- **C Programming**
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- - **Optimization of if**
(*http://www.velocityreviews.com/forums/t446110-optimization-of-if.html*)

Optimization of ifHi all,
I have a code like this. if(X) { return retVal1; } if(Y) { return retVal2; } if(Z) { return retVal3; } The above if can be written like this int retVal = 0; if(X) { retVal=retVal1; } if( retVal == 0) { if( Y) retVal=retVal2; } if( retVal == 0 ) { if(Z) retVal=retVal3; } return retVal; which has only one exit point. But it has more If's How do i Optimize this? Please give me some suggestions. |

Re: Optimization of ifRajen wrote:
> Hi all, > I have a code like this. > if(X) > { > return retVal1; > } > > if(Y) > { > return retVal2; > } > > if(Z) > { > return retVal3; > } > > > The above if can be written like this > int retVal = 0; > if(X) > { > retVal=retVal1; > } > > if( retVal == 0) > { > if( Y) > retVal=retVal2; > } > > if( retVal == 0 ) > { > if(Z) > retVal=retVal3; > } > > return retVal; > which has only one exit point. But it has more If's > > How do i Optimize this? Please give me some suggestions. Optimise? What's to optimise? I didn't see anything non-optimal with the first one. (There's a missing case for when none of X, Y, and Z are true.) The second form seems to assume that neither `retVal1` nor `retVal2` are 0. Maybe that's true but it seems to be asking for trouble to rely on it. And that second form is clumsier and harder to understand. Myself I prefer the form: return X ? retVal1 : Y ? retVal2 : Z ? retVal3 : whateverYouWantForNoneOfXYZ ; laid out in whatever way fits your coding style. Optimisation? This is really the bottleneck in your code? Colour me gobsmacked. -- Chris "green/purple hedgehog" Dollin "We live for the One, you die for the One." Unsaid /Babylon 5/. |

Re: Optimization of ifIn article <1170337541.911117.28970@a75g2000cwd.googlegroups. com>,
Rajen <rajenpn@gmail.com> wrote: > I have a code like this. > if(X) > { > return retVal1; > } > > if(Y) > { > return retVal2; > } > > if(Z) > { > return retVal3; > } > > >The above if can be written like this > int retVal = 0; > if(X) > { > retVal=retVal1; > } > > if( retVal == 0) > { > if( Y) > retVal=retVal2; > } > > if( retVal == 0 ) > { > if(Z) > retVal=retVal3; > } > > return retVal; >which has only one exit point. But it has more If's Your first form is much clearer, so use that. If your employer insists on less clear code to satisfy some rules, find another employer. -- Richard -- "Consideration shall be given to the need for as many as 32 characters in some alphabets" - X3.4, 1963. |

Re: Optimization of ifRajen wrote:
> Hi all, > I have a code like this. > if(X) > { > return retVal1; > } > > if(Y) > { > return retVal2; > } > > if(Z) > { > return retVal3; > } > > which has only one exit point. But it has more If's > > How do i Optimize this? Please give me some suggestions. Nothing says the first case generate more than one exit point of your function. BTW some compilers will effectively generate only one exit point and jump to it from each return point, so it far from obvious that first case will not be optimal. Clarity is much more important than implementation dependant low-level assumption. a+, ld. |

Re: Optimization of ifRajen wrote:
> Hi all, > I have a code like this. > if(X) > { > return retVal1; > } > > if(Y) > { > return retVal2; > } > > if(Z) > { > return retVal3; > } > > > The above if can be written like this > int retVal = 0; > if(X) > { > retVal=retVal1; > } > > if( retVal == 0) > { > if( Y) > retVal=retVal2; > } > > if( retVal == 0 ) > { > if(Z) > retVal=retVal3; > } > > return retVal; > which has only one exit point. But it has more If's > > How do i Optimize this? Please give me some suggestions. First, it's extremely unlikely that any optimization is needed. Second, learn about the `else' keyword. -- Eric Sosman esosman@acm-dot-org.invalid |

Re: Optimization of ifLaurent Deniau wrote:
> Rajen wrote: >> Hi all, >> I have a code like this. >> if(X) >> { >> return retVal1; >> } >> >> if(Y) >> { >> return retVal2; >> } >> >> if(Z) >> { >> return retVal3; >> } >> > >> which has only one exit point. But it has more If's >> >> How do i Optimize this? Please give me some suggestions. > > Nothing says the first case generate more than one exit point of your > function. There are three exit points show: each return is an exit point. (It doesn't matter how the compiler compiles it [1]; I suspect the OP is afflicted by Code Style Rules that values Conformance over Clarity.) [1] EG the compiler [2] might translate `return 0;`, `return 1;` and `return -1;` to jumps into the implementation library code to load the constant and then return. One might then say there were /no/ exit points in a function with those return values ... [2] OK, it was an RTL/2 compiler, not a C compiler, optimising for space. -- Chris "electric hedgehog" Dollin There' no hortage of vowel on Uenet. |

Re: Optimization of if"Rajen" <rajenpn@gmail.com> wrote:
> if(X) > { > return retVal1; > } > > if(Y) > { > return retVal2; > } > > if(Z) > { > return retVal3; > } > > The above if can be written like this > int retVal = 0; > if(X) > { > retVal=retVal1; > } > > if( retVal == 0) > { > if( Y) > retVal=retVal2; > } > > if( retVal == 0 ) > { > if(Z) > retVal=retVal3; > } > > return retVal; > which has only one exit point. But it has more If's > > How do i Optimize this? Please give me some suggestions. As others have noted, you're missing a default value. Try this: retval=default_value; if (Z) retval=retval3; if (Y) retval=retval2; if (X) retval=retval1; return retval; Then immediately return to the first, maintainable version, but lay it out a bit more legibly: if(X) return retVal1; if(Y) return retVal2; if(Z) return retVal3; or if you like braces: if(X) { return retVal1; } if(Y) { return retVal2; } if(Z) { return retVal3; } Richard |

Re: Optimization of if"Rajen" <rajenpn@gmail.com> wrote in message
news:1170337541.911117.28970@a75g2000cwd.googlegro ups.com... > Hi all, > I have a code like this. > if(X) > { > return retVal1; > } > > if(Y) > { > return retVal2; > } > > if(Z) > { > return retVal3; > } > > > The above if can be written like this > int retVal = 0; > if(X) > { > retVal=retVal1; > } > > if( retVal == 0) > { > if( Y) > retVal=retVal2; > } > > if( retVal == 0 ) > { > if(Z) > retVal=retVal3; > } > > return retVal; > which has only one exit point. But it has more If's You have three options for optimization. a)First, note in your second example that this is a control flow problem. Your problem is keeping the value of retVal from being multiply assigned. It is more economical to do this with an else-if statement, i.e. if (X) retVal = retVal1; else if (Y) retVal = retVal2; else retVal = retVal3; This suppresses the extra "0" tests and does what you want. Also, if your possibilities are mutually exclusive and/or exhaustive, you can suppress the final Z test. b)Else-if has the limit that in order to get to clause N, all of the preceding N tests must have been evaluated. If your problem is amenable to it, try using the switch() statement. c)Finally, in special cases where switch() can't be used, there are sometimes relationships that let one optimize. For example, assume that I want a fast implementation of floor(log_10(x)) up through 10,000. I could do it like this: if (x < 100) { if (x < 10) { } else { } } else { if (x < 1000) { } else { } } When the domain is "paneled", you can often build an if() construct where the time to reach the innermost clause is related to log(N) where N is the number of cases. Notice that the construct above gets there in 2 tests (rather than up to 4). -- David T. Ashley (dta@e3ft.com) http://www.e3ft.com (Consulting Home Page) http://www.dtashley.com (Personal Home Page) http://gpl.e3ft.com (GPL Publications and Projects) |

Re: Optimization of ifRajen wrote:
> Hi all, > I have a code like this. > if(X) > { > return retVal1; > } > > if(Y) > { > return retVal2; > } > > if(Z) > { > return retVal3; > } [snip] > How do i Optimize this? Please give me some suggestions. The more typical way to write this if you want to have only one return statement is with an if-else if ladder. int retVal = 0; /* or other default value */ if(X) { retVal = retVal1; } else if(Y) { retVal = retVal2; } else if(Z) { retVal = retVal3; } return retVal; This method probably is about as efficient as the one with returns in the if() statements, understanding that such things are up to the implementation. You could also skip the initialization of retVal and put a terminating else that sets it to the "no hit" value, but I'd prefer the method above. If the return value must be one of the three, then set retVal to one of the values and use only two if()'s. Brian |

Re: Optimization of ifrlb@hoekstra-uitgeverij.nl (Richard Bos) writes:
> "Rajen" <rajenpn@gmail.com> wrote: > > if(X) > > { > > return retVal1; > > } > > > > if(Y) > > { > > return retVal2; > > } > > > > if(Z) > > { > > return retVal3; > > } [...] > As others have noted, you're missing a default value. > > Try this: > > retval=default_value; > if (Z) retval=retval3; > if (Y) retval=retval2; > if (X) retval=retval1; > return retval; That's likely to be (slightly) *less* efficient, since it can assign to retval multiple times. It can also behave differently if X, Y, and Z are expressions that depend on each other's evaluation; for example, evaluating Y might not work if X hasn't already been evaluated. > Then immediately return to the first, maintainable version, but lay it > out a bit more legibly: > > if(X) return retVal1; > if(Y) return retVal2; > if(Z) return retVal3; > > or if you like braces: > > if(X) { return retVal1; } > if(Y) { return retVal2; } > if(Z) { return retVal3; } I'd probably lay it out that way if the conditions and result expressions really were that terse, except that I always put a blank after an "if" (it's not a function call, so it shouldn't look like one): if (X) return retVal1; if (Y) return retVal2; if (Z) return retVal3; But in real life, everything is likely to be longer, perhaps too long to fit on a single line. And if the conditions really were "X", "Y", and "Z", I'd seriously consider using names that make some actual sense. -- Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst> San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst> We must do something. This is something. Therefore, we must do this. |

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