1. The for loop

The for loop

So far we have used while loops whenever we needed to carry out a sequence of 
steps over and over. A while loop was often controlled by a single variable and 
was characterized by 3 elements:

- initialization of control variable
- loop condition
- update of control variable

Here is one of the earlier example where the three elements are highlighted:

void printSpelling( String phrase )
{
    int index = 0;
    while ( index < phrase.length() )
    {
        char curLetter = phrase.charAt( index );
        System.out.println( curLetter + ",  " );
        index = index + 1;
    }
}


In the loop above the control variable has a very well-defined:

- starting value  --  index = 0
- loop condition  --  index < phrase.length()
- update increment -- index = index + 1

Such loops are so common that Java offers a new construct that puts all 3 loop
elements on a single line. This makes it easy to see how the loop is controlled
and how it works. 

A for loop can make the the intentions of the programmer much clearer, so it is
good to use it whenever possible. Typically this would be in situations where
the loop is controlled by a variable that has well-defined starting value and
updates via fixed increments. On the other hand, the guessing game is a good
example of appropriate use of while loop.

Here is a comparison of the structure of while and for loop. One is simply a
rearrangement of the other.




    initialize control variable

    while ( loop condition )
    {
        LOOP BODY -- THE ACTUAL WORK

        update control variable
    }



    for ( initialize control variable ; loop condition ;  update control variable )
    {
        LOOP BODY -- THE ACTUAL WORK
    }





Here are a few of the earlier examples re-written with for loops:

Spelling a word




void printSpelling( String phrase )
{
    int index = 0;
    while ( index < phrase.length() )
    {
        char curLetter = phrase.charAt( index );

        System.out.println( curLetter + ",  " );

        index = index + 1;
    }
}



void printSpelling( String phrase )
{
    for ( int index = 0 ; index < phrase.length() ; index = index + 1 )
    {
        char curLetter = phrase.charAt( index );

        System.out.println( curLetter + ",  " );
    }
}





Counting character in a string




int countCharacter( String phrase, char letter )
{
    int result = 0;

    int index = 0;
    while ( index < phrase.length() )
    {
        char curLetter = phrase.charAt( index );

	if ( curLetter == letter )
        {
            result = result + 1;
        }

        index = index + 1;
    }

    return result;
}




int countCharacter( String phrase, char letter )
{
    int result = 0;

    for ( int index = 0 ; index < phrase.length() ; index = index + 1 )
    {
        char curLetter = phrase.charAt( index );

        if ( curLetter == letter )
        {
            result = result + 1;
        }
    }

    return result;
}