If-else-if versus map

Question

Suppose I have an if else-if chain like this:

if( x.GetId() == 1 )
{
}
else if( x.GetId() == 2 )
{
}
// ... 50 more else if statements

What am I interested in if I save the map, will it be better in terms of performance? (assuming the keys are integers)

Answer 1

Maps (usually) are implemented using red-black trees, which give an O (log N) search, since the tree is constantly in equilibrium. Your linear list of if statements will be the worst case of O (N). So, yes, the map will be significantly faster to search.

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unsigned char single_bit_index(unsigned char bit) {
    // Hard-coded balanced tree lookup
    if(bit > 0x08)
        if(bit > 0x20)
            if(bit == 0x40)
                return 6;
            else
                return 7;
        else
            if(bit == 0x10)
                return 4;
            else
                return 5;
    else
        if(bit > 0x02)
            if(bit == 0x04)
                return 2;
            else
                return 3;
        else
            if(bit == 0x01)
                return 0;
            else
                return 1;
}

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Answer 2

?

swich(x.GetId())
{
  case 1: /* do work */ break; // From the most used case
  case 2: /* do work */ break; 
  case ...: // To the less used case
}

EDIT:

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Answer 3

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Answer 4

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Answer 6

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#include <cstdio>
#include <cstdlib>
#include <ctime>

const unsigned int Max = 4;

void f1();
void f2();
void f3();
void f4();
void (*vp[Max])();

int main()
{
    vp[ 0 ] = f1;
    vp[ 1 ] = f2;
    vp[ 2 ] = f3;
    vp[ 3 ] = f4;

    srand( std::time( NULL ) );
    vp[( rand() % Max )]();
}

void f1()
{
    std::printf( "Hello from f1!\n" );
}

void f2()
{
    std::printf( "Hello from f2!\n" );
}

void f3()
{
    std::printf( "Hello from f3!\n" );
}

void f4()
{
    std::printf( "Hello from f4!\n" );
}

Answer 7

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thisvar = mymap[x.getID()];

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if(x.getID() == ...){thisvar = ...;}

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