Replacing nested if statements

Question

This is related to a chapter from beautiful code. And in that chapter I read about the nested ifs.

The author was talking about deeply nested if

Answer 1

Decision tables are where you store the conditional logic in a data structure rather than within the code itself.

So instead of this (using @Pax's example):

if (i == 1) {
    // action 1
} else {
    if (i == 2) {
        // action 2
    } else {
        if (i == 3) {
            // action 3
        } else {
            // action 4
        }
    }
}

you do something like this:

void action1()
{
    // action 1
}

void action2()
{
    // action 2
}

void action3()
{
    // action 3
}

void action4()
{
    // action 4
}

#define NUM_ACTIONS 4

// Create array of function pointers for each allowed value of i
void (*actions[NUM_ACTIONS])() = { NULL, action1, action2, action3 }

// And now in the body of a function somewhere...
if ((i < NUM_ACTIONS) && actions[i])
    actions[i]();
else
    action4();

If the possibilities for i are not low-numbered integers then you could create a lookup table instead of directly accessing the ith element of the actions array.

This technique becomes much more useful than nested ifs or switch statements when you have a decision over dozens of possible values.