Why the switch statement and not if-else

if-statementlanguage-agnosticswitch-statement

I've been wondering this for some time now. I'm by far not a hardcore programmer, mainly small Python scripts and I've written a couple molecular dynamics simulations. For the real question: What is the point of the switch statement? Why can't you just use the if-else statement?

Thanks for your answer and if this has been asked before please point me to the link.

EDIT

S.Lott has pointed out that this may be a duplicate of questions If/Else vs. Switch. If you want to close then do so. I'll leave it open for further discussion.

Best Solution

A switch construct is more easily translated into a jump (or branch) table. This can make switch statements much more efficient than if-else when the case labels are close together. The idea is to place a bunch of jump instructions sequentially in memory and then add the value to the program counter. This replaces a sequence of comparison instructions with an add operation.

Below are some extremely simplified psuedo-assembly examples. First, the if-else version:

    // C version
    if (1 == value)
        function1();
    else if (2 == value)
        function2();
    else if (3 == value)
        function3();

    // assembly version
    compare value, 1
    jump if zero label1
    compare value, 2
    jump if zero label2
    compare value, 3
    jump if zero label3
label1:
    call function1
label2:
    call function2
label3:
    call function3

Next is the switch version:

    // C version
    switch (value) {
    case 1: function1(); break;
    case 2: function2(); break;
    case 3: function3(); break;
    }

    // assembly version
    add program_counter, value
    call function1
    call function2
    call function3

You can see that the resulting assembly code is much more compact. Note that the value would need to be transformed in some way to handle other values than 1, 2 and 3. However, this should illustrate the concept.