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From: conor@lion.inmos.co.uk (Conor O'Neill)
Newsgroups: comp.lang.c
Subject: Re: When do you use "if ( a = b )"? (was Re: Funny mistake)
Message-ID: <15053@ganymede.inmos.co.uk>
Date: 25 Mar 91 11:52:23 GMT
References: <8148@rsiatl.Dixie.Com> <15481@smoke.brl.mil> <775@camco.Celestial.COM> <65837@eerie.acsu.Buffalo.EDU> <1991Mar18.195351.11985@unlv.edu> <11109@dog.ee.lbl.gov>
Sender: news@inmos.co.uk
Reply-To: conor@inmos.co.uk (Conor O'Neill)
Organization: INMOS Limited, Bristol, UK.
Lines: 46

To me it is clear that using assignments inside conditions is very bad
style because it makes the intention of the original programmer unclear.

The only sort of situation which I can think of where I _might_ consider
using it is something like the following:

if ( (x != 0) && ((a = expensive_function(x)) != 0) )
  {
    ...  do something involving 'a'
  }
else if ...  more complicated tests

This example cannot simply move the assignment to 'a' _out of_ the 'if',
because it is conditional on another condition, which may in itself be
complicated. The assignment cannot be moved _into_ the 'if', because
that would change the behaviour of the 'else' clauses.

Of course, there are a multiplicity of ways in which this small example
can be re-written so that the assignment is not inside the condition,
and most of them are probably clearer to read. I would use an alternative.

PS - The INMOS ANSI C compiler produces identical code for the following,
and warns about one of them:

lion/test(48)% cat -n c.c
     1  int main(void)
     2  {
     3    int a, b ;
     4    a = b;
     5    if (a)            b = 99;
     6    a = b;
     7    if (a != 0)       b = 99;
     8    if (a = b)        b = 99; /* This is the confusing one */
     9    if ((a = b) != 0) b = 99;
    10    return 99;
    11  }
lion/test(49)% icc c.c
Warning-icc-c.c(8)- use of '=' in condition context
lion/test(50)%

This warning can be suppressed by a command line switch.

---
Conor O'Neill, Software Group, INMOS Ltd., UK.
UK: conor@inmos.co.uk		US: conor@inmos.com
"It's state-of-the-art" "But it doesn't work!" "That is the state-of-the-art".