# # Python Review # # # Comments # # Anything after a # symbol is ignored # by the Python interpreter # # Use comments to make your code more readable # You can also use them as a design tool; # they can be used as place holders for detail # to put in later # # Generally, put comments before code blocks, # such as before loops, ifs, elifs, elses, # and function definitions. Describe in # plain English what the block is doing # rather than specifically what each line does. # # Data types and variables # # Create a variable with a specific data type by initializing it to a value # Here is an example of creating an integer variable. integer_variable = 1 # Here is an example of creating a float variable (number with decimals) float_variable = 1.0 # Here is an example of creating a string variable (character list) string1_variable = "Hello World" string2_variable = 'Hello World' # you can use single quotes, too # Here are examples of booleans - these variables can only be True or False # (and case matters = both True and False must have the first letter uppercase) boolean1_variable = True boolean2_variable = False # You MUST create the variable before you use it! x = 0 x += 1 # Add one to what is in x. Will not work if you don't set x to something first! # # Console input and output # # Use the input function to get data the user types into the console # It returns a string variable user_supplied_value = input ("What is your name?") # The print function will put data on the console # It takes a string as an input print (user_supplied_value) # You can format strings using the + operator to join strings together print ("You typed '" + user_supplied_value + "' into the console") # # Casting # # Casting lets you convert a variable's data type to another data type # # An example might be if you want to create a string with a number in it # You would use the str() function to convert the number to a string # (See what happens if you DON'T use the str() function in the statement below) int_var = 5 print ("The value of int_var is " + str(int_var)) # Example of converting a string to an integer using the int() function # This is helpful if you ask a user to input a number with the input function; # input will return a string, but we want it to be a number input_number = input("Enter a number (for real - don't try to trick it!)") x = int(input_number) + 4 # Take away the int() call to see what happens!! print (str(x)) # # Conditionals # # Conditionals allow you to run different lines of code based on a # condition being True or False # If statement - if the condition (the expression after the IF) is True # the indented block under the IF will run. Otherwise it will not. # You must have a colon (:) after the IF statement and all lines # underneath are indented 4 spaces # In the example below, the if condition will always be true if True: pass #pass doesn't do anything, but if you have an IF you must have at least # one statement after it; use pass if you don't know what to put yet # but you still want your code to be able to run # in this example, the IF will execute if the variable has the indicated value x = int(input("Enter a number (please make sure it's a number!)")) if x == 1: # Notice the : and the 2 equal signs print ("The code here is run") # You can also have an ELSE, which provides code to run if the condition is False x = int(input("Enter a number (please make sure it's a number!)")) if x == 1: # Notice the : and the 2 equal signs print ("The code here is run when the condition is TRUE") else: # Notice the : print ("The code here is run when the condition is FALSE") # You can use ELIF you have more than just 2 possibilities x = int(input("Enter a number (please make sure it's a number!)")) if x == 1: # Notice the : and the 2 equal signs print ("The code here is run when 1 is entered") elif x == 2: print ("The code here is run when 2 is entered") else: # Notice the : print ("The code here is run when anything other than 1 or 2 entered") # # Functions # # Functions allow you to create executable modules that can be "called" # by your code # # Like variables, you have to define functions before you can use them # You may optionally pass arguments (values) into functions and # optionally return values # # Below is an example of a simple function definition and call # Use keyword def in front of a function definition def add_numbers (a, b): # Notice colon (:) after definition statement # Always put a comment describing what the function does # This function takes two numbers as inputs and returns their sum x = a + b return x # Call the function (notice that the call is NOT indented, which means # it is outside the "scope" of the function. You must define the function # first before calling it sum = add_numbers (2, 5) print ("The sum is " + str(sum)) # # Loops # # Loops allow you to repeat code in a circular way # There are several different kinds of loops # We've looked at the just a few of them # The loop with a range loops through a specified number of times # The range number can be a constant or a variable set with an integer value # The loop counter variable keeps track of the iteration number, that is, # it goes up by one each time the loop repeats. # The tricky thing here is that the counter starts at 0. # # In this example the loop will repeat 5 times; the counter values are 0, 1, 2, 3, 4 # each time through the loop for counter in range(5): # print out the counter value each time through the loop print (counter)