Session 3
Loops

At the heart of their functionality, loops enable blocks of code
to be executed more than once.

Loops represent a simple concept that can be used to create
extremely dynamic and flexible programs.

Loops are very important for two reasons: efficiency and
flexibility.

In the second part of the project from the previous session,
you were instructed to basically run the program twice. In
order to accomplish this, you had to copy the existing blocks
and append them to the end of the program effectively
doubling the number of blocks present. It worked, but just
imagine what would happen if you had to go through the
program ten, one-hundred, or even a thousand times!
Assuming you were able to create a program of that
magnitude, what if a change had to be made? You would be
required to make that change for each copy! Loops allow the
program to contain only one copy of the code and go through
it as many times as you need to. This is much more efficient.
The Loop Block

In Alice, the simplest type of loop is the loop block. It closely
resembles the “for” loop found in other programming
languages.

The loop block expects the programmer to specify a number
which will indicate exactly how many times it will repeat.

In this example, the move block will be repeated ten times. To
find the total distance the cat will move, all we have to do is
some quick multiplication to realize it will move 100 meters.

This block is used when the number of times to repeat is
known before the block is encountered, but sometimes, there
is no way of knowing exactly how many times a loop must
execute. In cases of uncertainty, a slightly different loop must
be used.
The While Loop

The while loop, as the name implies, keeps repeating while
a given Boolean expression is true.

It is identical to the while loop found in other programming
languages.

While loops are perfect when combined with user input.

This allows the user to interact with the program
indefinitely; loops such as these are found in almost every
program in some form.

Although the two loops mentioned thus far appear to
behave differently, they are the same loop. With the use of
variables (discussed in session 4), the while loop can be
made to work exactly like a regular loop.
Infinite Loops

In some cases, a loop must continue repeating the entire
time the program is running. When this is required an
infinite loop is often used.

Like the while loop, the infinite loop is commonly used in
conjunction with user input.
Nested Loops

Like the conditional branch structures in the previous
session, loops can, and sometimes must, be nested. The
behavior of these loops is often trivial, but sometimes they
can be very complex.

This script moves the sprite 100 meters (2 * 10 * 5).
What does this one do?
For All

There are two additional loops in Alice which will not be
covered in this session:

These loops are for use with lists and arrays which will be
covered in session 7.
Project 3

While the user chooses to continue the program, ask the
user how many times to move an object 0.2 meters. Use
the number provided by the user in the condition of a for
loop which moves the object 0.2 meters that many times.

Flow of program:
Would you like to continue? Yes/No
(if yes) How many times would you like to move the
object 0.2 meters? Answered with a number
Uses a number with a for loop to move the object that
many times.
Day 2 Questions
1.
What part of an If/Else control structure determines which
sequence of blocks is executed?
2.
The expression in the predicate part of an If/Else structure
evaluates to a
or
value?
3.
What part of the structure would execute if the predicate
value was true.
4.
If a certain block or number of blocks needed to be executed
several times, what control structure would you use?
5.
If a loop executes 5 times and there was a block inside that
moved the object 3 meters, how many meters would it move
in total?
6.
The expression in the predicate part of an If/Else structure
evaluates to what type of value?
7.
If the condition of a while loop was set to “true” how many
times would this loop execute?
Day 2 Answer Key

What part of an If/Else control structure determines which sequence
of blocks is executed? predicate or conditional, Boolean expression,
anything similar.

The expression in the predicate part of an If/Else structure evaluates
to a true or false value?

What part of the structure would execute if the predicate value was
true. if part, consequent

If a certain block or number of blocks needed to be executed several
times, what control structure would you use? loop,while loop

If a loop executes 5 times and there was a block inside that moved
the object 3 meters, how many meters would it move in total? 15
meters

The expression in the predicate part of an If/Else structure evaluates
to what type of value? Boolean, true or false

If the condition of a while loop was set to “true” how many times
would this loop execute? Infinity times, until the user stops the
program from running