Trace Tables

Understanding Trace Tables

A trace table is a tool used to track the values of variables as each line of code is executed. It helps in debugging and understanding the flow of a program.

Components of a Trace Table

A trace table typically includes the following columns:

Line Number: Indicates the line of code being executed.
Variables: Columns for each variable to track their values.
Output: Any output produced by the program.

Example Program

Consider the following simple program:

x = 5
y = 10
z = x + y
print(z)

Creating a Trace Table

Initialize the Table: Create columns for the line number, each variable, and output.
Track Variable Changes: As you go through each line, update the values of the variables.
Record Output: Note any output produced by print statements.

Line	x	y	z	Output
1	5
2		10
3			15
4				15

Steps to Create a Trace Table

Read the Code: Understand the program to identify all variables and outputs.
Set Up the Table: Draw the table with appropriate columns.
Execute Line by Line: Follow the code, line by line, and update the table accordingly.
Check Output: Verify if the recorded outputs match the expected results.

Benefits of Trace Tables

Debugging: Helps identify where errors occur by tracking variable values.
Understanding Flow: Provides a clear view of how data changes through the program.
Teaching Aid: Useful for explaining how algorithms work step-by-step.

Using trace tables effectively can greatly enhance your debugging skills and your understanding of how programs execute. Practice creating trace tables with different programs to become proficient in their use.

For Loop example 1

total = 0
for i in range(1, 5):
    total += i
    print(total)

Explanation

Line 1: Initialize total to 0.
Line 2: Start the for loop, setting i to 1 initially.
Line 3: Add i to total, then print total. Repeat for i = 2, 3, 4.

Each iteration the line numbers will repeat until the loop condition is broken

Line	i	total	Output
1		0
2	1
3		1	1
2	2
3		3	3
2	3
3		6	6
2	4
3		10	10

For Loop example 2

In this second example the output occurs outside of the for loop. This is a common mistake that people miss in the exam.

total = 0
for i in range(1, 5):
    total += i

print(total)

Line	i	total	Output
1		0
2	1
3		1
2	2
3		3
2	3
3		6
2	4
3		10
5			10

Trace tables without line numbers

Sometimes you see trace tables in the exam without line numbers In this case you would tend to group constructs together

total = 0
i = 1
while i < 5:
    total += i
    i += 1
print(total)

i	total	Output
	0
1	1
2	3
3	6
4	10
5
		10

Explanation

Initialization: total is set to 0 at the start in the sequence
Loop Iteration: i is set to 1 in the loop, For each iteration, add i to total, then increment i on the next line.
Output: After the loop finishes, print the final value of total

Usually the exam paper will provide you with a table, and often the trace is started for you so you just need to follow their lead.