Section 6.1 Updating variables

Commonly, assignment statements are used to update a variable, where the new value of the variable depends on the old.

x = x + 1

This means “get the current value of x, add 1, and then update x with the new value.”

If you try to update a variable that doesn’t exist, you get an error, because Python evaluates the right side before it assigns a value to x:

Before you can update a variable, you have to initialize it, usually with a simple assignment:

When you update a variable by adding 1 it’s called an increment; subtracting 1 is called a decrement.

Activity 6.1.1.

Consider the code block below. What happens when you run this program?

y = 1
x = x + 1
print(x)

The integer "2" prints.
Incorrect! Take another look at the second line. Try again!
We get a TypeError.
Incorrect! This will not cause a TypeError because x, y, and 1 are all integers. Try again!
We get a NameError.
Correct! This will cause a NameError because x has not been initialized yet.
The program compiles with no errors but nothing prints.
Incorrect! This program will not compile. Try again!

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