Trace tables
A trace table records every variable at each step, to check the algorithm works.
| Pass | n | n > biggest? | biggest after |
|---|---|---|---|
| 1 | 7 | No | 7 |
| 2 | 3 | No | 7 |
| 3 | 12 | Yes | 12 |
| 4 | 5 | No | 12 |
e.g.Example
Output: 12. Correct.
Iterating with for
PracticalPresent ▶
What this lesson covers
Topic 11 Algorithms
KS3 Computing · An algorithm is a precise, step-by-step plan to solve a problem. Computational thinking turns a messy problem into one. Every algorithm is built from just three things: sequence, selection and iteration.
Topic 13 Programming basics
KS3 Computing · A program stores information in variables, works on it with operators, and controls the order with sequence, selection and iteration. Examples are in Python, with Scratch alongside for Year 7.
Programming Computational ThinkingCambridge Lower Secondary Computing 0860
You will learn to
- Use a for loop to visit every item in a list, one at a time.
- Total a list of prices with a running total that starts at 0.
- Find the biggest value by keeping the 'biggest so far', and trace it.
- Count how many prices are over 10, using a for loop with an if inside.
Success criteria
- I can total a list of prices with a for loop, without using sum().
- I can fill in a trace table for the 'biggest so far' program.
- I can count the prices over 10 and check the answer by hand.
Key words
Say each word, then read what it means. Every one is in the glossary too.
- trace table
- Records variable values step by step to check logic.
- infinite loop
- A loop whose condition never becomes false.
- Algorithm
- Precise, ordered, finite steps to solve a problem.
- Decomposition
- Breaking a problem into smaller ones.
- Abstraction
- Ignoring detail that doesn't matter for the problem.
- Sequence · selection · iteration
- Steps one after another. First this, then this.
- Pseudocode
- Code-like writing for algorithms, without strict rules.
- Variable
- A named place in memory that holds a value.
- Assignment
- Storing a value in a variable. = in Python, ← in pseudocode.
- Casting
- Changing a value from one data type to another, like int("13").
- Concatenation
- Joining strings together: "7" + "7" = "77".
- Selection
- Choosing a path with if / else.
- Iteration
- Repeating steps: a loop.
- Boolean
- A value that is only True or False.
The explanation
Read it here, or open the study page below to go one step at a time with things to try.
Loops: for
for i in range(5):
print("Lap", i)Prints Lap 0 to Lap 4. range(5) gives 0, 1, 2, 3, 4: it starts at 0 and stops before 5.
e.g.Example
Scratch: 'repeat 5'.
Loops: while
password = ""
while password != "dragon":
password = input("Password: ")
print("Access granted")Repeats while the condition is true. As soon as the user types dragon, the loop ends.
!Watch out
If the condition can never become false, it loops for ever: an infinite loop.
Study it
Open the study page and go one step at a time.
The steps for this lesson
- Topic 11, step 9: Trace tables (Trace 'find the biggest')
Try it
Check yourself
One question at a time. The reason shows after each answer.
Saved on this device.