39. For loops

As we have seen, while loops can be used with numbers using the following pattern:

var n = initial_value;  // initialization
while (n < max_value) {  // condition to end the loop
    ...
    n++;  // increment
}

Another way to write exactly the same program meaning is to use a for loop:

for ( var n = initial_value; n < max_value; n++ ) {
    ...
}

Thus, instead of having the variable initialization, the condition test for ending the loop and the increment steps on three different lines, they appear on the same line:

for (initialization; condition; increment) {...}

However, for loops and while loops are completely equivalent. Deciding which type of loop to choose is usually done based on how much easier it makes it to read and understand a program: for programs that use numerical values that are incremented each steps until a specific value is reached, for loops are often preferred because the three steps (initialization, condition, increment) can be seen at a glance. Note that any one (or more) of these three steps can be omitted in a for loop. For example, one could write:

for ( ; !front_is_clear(); ) {...}

instead of:

while ( !front_is_clear() ) {...}

However, in this case, the while loop syntax is considered by most people to be easier to read.

Using the for loop syntax, we can have a different definition for a repeat-like function:

function my_repeat(some_function, max_value){
    for(var n = 0; n < max_value; n++) {
        some_function();
    }
}

my_repeat(move, 9);
my_repeat(turn_left, 4);

Try it!

Try the above program with world Around 1.

39.1. Reeborg’s repeat()

Open Reeborg’s diary and have Reeborg execute the single line program:

view_source( repeat );

to find out which definition Reeborg uses for the repeat() function.

Note that you could also use:

write( repeat );

but the code would not be as nicely formatted!