce

Threads like this are what makes A.org great. People helping people out.

There are loads of great programmer on A.org... Odd actually that there isn't a dedicated forum for programming issues?

[Zac67]

Actually, comparing against an array uses less RAM all in all (just the values and the loop, you save the space of all but one cmp and blt ops minus loop ops). On a somewhat caching CPU it should also perform better since the loop needs only be fetched once and then just the array values get read - the slowest is always the memory. Without cache your approach will be faster, saving the loop overhead.

Also depends on the indexing range the CPU is capable of, of course...

You could also combine both methods with a larger loop (did that once on a 6502 to save the few cycles I needed).