Expressions

An assignment statement tells the computer to store a value in a variable.

Programming

Watching a Variable Change

Programs spend most of their time doing one thing over and over: taking a value that already exists, doing something with it, and keeping the result. An assignment statement is the piece of program code that makes this possible. It tells the computer to store a value in a variable, and when the value being stored is produced by a computation, the statement connects the two actions together: computing with data and storing the result.

What do you think happens?

If x holds 5 and then the statement x = x + 1 runs, what is the value of x afterward?

5
6
x + 1

Reveal answer

Answer:

The right side x + 1 is computed first, using the value 5 that x currently holds, which gives 6. That 6 is then stored back into x, replacing the old 5.

How a Computed Result Is Stored

Trace the statement x = x + 1 one step at a time. The variable on the left of the equals sign is the destination. The expression on the right side is evaluated first. Evaluating it means reading the variable's current value, performing the addition, and producing a new value. Only after that new value exists does the assignment store it in the destination. The same name, x, appears on both sides, but the two appearances play different roles: on the right it supplies the old value, and on the left it receives the new one.

Before

x = 5

x holds 5

After

x = x + 1

x holds 6

Code walkthrough

x = 5
x = x + 1
print(x)

1
Line 1 the destination x receives the value 5.
2
Line 2 the expression x + 1 reads 5, computes 6, and the result is stored back into x.
3
Line 3 reading x now gives 6, showing the assignment replaced the old value.

The first line stores the literal 5 in the variable x.
The second line is the assignment: the right side x + 1 is computed from the current value of x.
print(x) shows the value stored after the assignment finished.

Output

What is the value of x before the assignment runs, and how does the old value feed into the new one?

Anatomy of an Assignment Statement

An assignment statement has the general form variable = expression. The left side is the destination that receives the value. The right side describes the value-producing expression. When tracing or debugging, it helps to separate the computation from the destination that receives its result: first ask what the right side computes, then ask where that result is saved.

python

interest = principal * 0.07

interest = principal * 0.07

Notes on the code

interest is the destination: the variable that receives the result.
principal is a variable whose existing value takes part in the computation.
0.07 is a literal value written directly in the code.
The whole right side principal * 0.07 is the expression, and its result is stored in interest.

In the source example interest = principal * 0.07, principal participates in a multiplication, and interest is the destination for the result. The assignment therefore combines two actions: it computes with data that already exists and stores the computed value. The diagram below breaks the statement into its parts.

How are the variable, operator, and literal combined into one expression, and which part is the destination?

What Counts as an Expression

Definition

An expression is program code that represents or computes a value.

Key point

When reading a piece of program code, ask one simple question: what value does this piece represent or compute? If the code has an answer, it is an expression. An expression does not have to be complicated. It might be one literal, one variable, one function call, or a combination of these parts.

Think of an expression as a value-producing unit assembled from smaller pieces. The smallest pieces are literals, variables, and function calls. Operators such as + and > can connect these pieces into a more complicated expression. Once the larger expression has a value, that value is the thing being passed along, whether it is assigned, passed as a parameter, or combined again.

Function Calls Inside an Expression

Consider the statement total = double(4) + 1, where double is a function defined as a subroutine that returns twice its input. When the computer reaches the function call, control moves into the function. The function returns 8, and that returned value takes the place of the call in the larger expression. The expression is now 8 + 1, which produces 9, and 9 is stored in total.

Code walkthrough

def double(n):
    return n * 2

total = double(4) + 1
print(total)

1
The assignment starts with its right side: double(4) + 1.
2
Control transfers into double with n equal to 4, which returns 8.
3
The expression 8 + 1 produces 9.
4
The assignment stores 9 in total, and print shows it.

double is a subroutine that returns a value.
double(4) is a function call used as one building block of the larger expression.
The returned value replaces the call, so the expression becomes 8 + 1.

Output

What happens step by step when a function call appears inside an expression: where does control go, and how does the returned value replace the call?

Where an Expression's Value Goes

Once an expression has computed a value, that value has several possible uses. It can be assigned to a variable, supplied as a parameter in a subroutine call, or combined with another value to build a more complicated expression. Programming does not require every computed value to be saved. In some situations the value can be ignored.

Piece of code	What it does	Example
Expression	Represents or computes a value	principal * 0.07
Assignment statement	Stores a value in a variable	interest = principal * 0.07
Destination	The variable that receives the value	interest

An expression produces a value; the assignment statement saves it.

Mistakes Beginners Make with Expressions

Common mistake
Treating the destination as part of the computation
What it looks like
In interest = principal * 0.07, reading interest as an input to the multiplication.
Why it fails
The variable on the left receives the result. The computation is the right side only.
Do this instead
Evaluate the right side first, then ask where the result is stored.
Common mistake
Assuming every computed value must be saved
What it looks like
Writing a function call whose result is never assigned and expecting a variable to change.
Why it fails
A computed value can be used directly, passed as a parameter, or ignored. Only an assignment stores it.
Do this instead
Check whether the code assigns the value to a variable before expecting a variable to change.
Common mistake
Thinking only literals and variables can be part of an expression
What it looks like
Believing that a call such as double(4) cannot appear inside x + 1.
Why it fails
A function call supplies a returned value, so it can appear wherever an expression needs a value.
Do this instead
Replace the call mentally with the value it returns, then evaluate the rest of the expression.
Common mistake
Forgetting that the old value of a variable is used on the right side before it is replaced

Practice Tracing Assignments

EASY

Trace the following program. After each line, write down the value stored in a. Then state what a holds at the end. a = 3, then a = a * 2 + 1.

Hints

Start by finding the value of a on the right side of the second line, using the value stored after line 1.
Compute the whole right side first: multiply the old value by 2, then add 1.
The result replaces the old value of a in the destination.

Your attempt (not saved)