Pecobians.com :: Just A Small C Question [JustASC] 2

shantanu · 11 Feb 2005

Whats the most concise way you can replace an if statement of the sort:

if(a==b)
{
c=d;
}
with. Remember, there is no else part here...
Hint: The solution lies in a very basic property of C compilers...

shantanu · 11 Feb 2005

Whats the matter guys...no replies today???
atleast try....

Rahul Bhandari · 25 May 2005

is it by DO WHILE and if not then plz tell the answer on my private message

shantanu · 11 Feb 2005

@rahul: the answer that i have in mind doesnt have anything to do with a do while statement, neither can i think of a way with which i can make a more conciser solution using do while...Please share with us what u exactly had in mind cuz I may be wrong and that'll make me learn too...

Neways, the answer that i have will be put on the forum in a day or two, so can't PM u right now...just wait a bit...

kashyap9_navjot · 17 Mar 2005

Don't know dude...
Only thing I can think of is to remove the braces..
if(a== b) c=d;

shantanu · 11 Feb 2005

well, i'll give you a hint...
The solution will convert the if block into a single statement. This is true for ur answer too, navjot, but only when there is a single statement inside the braces. The solution that i've in mind will work for multiple statements inside the braces too, and convert the whole block into a single statement...(BTW, the "if" keyword won't be there in that solution, that much i can tell u.)

Whacko · 09 Jul 2005

maybe something of sort

( a==b ? c ? x)

plz make clear that can we assume a value in else case

shantanu · 11 Feb 2005

@whacko: I have already mentioned it there should is "no" else statement, so you can't assume anything for that..

Whacko · 09 Jul 2005

using the macros

#define (a==b) (c=d);

shantanu · 11 Feb 2005

@whacko: I cudnt understand how ur last solution wud achieve the purpose...
Neways, the solution has to support multiple lines withing the if block too..

nmadnani · 14 Feb 2005

OK, so I just saw this and I have some comments and some questions.

1) Are you asking us what the compiler *could* do to optimize this code block or what it definitely *will* do?

2) Are you sure we don't need to know any other information about the relationships between a,b,c and d?

In any case, here are a few comments:

* If the compiler can figure out earlier in the program (by constant folding, CSE, SSA and other techniques) that a will *never* be equal to b, then it can do what's called "dead code elimination" and basically remove that entire block (since it's dead code) but this doesn't seem like what you want.

* Hmm ... if you don't have any information about whether the boolean expression (a==b) evaluates to True or False, then I can't think of what sort of optimizations you can apply at compilation time.

If you have given us all the information that we needed to know, then I am stumped ! Smile

Wicked Sunny · 11 Feb 2005

c=a=b?d:c;
not my answer.. one of my cubicle mate saw t his post and said this will be the reply!!!!

shantanu · 11 Feb 2005

@wicked_sunny:
sorry, wrong answer on four accounts:
1. This statements will always give a true result and c will always be equal to d.
It should be written as c=(a==b)?d:c;

2. Even then, it includes an "else" part (":c") which i have specified in the problem that it shouldn't be there...

3. It is only true for the simple case of an assignment statement present within the if block (which i gave as an example) while i've specified that the block can have any type of arithmetic statements.

4. It can't be used for multiple statements within the if block....

nmadnani · 14 Feb 2005

Oh you have to got to be kidding me !! That's all you wanted ... boy, do I feel dumb !! I think the answer should be this:

(a==b) && (c=d);

The first boolean subexpression short-circuits the entire thing if it's not true and therefore 'c=d' is never evaluated unless a is equal to b.

shantanu · 11 Feb 2005

@nitin: Yupz u got it right once again...simple, isn't it?? Smile

and the beauty is that u can chain any no. of expressions to convert the whole block into a single statement...

like

(a==b) && (c=d) && (e=f+g) && (h=i*4); etc etc....

Since after the first condition (u can even chain conditions), all the expressions evaluate to true so all of them will be evaluated...

New question soon...