In this section, we are going to learn how to use if statements to affect the way our code runs in Python. Let's write a program to check if a number is positive or negative to see this in action:

number = int(input("Enter a number: "))

First, we can use an if statement to check if a number is positive. To start an if statement, we use the Python keyword if. After that, we include the condition that we learned about last time. In this case, our condition will be something like number > 0, because all positive numbers are larger than 0. After the condition, we always have to put a colon to indicate the end of the line. On the next lines, we put whatever we want to happen if the condition is true, making sure to indent each line. Here, if the condition is True, we want to inform the user that their number is positive:

number = int(input("Enter a number: "))

if number > 0:
  print(f"{number} is positive")

Notice that if we enter a number that is greater than 0, the condition is True, and the line is printed to the console. But if we enter a number less than 0, the condition is False, the computer skips over everything in the if statement, and nothing gets printed to the console.

Now, let's also tell the user if their number is negative. To do that, we can add on an elif statement to our if statement. elif is short for "else if", and this section acts just like our original if section, except the computer only looks at it if the condition in the if statement is False. Otherwise, the computer skips right over it. So if number > 0 is False, we can then check if number < 0 and inform the user that their number is negative:

number = int(input("Enter a number: "))

if number > 0:
  print(f"{number} is positive")
elif number < 0:
  print(f"{number} is negative")

Now if we test this again, we see we get proper outputs for both positive and negative numbers, but nothing for 0, which is neither positive nor negative. We can use one last tool to include it in our if block.

The else statement is a final catch-all section of an if block. It doesn't have any conditions, and it only gets run if all the other sections fail. In our case, we know that if a number is not positive and is not negative, it must be 0. So if an input fails our first two conditions, it has to be 0. Let's add one final update to our code to reflect this new information:

number = int(input("Enter a number: "))

if number > 0:
  print(f"{number} is positive")
elif number < 0:
  print(f"{number} is negative")
else:
  print("0 is not positive or negative")

A few reminders about if statements:

• An if statement must start with the keyword if, and can have exactly one only.
• You can have as many elif sections as you want after an if statement.
• There can be one else section only, and it must be at the very end of the if statement.
• Always put a colon at the end of the statement, and indent everything inside the body.