Decision Structures, String Comparison, Nested Structures

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The IF-ELSE Statement

– Previously, we wrote “IF” statements to check for a condition, and our programs followed the order of the program according to whether the conditions held “True” or “False”

– Now, we will add in another branch of execution based on the value of the Boolean expression

The IF-ELSE Statement

– We can do this by adding the reserved word “else”

– This tells the program to check for the “if” condition and if it is not satisfied, run the code branched under the “else” statement

– You need to add a colon at the end of each else statement

The IF-ELSE Statement

– Previously, we had to use two “if” statements like this:

if temperature < 32:

print(“It’s freezing outside”)

if temperature >= 32:

print(“It’s a normal day in October … ”)

The IF-ELSE Statement

– Now, with the “else”, we can write our codes like this:

if temperature < 32:

print(“It’s freezing outside”)

else:

print(“It’s a normal day in October … ”)

The IF-ELSE Statement

The IF-ELSE Statement

hours = float(input(“how many hours did you work?”))

rate = float(input(“what’s your hourly rate?”))

if hours > 40:

print(“Your total is”, (40*rate) + ( (hours - 40)*1.5*rate) )

else:

print(“Your total is”, hours*rate)

String Comparison

– So far, we’ve been comparing data as numeric types

– We can also compare strings

– The way we do this is to convert all characters back to their basic data type, all one’s and zero’s using the ASCII/UNICODE table

ASCII Table

String Comparison

– When comparing strings, Python will compare one letter at a time, in the order that they appear

– Example:

“animal” < “banana” # True

– Because the letter “a” in animal has a lower ASCII value than the letter “b” in banana, this Boolean expression holds True

String Comparison

“dog” > “cat” # is “dog” greater than “cat”?

“fish” < “alligator” # is “fish” less than “alligator”?

“elephant” == “tiger” # are “elephant” and “tiger” equal?

“Mr. Seok” != “Donald # are these different strings?

Practice: Passwords

– Write a program that asks the user to input a new password and store it in a variable

– Then, ask the user to confirm the password they just typed

– If they are exactly the same, then print out “Great! Password was saved!”

– Otherwise, print out “Sorry, the two passwords did not match.”

Basic String Manipulation

– Python has a huge list of string manipulation commands written in a package. We will take a look at many of them, but for now we’ll take a look at:

– lower() # this changes all characters into lower case

– upper() # this one changes into upper case

Basic String Manipulation

– These two commands are not actually built into Python’s function library, therefore we need to refer to a separate module

– We call this the “str” module

– Examples:

string_lower = str.lower (“Hello, World!”) # hello, world!

string_upper = str.upper (“Hello, World!”) # HELLO, WORLD!

Basic String Manipulation

– A module is a file containing functions defined in Python and other statements

string_lower = str.lower (“Hello, World!”)

– The characters “ str. ” calls on a module entitled “str” that Python can access but is not directly built into the shell

– The word “lower()” calls the function within that specific module

Practice: Passwords

– Rewrite your password program so that it is not case sensitive

– Such that, “password” and “PaSsWoRd” are considered valid entries

Practice: Alphabetizing Strings

– Write a program that asks the user to input two names

– Then print back these names in alphabetical order

String Length

– Python also has a function that counts the number of characters in a string

– We call it the len() function and it returns an integer

– Example:

count = len(“Donald”) # count will hold value 6

Practice: Compare Size of Strings

– Write a program that asks the user for two names

– Sort them in size order and print them out

Nested Decision Structures

– Sometimes, one question isn’t enough. We may need to ask “follow-up” questions.

– Python allows us to nest decision structures inside one another, allowing you to evaluate additional conditions once a “higher” condition is satisfied

– In other words, we can have an “if” statement inside another “if” statement

Practice: Magic Number Game

– Rewrite the magic number game

– Set a magic number between 1 and 10

– Ask the user to guess the number, but this time, if the guess is above the magic number, print out “too high!” and if the guess is below the magic number, print out “too low!”

Practice: Magic Number Game

magic = 5

guess = float(input (“Guess the magic number: ” ))

if guess == magic:

print (“Woah! You were right.”)

else:

if guess > magic:

print(“Too high!”)

else:

print (“Too low!”)

Nested Decision Structures

– Remember, indentations are important in decision structures

– Make sure to indent accordingly for each decision structure

ELIF

– There is one more reserved word for our decision structures

– We call it the “elif” statements

– The “elif” word allows you to check for an multiple conditions at a time

– This is different from checking additional conditions within one another

ELIF

if guess == magic:

print (“You got it!”)

elif guess > magic:

print (“Too high!”)

else:

print (“Too low!”)

Practice: Alphabetizing Strings

– Now, try writing a program that asks the user to input three names

– Then print these names back in alphabetical order

– You should also try four and so on and so forth …

Challenge: Grade Generator

– Write a program that asks the user for their grade on the last test they took

– If the user inputs a number greater than 100, or less than 0, tell them to put in a different number

– Then, according to the grade they give you, print out their letter grade

Challenge: Asian Standards

100 # A

95 # B

88 # C

80 # D

75 # F