A program that works the same way each time is no use. That's why we learned how to get user input. However, there are more ways to change how the program works. The way that you'll learn here is the if block, a very powerful method. But first, we have to learn about something else called blocks.
In Python, whitespace matters. Try running the code below:
print("Here's some text") print("Here's more text")Python produces an indentation error. Python didn't think that the second line would have more spaces in front of it than the first.
Python think in terms of blocks. Look at the example below:
print() # Block 1print() # Block 1 print() # Blocks 1 and 2 print() # Blocks 1 and 2 print() # Blocks 1, 2, and 3 print() # Blocks 1 and 2print() # Block 1As you can see, code with the same indentation level is in the same block. Here's another example:
print() # Block 1 print() # Blocks 1 and 2 print() # Blocks 1 and 2print() # Block 1 print() # Blocks 1 and 3print() # Block 1Wait! Lines 2 and 5 have the same indentation level. Why aren't they in the same block? Answer: Since there is a block with less indentation between them, Python considers them to be seperate blocks.
Try making a block with nothing in it. Python should produce an error. One way to prevent this is by writing something in it. Another way is with the pass statement. pass does nothing at all, and is just there to help prevent errors. Example:
if a < 2: passelse: code... code...Note that pass does not stop errors from occurring:
passstr = 2passPython will still produce an error. pass is just meant to fill space.
Now that we have learnt about blocks, let's learn about if statements. Here is the basic format:
if statement: statements statementsHere's an example where we get a number, then display something if the number is less than 10:
num = int(float(input("Enter a number:")))if num < 10: print("Your number is less than 10.")The statements inside "indented within" the if block will only be run if the statement is true.
A lot of times, you want to do something if something is True, and something else if something is False. Good news: the else statement is here! Here's the format:
if statement: code # This code is run if statement is True.else: code # This code is run if statement is False.Improving on our last example, let's display something if the number is NOT less than 10.
num = int(float(input("Enter a number:")))if num < 10: print("Your number is less than 10.")else: print("Your number is not less than 10.")What if we want to check through several boolean statements? We could use a nested if-else block:
if statement: codeelse: if statement 2: code else: codeThe better way is with an elif statement, short for else if. Here's the format:
if statement: code # This code is run if statement is True.elif statement 2: code # This code is run if statement is False and statement 2 is True.else: code # This code is run if neither statement is True!You can have as many elifs as you want. You don't even need an else statement! Python will go through each statement until it finds one that is True. Then it stops the if statement and runs the code inside.
Improving on our already improved example, let's add an elif or two.
num = int(float(input("Enter a number:")))if num < 10: print("Your number is less than 10.")elif num < 20: print("Your number is less than 20.")elif num < 30: print("Your number is less than 30.")else: print("Your number is not less than 10, 20, or 30.")