Init() Lesson 2

ACM init()
Day 2: October 29, 2014
Happy Birthday Internet!

Any questions?
• A little bit about Ruby
• How to use Koding
• Data types: numbers, string, and booleans
• Variables
• Math
• How to print to the screen
• String methods
• How to write good code

No code. This is your
homework.

Interacting with the user
• Programs can change
based on user input
• We want a way to store
and use user-inputted
values

Interacting with the user
• Sometimes we write programs
that change depending on
what the user types in
• For example, the Mad Libs
program produced a different
story based off of the words
we entered
• We want a way to store and
use user-inputted values

Getting user input
First, we need to tell the user that we expect something from
them. Prompt the user to enter their name:
Now, we need to store the name in a variable. The syntax for
this is 'variable_name = gets.chomp'
Here we used gets.chomp to store the user's name in a
variable called 'name'

Using user-inputed values
Now we have a variable with a name stored in it. How can we
use this? Let's print it out:
Notice how we accessed our name variable? The syntax to
access a variable is "#{name}"
Here's how to print out the contents of a variable:
Don't forget the " " when accessing a variable or Ruby will think you
are trying to write a comment.

Let's look at the full program:

Any questions on user
input?
Hint: You now know enough to write a Mad Libs program.

Control Flow
• Programs need to make decisions based on
different situations - user input, calculations, etc.
• Four types of control flow
• 'if', 'elsif', 'else', and 'unless'

Tells Ruby input should be an integer
'if' to start
the decision
optional one
or more 'elsif'
optional 'else'
'end' to end decision
We'll go into more detail
on the next few slides

if
An if statement looks at an expression then exectutes a block of code if the
expression is true. Otherwise, the block of code is skipped.
if statement
do something
end
An if statement can stand alone. If the statement is not true, the rest of the
code will be skipped until 'end' is reached.

if example
What will this print out?

else
An else statement must always be paired with an if statement. If the if
statement is not true, the else statement will be executed.
if statement
do something
else
do something else
Note that if the if statement is not true the else statement will always be
executed. In an if-else code block one of the two cases will be true.

else example

elsif
elsif must always be paired with an if statement and en else statement.
However, you can have as many elsif statements as you want.
if statement
do something
elsif other statement
do something else
else
do something else
Remember that you can have more than one elsif statement, but only one if
statement and one else statement. If and elsif statements have arguments,
but else is at the bottom and is the fall-through case.

elsif example
If n = 5 and m = 1, what will this print out?

unless
An unless statement is used to check if something is false. It is paired with
an else statement. Unless the statement we are evaluating is true the else
statement will be executed.
unless statement
do something
else
do something else

unless example
What type of variable is 'studied'?

Answer
"The string length is 9!"

Answer
"The string length is 7 or less!"

Answer
"The string length is 10 or greater!"

Trick Question!
There was no 'end' after the unless-else block. This will cause
an error.
Now what will this print out?

Any questions on if,
else, elsif, or unless?

Comparison Operators
• A lot of code we write depends on comparing two
values.
• Ruby has operators that do the comparison for us.
• However, we have to be very careful to make sure
we write the operators correctly or the comparison
could be misinterpreted.

Equal/Not Equal
• While it would nice to write
x = y to check if two things are
equal, we can't. Remember
that we used 'x = ' to define
variables. We need a different
way to check if two things are
equal.
• The syntax for checking if two
things are equal is : 'x == y'.
Notice the double equals.
• To see if two things are not
equal we type: 'x != y'. The '!'
means 'not'.
Note that the above example
could be written much more
compactly in an if-else block.

Greater/Less Than...
• We already saw an example of
less than and greater than in
previous slides
• Less than: <
• Greater than: >

... or Equal To
• Less than or equal to and
greater than or equal to are
written by just adding an
equals sign on to the end of
the expression
• Less than or equal to: <=
• Greater than or equal to: >=

Boolean Operators
• Remember the variable boolean that was either true
or false?
• Ruby has operators that can determine if a
statement is true or false
• These operators are 'and', 'or', and 'not'

and
The symbol for 'and' is &&
!
An && will only result in a true value if the statements on both
sides of the && are true.
Let's look at some examples.

Answer
"The boolean is true!"

or
The symbol for 'or' is ||
!
|| evaluates to true if one or both statements on either side of
the || are true

not
The symbol for 'not' is !
!
! makes true values false and false values true

Answer
"Two is less than three."

Combining Operators
• We can combine &&, ||, and ! to evaluate
statements.
• If the expression looks confusing, we can add
parenthesis to clarify things. Expressions inside ()
will be evaluated first.
• Let's look at some examples.

What we did today
• Getting user input
• Control flow: if, else, elsif, unless
• Making comparisons: ==, !=, <, >, <=, >=
• Boolean operators: &&, ||, !

Any questions before we
move on to homework?

Make your own Mad Libs!
Your assignment is to make your own Mad Libs game. Here
are the details:
1) You should be able to make the game using only what
you have learned so far.
2) If you don't know what Mad Libs is or don't want to write
your own here is a site with some samples:
http://www.madglibs.com/
3) This is optional, but highly recommended. The best way
to learn programming is to practice.
4) If you have any questions or get stuck post on the
Facebook group or email one of us. We will go over the
solution next time we meet.

Init() Lesson 2

More Related Content

Similar to Init() Lesson 2 (20)

More from UCLA Association of Computing Machinery (13)

Recently uploaded (20)

Init() Lesson 2