Working Out Program Output

To work out a program's output, follow the code one statement at a time, from top to bottom. For each statement, update the value of any variable it changes. When you reach a print() call, work out what it would show — quoted text as written, a variable's current value, or an expression calculated first — and note that down before moving on.

The order matters because a variable can hold one value early in a program and a different value later. A print() call never changes a variable, but an assignment such as boxes = boxes + 3 does. Keep a running record of every value so you never use an old one by mistake.

boxes = 6
boxes = boxes + 3
print("Boxes counted")
print(boxes)
boxes = boxes * 4
print(boxes - 5)

Output:

Boxes counted
9
31

Track values with a trace table

A trace table records the value of each variable, and any output, after every statement. That makes it clear which value each print call uses.

At the print(boxes) call, boxes holds 9, so 9 appears on screen. The next assignment changes it to 36, so the last print call starts from 36, works out 36 - 5, and prints 31. Reading the print calls out of order, or skipping an assignment in between, gives the wrong output.