How should I order an if statement?

Question

[I thought this would have been asked already, but I can't find an answer].

When writing an if statement, should the thing I'm comparing against come first or second? (I am specifically curious about both C / C++ and Java).

Does it matter, or is it just stylistic? (Convention seems to be "variable first, value second").

Consider the following code:

#include <iostream>

int main() {
    int x = 5;

    if ( 5 == x ) {
        std::cout << "X == 5, true." << std::endl;
    }
    if ( x == 5 ) {
        std::cout << "5 == X, true." << std::endl;
    }

    return 0;
}

which outputs:

X == 5, true.
5 == X, true.

so there doesn't appear to be any difference. Are there subtleties I'm missing?

Annoying statements of the second kind Yoda conditions are called :-) — Sergey Kalinichenko, Jul 12 '13 at 01:13
@dasblinkenlight Brilliant, I didn't see the linked question. Once you know what it's called, it's easy to search for. — simont, Jul 12 '13 at 01:17
In C++, there *can* be a difference if operator overloading is involved. (But that won't be an issue in your particular example.) — jamesdlin, Jul 12 '13 at 01:25

Nobilis · Accepted Answer · 2013-07-12T01:44:10.413

10

These are called Yoda Conditions and to my knowledge the main idea is to guard against accidental assignment (i.e. you can't assign to literals).

Yes, worth mentioning that something like if (a = 5) where a is not a bool is not allowed in languages such as Java (though OK in C/C++), but as the Wikipedia article points out it is still possible to guard against unwanted behavior:

It can also solve some types of unsafe null behavior.

String myString = null;
if (myString.equals("foobar")) { /* ... */ }
// This causes a NullPointerException in Java

With Yoda Conditions:

String myString = null;
if ("foobar".equals(myString)) { /* ... */ }
// This is false, as expected

edited Jul 12 '13 at 01:44

answered Jul 12 '13 at 01:12

Nobilis

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+1 for `equals` example - that's a basic trap most people fall :) – exexzian Jul 12 '13 at 01:29
But `if(foo = true)` would be allowed in Java. – Jeffrey Jul 12 '13 at 01:42
@Jeffrey yes, should clarify that bools are an exception. – Nobilis Jul 12 '13 at 01:43

score 5 · Answer 2 · answered Jul 12 '13 at 01:12

5

Putting the constant first (aka "Yoda style") prevents problems from a simple typo:

if (x = 5) // if (x == 5) was intended

...still compiles but does the wrong thing (though most current compilers will warn about the problem).

if (5 = x) // if (5 == x) was intended

...absolutely cannot compile on anything even approaching a properly functioning compiler.

answered Jul 12 '13 at 01:12

Jerry Coffin

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Yoda style will benefit only for c/c++ as in java `x=5` will not compile – exexzian Jul 12 '13 at 01:17
yeah in java too, think if some one want to compare and put `x=5` will compile and don't know about his miss, if put in yoda way not gonna to compile :) – nachokk Jul 12 '13 at 01:24
@Bingo Yoda style benefits Java with boolean expressions: `if(foo = true)` compiles. – Jeffrey Jul 12 '13 at 01:41
@Jeffrey yeah that's the case. Good Point – exexzian Jul 12 '13 at 01:45
Who writes `foo == true` anyway? (And FWIW, C and C++ compilers warn against this so there isn't a solid reason to prefer less readable code) – R. Martinho Fernandes Jul 12 '13 at 02:17

score 2 · Answer 3 · answered Jul 12 '13 at 01:11

No, there is no difference. It doesn't matter. Do whatever you like. Personally I'd do:

x == 5

Because it reads better (or so I think). 5 == x just doesn't sound right. But again, personal opinion.

I think you'll find that most people do x == 5 because you are comparing x, rather than comparing 5.

score 1 · Answer 4 · answered Jul 12 '13 at 01:11

1

Both are ok. But it is more accepted to do so when the parameter on left side. example:

if ( x == 5 )

Anyway. it doesn't matter, do whatever you pleased and feel more comfortable with.

answered Jul 12 '13 at 01:11

Novo · Answer 5 · 2013-07-12T02:13:04.833

-4

In most cases you'll find the common practice is to have the variable come first because the operator == means assignment and here you are assigning the value 5 so it only makes sense to have the variable come first.

Your if statement should then look like this:

    if ( 5 == x ) {
        std::cout << "X == 5, true." << std::endl;
    }

edited Jul 12 '13 at 02:13

answered Jul 12 '13 at 01:54

Novo

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operator == is *not* assignment. – Borgleader Jul 12 '13 at 02:16
The word you were looking for is "equality", _not_ "assignment". – Etienne de Martel Jul 12 '13 at 02:35

How should I order an if statement?

5 Answers5