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From: Dan Purgert <dan@djph.net>
Newsgroups: comp.lang.c
Subject: Re: else ladders practice
Date: Thu, 31 Oct 2024 14:14:49 -0000 (UTC)
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On 2024-10-31, fir wrote:
> somethins i got such pices of code like
>
> if(n==1) {/*something/}
> if(n==2) {/*something/}
> if(n==3) {/*something/}
> if(n==4) {/*something/}
> if(n==5) {/*something/}
>
> technically i would need to add elses - but the question is if to do that
>
> do teh code has really chance to be slower without else (except some 
> very prmitive compilers) ??

In the above, all conditionals are always checked -- that is the truth
of a previous conditional statement has no bearing on subsequent tests.
This leads to the potential of tests going off in directions you hadn't
necessarily anticipated.

However, 'if..elseif..else' will only check subsequent conditionals if
the prior statements were false.  So for the case that "n=2", you're
only ever testing the two cases "if (n==1)" (which is false) and
"elseif(n==2)".  The computer just skips to the end of the set of
statements.


Given this MWE (my own terrible code aside ;) ):

  int main(){
    int n=0;
    printf ("all if, n=%u\n",n);
    if (n==0) { printf ("n: %u\n",n); n++;}
    if (n==1) { printf ("n: %u\n",n); n++;}
    if (n==2) { printf ("n: %u\n",n); n++;}
    if (n==3) { printf ("n: %u\n",n); n++;}
    if (n==4) { printf ("n: %u\n",n); n++;}
    printf ("all if completed, n=%u\n",n);
 
    n=3;
    printf ("with else if, n=%u\n",n);
    if (n==0) { printf ("n: %u\n",n); n++;}
    else if (n==1) { printf ("n: %u\n",n); n++;}
    else if (n==2) { printf ("n: %u\n",n); n++;}
    else if (n==3) { printf ("n: %u\n",n); n++;}
    else { printf ("n: %u\n",n); n++;}
    printf ("with else if completed, n=%u\n",n);
  } 


You'll get the output:

all if, n=0
n: 0
n: 1
n: 2
n: 3
n: 4
all if completed, n=5
with else if, n=3
n: 3
with else if completed, n=4

HTH :)

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