COMP 364: Conditional Statements Control Flow Carlos G. Oliver, - - PowerPoint PPT Presentation

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COMP 364: Conditional Statements Control Flow

Carlos G. Oliver, Christopher Cameron September 15, 2017

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Outline

• 1. New midterm date → Tuesday October 24 19:00-21:00

ENGMC 204

• 2. Recap
• 3. Conditionals (if, else, elif)
• 4. User input

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iPython vs command line

◮ The interactive Python interpreter is not the same as the

Terminal/Command Prompt

◮ Interactive Python prints the result of evaluating an

• expression. (more later)

1

>>> "hello"

2

"hello" #interactive python automatically prints result of expression

֒ → 3

>>> print("hello") #this is the same as above

4

"hello"

5

>>> s = "hello" #will not print anything

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Refresher: the OS and file system

◮ The Terminal / Command line lets you execute programs (e.g.

ls, cd, dir, python, mkdir)

◮ How to open terminal in Mac and in Windows ◮ Syntax: $ [program name] [program options] ◮ These functions include

◮ Launch the Python interpreter $ python ◮ List files in your current directory $ dir $ ls ◮ Change directories $ cd [destination path]

◮ A program you are executing can only see files in your current

directory.

◮ If you want to access a file outside of your current directory

you must specify its path in [program options]. (quick demo)

◮ e.g. $ python /Users/carlos/Desktop/hello.py will

work from anywhere because it has the full path to the file.

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Functions are objects

◮ A function is also an Object which can execute some

commands given an (optional) input to produce an output.

◮ function name(input) ◮ e.g. >>> print("Hello world")

Input Function Output

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Names, Namespaces, Objects

class / type

• 2
• 1
• 3

bob alice eve Names Objects Namespace

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Making decisions: conditional statements

◮ Programs often have to make decisions that depend on some

conditions.

◮ For example, depending on what you type into a Google

search, it executes a different set of commands

◮ Python lets us accomplish this using conditional statements 1

1Rick and Morty

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An example:finding genes

◮ Genes are DNA sequences made up of codons that get

converted into proteins.

◮ We know that they always start with the letters: “AUG” ◮ We can use the if statement to check if an object is a start

codon.

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codon = "AAG"

2 3

if codon == "AUG":

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print("This is a start codon!")

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else:

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#if we have a start codon, this NEVER executes

֒ → 7

print("This is not a start codon!")

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A little terminology: Expressions and Operators

◮ Expressions are any line of code that can be evaluated to

some value and stored as an object.

◮ Operators do computations on expressions (e.g. +,-,==,

<=, >=, %) to produce new expressions.

◮ You can think of operators as special tokens for functions.

Life Hack 1 Don’t worry too much about all this terminology. It’s just so we can have a common language when talking about code.

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A little more terminology: Statements

◮ Statements are “special” instructions that tell Python how

to execute code. (e.g. x = 2, if, else)

◮ They do not evaluate to a value. (e.g. if is a statement, x =

4 is also a statement)

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>>> x = 3 #statement

2

>>> y = 9 # statement

3

>>> x + y # expression

4

12 Life Hack 2 If you can print it or assign a name to it it’s an expression,

• therwise it is a statement.

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Back to the if statement

◮ Python executes line by line going down your code. However,

if statments (and others) let us intervene.

◮ The if statement lets us split our commands into “separate”

units that execute based on the value of some boolean expression.

◮ Code belonging to an if/else is denoted by a tab (4 spaces) ◮ Syntax: if (boolean expression) :, else: ◮ Note: here, the else is not mandatory but can come in handy.

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Small example

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sequence = "AACGAAgU"

2 3

if not s.isupper():

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#inside first case

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#DNA sequence not capitalized

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print("You forgot to capitalize")

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sequence = sequence.upper()

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print("I fixed it")

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else:

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#inside second case

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# DNA sequence correct

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print("Sequence OK")

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#outside if statement

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print(sequence)

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Control Flow

s = "AAgU" if not s.upper() print("OK") s.upper() print("done")

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Control flow

s = "AAGU" if not s.upper() print("OK") s.upper() print("done")

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What if we have more than two cases? → elif

◮ Let’s try to figure out if we have a natural disaster.

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ground_shaking = True

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flooding = False

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if(ground_shaking and not flooding):

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print("Earthquake!")

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elif(not ground_shaking and flooding):

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print("Hurricane!")

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elif(ground_shaking and flooding):

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print("End of the world!")

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else:

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print("Everything is okay.")

◮ Note:

as soon as ONE of the cases is True the rest does not execute (even if they happen to be True as well. Try it!).

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Else if: elif

◮ elif always comes after an if or another elif and the last

statement must be an else

◮ “If the previous if or elif did not evaluate to True AND

this elif is True execute this block of code.”

◮ Useful for when you have multiple cases but only one should

execute.

◮ Important: vertical alignment is crucial.

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Nested if/else/elif statements

◮ We can put if/else/elif statements inside other

if/else/elif statements.

◮ Note:

can lead to repetitive code so elif is preferred when applicable.

2

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//wallpoper.com/images/00/21/28/83/movie-inception_00212883.jpg

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Nested if statements

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ground_shaking = True

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flooding = False

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if ground_shaking:

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if flooding:

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print("End of the world!")

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else:

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print("Earthquake!")

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else:

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if flooding:

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print("Hurricane!")

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else:

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print("Everything is okay.")

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An interlude: interacting with the user

◮ Code is not very useful if it can’t receive information from a

user.

◮ Receiving information → input. 3

3http://www4.pictures.zimbio.com/gi/Kim+Kardashian+eBay+

Holiday+Store+opmeLl_yL8ql.jpg

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Interactive user input: input()

◮ The input() function lets us store values into string objects

based on text given to us while the program is running.

◮ a.k.a at “runtime” ◮ Synatx: x = input("Message to user ") ◮ Convert the input to the appropriate data type using: int(),

float(), bool()

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flooding = input("Is there flooding? (Y/N)")

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if flooding == "Y":

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flooding = True

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elif flooding == "N":

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flooding = False

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else:

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print("Incorrect input format, enter Y/N")

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Example

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flooding = input("Is there flooding? (True/False) ")

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ground_shaking = input("Is the ground shaking? ")

3 4

#leaving out the type conversions for brevity (see prev slide for example)

֒ → 5 6

if(ground_shaking and not flooding):

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print("Earthquake!")

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elif(not ground_shaking and flooding):

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print("Hurricane!")

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elif(ground_shaking and flooding):

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print("End of the world!")

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else:

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print("Everything is okay.")

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Sanity checks: assert statements

◮ Often you want to make sure your code doesn’t do something

that makes no sense.

◮ This comes up a lot in user input since we can’t predict what

the user will input.

◮ The assert statement takes a boolean expression. If it

evaluates to false, the program terminates.

1

#this code divides two user input numbers

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numerator = int(input("Give me a number: "))

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denominator = int(input("Give me a number: "))

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assert denominator != 0

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print(numerator / denominator)

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In-class problem: mini medical diagnosis program

◮ Let’s write a program that takes info on a patient’s symptoms

and outputs a diagnosis.

4

4http://house.wikia.com/wiki/File:House328.jpg

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In class problem: house.py

◮ Since it may get a little long and we would like to reuse it, we

should save our code in a file house.py (or a notebook).

◮ Input:

age, sex, temperature, coughing, headaches, nausea

◮ Output:

"healthy", or "infection", or "hypothermia",

• r "pregnant", or "food poisoning"

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RTFM: Documentation

◮ Everything I am showing you and much much more is

documented in the python docs for bulit-in functions.

◮ The entire set of default python commands and data types is

here

◮ How to read documentation syntax (live demo) 5

5http://i0.kym-cdn.com/photos/images/newsfeed/000/131/662/

22711800_646849b145.jpg

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Collections

◮ The world is full of collections of things. We would like to be

able to work with such things efficiently.

6

6Rick and Morty

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How can we keep track of all the Jerrys?

◮ We know how to store data as objects. So let’s make an

• bject for each Jerry.

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jerry1 = "Original"

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jerry2 = "First clone"

3

jerry3 = "Second clone" This doesn’t seem like a very efficient way of doing things. Thankfully Python lets us store collections of things very easily.

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Lists

◮ Just like everything in Python, a list is an Object of type list. ◮ We store a list using square brackets [] and separate objects

with commas (,).

1

>>> jerrys = ["Original jerry", "Clone one", "Clone two"]

֒ → 2

>>> type(jerrys)

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<class ‘list’>

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>>> id(jerrys)

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4339082696