Decision Structures, String Comparison, Nested Structures Best Powerpoint Layout Ever!

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 Notice that the “else” statement works kind of like a safety net, that “catches” all other possibilities. In other words, the first “if” condition checks for one type of possibility and all other possibilities fall into the “else” block For example, any temperature that is NOT less than 32 degrees, must be equal to or greater than 32, and that is summed up in the “else” Another example might be if something is NOT green, it can be any other color whether purple, red or blue and all of those colors will fall under an “else” statement

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

Basic String Manipulation Keep in mind that the str.lower() function is one that “returns” a value and therefore must be stored in some place, or variable name new_word = str.lower (“Hello, World!”) print(new_word) >> hello, world!

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 Note: Keep in mind that lower case letters hold “greater” values than that of the capital letters

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 # as an integer

Practice: Compare Size of Strings Write a program that asks the user for two names Sort them in size order and print them out