Complexity of if statement

Question

I am wondering complexity of following if statement

if (isTrue()) //case 1

VS

if(isTrue()==true) //case 2

And isTrue defined as

boolean isTrue(){
//lots of calculation and return true false based on that.
 return output;
}

I was thinking, complexity of if (isTrue()) is lower then if(isTrue()==true) because on case 2 require additional comparison for equals.

What about space complexity?

Any different thought?

because compiler have to check function output with its right side value and then to test with if condition — minhaz, Mar 16 '13 at 19:24
@minhaz Most compilers would probably just optimize out those `if`s entirely, since `isTrue` will only ever return `true`. — Jeffrey, Mar 16 '13 at 19:28
I guess thats bad example, consider that function has one million lines of code. — minhaz, Mar 16 '13 at 19:29
@minhaz There's no difference between `if (foo)` and `if (foo == true)`. `if (foo)` still has to check `foo` for true. — Dave Newton, Mar 16 '13 at 19:32
@DaveNewton That's not correct. By the same logic you give, 'if (foo == true)' has to check 'foo == true' for being true. A compiler would be expected to optimise all the excess crud away but it's not the same thing. — user207421, Mar 17 '13 at 00:20

score 3 · Answer 1 · answered Mar 16 '13 at 19:20

3

Both of them are same in speed/space. But second way is weird for C/C++ programmers.

The different is, second way is just less readable.

answered Mar 16 '13 at 19:20

masoud

55,379
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5

and weird for Java programmers too. – TofuBeer Mar 16 '13 at 19:21

score 2 · Answer 2 · answered Mar 16 '13 at 19:20

2

They are equivalent. And when doing global optimizations condition is removed altogether.

answered Mar 16 '13 at 19:20

Valeri Atamaniouk

5,125
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if complier does the optimization that also increase processing right? – minhaz Mar 16 '13 at 19:39

score 1 · Answer 3 · answered Mar 16 '13 at 20:12

The second case (checking for ==true) can get problematic if you or someone else redefines the value of true.

Let's say that we have the following C code:

#define true 2

bool isEqual(int a, int b)
{
    return (a == b);
}

if (isEqual(5, 5)) {
    printf("isEqual #1\n");
}

if (isEqual(5, 5) == true) {
    printf("isEqual #2\n");
}

The output from this code will be

isEqual #1

So the shorter form where you leave out ==true is preferable not only because it leads to less verbose code but also because you avoid potential problems like these.

Complexity of if statement

3 Answers3