Lesson 01-07: Variable Reassignment¶
Learning Target: I can reference a variable in its own reassignment statement.
With lots of experience in math and little experience in programming, when you see something like this:
x = x + 1
Red flags should instantly go off in your head. How can x be equal to itself plus 1?? Can 5 be equal to 6? No, of course not!
Increasing or Decreasing a Variable¶
What you should not forget, however, is that the = symbol in programming is different from the = symbol in mathematics.
To review, the equal sign means variable assignment - it has a very specific purpose.
- Recall the meaning of the left and right side of a variable assignment statement:
- Left side: the variable to store into
- Right side: the value or expression to store
An important thing to note is that the right side is always evaluated before being saved into the variable on the left. This gives the line of code x = x + 1 a whole new meaning. Let’s break it down:
Given a statement x = x + 1, let’s assume the value of x is currently 0. Since it is an assignment statement, the right side of the equal sign, or x + 1, gets evaluated first. Since x is a variable being used in an expression, we use its actual value instead, which is 0. So the expression being evaluated is 0 + 1, which is just 1.
This leaves us with the statement x = 1, which then allows us to store 1 in x, basically increasing the value of x by 1. Up until the beginning of this paragraph, x was still 0! It doesn’t actually change until you complete the assignment statement.
However, this would have worked with any value of x! Let’s say x was 28. The expression x + 1 would then be 28 + 1, or 29, which then gets saved back into x. x is changed from 28 to 29, which is the same thing as increasing it by 1.
Let’s run through some example code below, step by step.
(cl_reassign_1)
x = 10
x = 10 * 5
x = x - 5
x = x / 9
What will the result of x be after the code above is run?
Warning: Error Ahead!¶
You’ll want to be careful about how you use variable reassignment. For example, take the following program:
Notice how you get an error. That’s because of how regular variable assignment works.
Remember that the right side of the equal sign is evaluated first. So it’s trying to evaluate x + 1, but x doesn’t have a value yet! So it will give you a NameError.
You would have to give variables an initial value. This is called initializing a variable. So before you can use x = x + 1, you will have to give x a value, using a regular assignment statement. Maybe with x = 0. The new code will look like this:
Always make sure that your variables are initialized!
Checks for Understanding¶
Q#1¶
Given a variable x, write an expression that will increase x by 1. Include no spaces in your solution.
Q#2¶
In the space below, write code on line 2 that will double the value of x. If done correctly, the output should be 10. Please do not just put x = 10, as your solution should work for all values of x.