ACM init() !

Day 2: April 22, 2015

From last time• A little bit about Python

• How to use Koding.com

• Data types: numbers, string, and booleans

• Variables and naming conventions

• Printing

• String methods

• Math operators

• Comments

Brief topic: getting inputWe often want to get input from a user that we can use later.

Here is how to do that:

variable = input("Prompt: ")

InputHere is an example of getting the user's name:

Practice: Write a mad libs program

Here is an example of how to string together text:

Write your own mad libs with the people sitting around you!

I guarantee you will get an error message. Work with those around you to fix it!

Madlibs conclusionsJust using the keyword 'input' gives an error for strings.

!It will work for numbers, but strings need to be in quotes.

We can fix this!

We use 'raw_input' instead of 'input' when we expect a string. This is due to the version of Python we are using.

Comparison operatorsWe are going to look at comparison operators. There are 6 to

know:

EqualWe assign variables like this:

What if we want to check if two variables are equal? Can we do this?

EqualWe cannot write x = y ! That would set x equal to the value of y.

If we want to compare two values we need to use '=='

This is correct syntax!

What type of variable is b? What is b's value?

Not equalThe way we denote not in Python is with an exclamation mark.

!Logically, not equal would be !=

b is still a boolean. What is its value?

Greater/less than...

• This is no different than regular math you have learned.

• 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: >=

Some bools

Boolean operators

• Remember the variable boolean that was either true or false? (Hint: it was just on the previous slide)

• Python has operators that can determine if a statement is true or false

• These operators are 'and', 'or', and 'not'

andThe symbol for 'and' is 'and'!

!An and will only result in a true value if the statements on both

sides of the and are true.

Let's look at some examples.

and examples

and examples

orThe symbol for 'or' is 'or'

!or evaluates to true if one or both statements on either side of

the or are true

Let's look at some examples.

or examples

or examples

notThe symbol for 'not' is 'not'

!not makes true values false and false values true

Let's look at some examples

not example

not example

Combining operators• We can combine and, or, and not 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.

True or false?

Answer

True or false?

Answer

Conditionals

• Programs need to make decisions based on different situations - user input, calculations, etc.

• Three types of control flow

• 'if', 'elif', 'else'

if to start

optional one or more elif

optional else

We'll go into more detail in the following slides

ifAn 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 'do something' will be skipped.

What will this print?

elseAn 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.

What will this print?

elifelif must always be paired with an if statement and en else statement.

However, you can have as many elif statements as you want.

if statement:! do something!elif other statement:!! do something else!else:! do something else

Remember that you can have more than one elif statement, but only one if statement and one else statement. If and elif statements have arguments,

but else is at the bottom and is the fall-through case.

What will this print?

What will this print?

What we did today

• Input- Madlibs!

• Comparison operators

• Boolean operators

• Conditionals

Practice

At home, try to make a calculator. !

Ask the user what operation they want to do. (+, *, etc) Ask the user for the numbers they want to do the calculations

on. !

Print the answer!