CODEWITHSUNDEEP
cconditional-statements
Vorac
Vorac

Reputation: 9114

Simple state machines. When does switch() beat if - else if - else

What guidelines can be followed when deciding between if - else if - else and switch - case?

Some examples of equivalent couples of structures. Or are they?

int a;
#define const1 42
#define const2 666

if(a == const1){};

switch(a){  
    case const1: {} 
    break;  
}  

if(a == const1){}
else {}

switch(a){  
    case const1: {} 
    break;  

    default: {}
    break;
} 

if(a == const1){}
else if(a == const2){}
else {}

switch(a){  
    case const1: {} 
    break;  

    case const2: {} 
    break;  

    default: {}
    break;
}

From here on, I think switch is definitely superior in terms of both readability and performance.

As a matter of fact, I am currently at the last situation, trying to decide which way to go.

Upvotes: 0

Views: 1050

Answers (1)

teppic
teppic

Reputation: 8195

If they just involve evaluating a variable, testing it and executing a statement depending on that, there's no difference in the logic, and any decent compiler can see that.

I tried the same test (a check for 32) with an if and a switch, and gcc, even with all optimisation turned off generated:

For the if:

movl    a(%rip), %eax
cmpl    $32, %eax
jne .L2
movl    $1, %eax
jmp .L3

for the switch:

movl    a(%rip), %eax
cmpl    $32, %eax
jne .L6
movl    $1, %eax
jmp .L3

Of course, if you're going to test the same integer for lots of values, a switch is more readable -- that's what it's intended for.

Upvotes: 3

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