Question

I was trying to understand how memoization works in C++, so I looked at an example of memoization used in Fib. sequence.

std::map<int, int> fibHash;

int memoized_fib (int n)
{
    std::map<int, int>::iterator fibIter = fibHash.find(n);
    if( fibIter != fibHash.end() ) return *fibIter;

    int fib_val;
    if( n <=1 )    fib_val = 1;
    else           fib_val = memoized_fib ( n-1 ) + memoized_fib ( n-2 );

    fibHash[ n ] = fib_val;
    return fib_val;
}

I was a little confused with how the fibHash[n] works. Does it just hold the individual values of each fib(#)? Also, the iterator traversess through the index to look for the correct value in the table and returns that? For example fib(6) = find fib(5) and fib(4), already stored and just add them?

Answer 1

The code indeed save each fib_val into the fibHash map. The find method called on fibHash searches the map to see if the value was previously computed. If so, find returns an iterator on this value and the function returns it (return *fibIter).

fibHash[ n ] = fib_val; adds a new value in the map.