Using Trace Tables

Posted on October 5, 2019 by Administrator Posted in A Level Concepts, Computer Science, Computing Concepts, GCSE Concepts

A trace table is a technique used to test an algorithm and predict step by step how the computer will run the algorithm. It can be used to understand or predict what an algorithm is doing and to identify potential logic errors (when the program compiles but does not produce the expected output).

The animation below demonstrate the use of a trace table used to track the values of variables as they change while the program is running.

Using a trace table to test an algorithm is called dry run testing.

Your Task

Check the following algorithms and complete their trace tables. (Click on any cells of these tables to add content)

Algorithm #1Algorithm #2Algorithm #3Algorithm #4

Algorithm #1: While Loop

i = 0
j = 10

WHILE i < j
   i = i + 1
   j = j - 1
END WHILE

OUTPUT(i)
OUTPUT(j)

Trace Table

Line Number	i	j	i < j
1	0
2		10
4			True
5	1
6		9
4			True
…

Algorithm #2: Factorial

number = 5
factorial = number

WHILE number>2
   number = number - 1
   factorial = factorial * number
END WHILE

OUTPUT(factorial)

Trace Table

Line Number	number	factorial
1	5
2		5
…

Algorithm #3: Fizz-Buzz Sequence

FOR i FROM 1 TO 20
   IF i MOD 3 == 0 AND i MOD 5 == 0 THEN
      OUTPUT "Fizz-Buzz"
   ELSE IF i MOD 3 == 0 THEN
      OUTPUT "Fizz"
   ELSE IF i MOD 5 == 0 THEN
      OUTPUT "Buzz"
   ELSE
      OUTPUT i
   END IF
NEXT i

OUTPUT("The End")

Trace Table

Line Number	i	i MOD 3 == 0	i MOD 5 == 0	OUTPUT
1	1
2		False	False
4		False
6			False
8
9				1
1	2
…

Algorithm #4: Max Function

FUNCTION max(a, b)
   IF a>b THEN
      RETURN a
   ELSE 
      RETURN b
   END IF
END FUNCTION

number1 = 7
number2 = 12
number3 = max(number1, number2)
OUTPUT(number3)

Trace Table

Line Number	number1	number2	a	b
9	7
10		12
11	7	12
1			7	12
…

Fibonacci Sequence using LMC

Posted on September 24, 2019 by Administrator Posted in Computer Science, Computing Concepts, Little Man Computer (LMC), Solved Challenges

Did you know?

The Fibonacci Sequence is the series of numbers: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 … where the next number is found by adding up the two numbers just before it.

The first 10 numbers of the Fibonacci number sequence are:

0, 1, 1, 2, 3, 5, 8, 13, 21, 34

The following table shows how we can calculate each Fibonacci number from this sequence:

Fibonacci Number	Calculation
0
1
1	= 0 + 1
2	= 1 + 1
3	= 1 + 2
5	= 2 + 3
8	= 3 + 5
13	= 5 + 8
21	= 8 + 13
34	= 13 + 21
…	…

Little Man Computer

Your challenge is to write a program using one of the following online LMC Simulators to calculate and output the first 10 numbers of the Fibonacci sequence.

LMC Instruction Set

Note that in the following table “xx” refers to a memory address (aka mailbox) in the RAM. The online LMC simulator has 100 different mailboxes in the RAM ranging from 00 to 99.

Mnemonic	Name	Description	Op Code
INP	INPUT	Retrieve user input and stores it in the accumulator.	901
OUT	OUTPUT	Output the value stored in the accumulator.	902
LDA	LOAD	Load the Accumulator with the contents of the memory address given.	5xx
STA	STORE	Store the value in the Accumulator in the memory address given.	3xx
ADD	ADD	Add the contents of the memory address to the Accumulator	1xx
SUB	SUBTRACT	Subtract the contents of the memory address from the Accumulator	2xx
BRP	BRANCH IF POSITIVE	Branch/Jump to the address given if the Accumulator is zero or positive.	8xx
BRZ	BRANCH IF ZERO	Branch/Jump to the address given if the Accumulator is zero.	7xx
BRA	BRANCH ALWAYS	Branch/Jump to the address given.	6xx
HLT	HALT	Stop the code	000
DAT	DATA LOCATION	Used to associate a label to a free memory address. An optional value can also be used to be stored at the memory address.

Solution...

The solution for this challenge is available to full members!
Find out how to become a member:
➤ Members' Area

Tagged with: Little Man Computer, LMC

Hardware Contest

Posted on September 20, 2019 by Administrator Posted in A Level Concepts, A Level Quiz, Computer Hardware, Computer Science, Computing Concepts, GCSE Concepts, GCSE Quiz

In this hardware contest, you will be presented with two computers at a time (desktops or laptops). Your task is to compare the technical specifications of both computers and discuss and justify which computer seems to have the highest specifications. You should discuss your findings with your peers.

Computer #1

CPU Clock Speed
Number of Cores
CPU Cache
RAM
Graphic Card
Hard Drive
Optical Drive
NIC Card
Screen Size

Computer #2

CPU Clock Speed
Number of Cores
CPU Cache
RAM
Graphic Card
Hard Drive
Optical Drive
NIC Card
Screen Size

New Contest

LMC Simulator

Posted on September 18, 2019 by Administrator Posted in Computer Science, Computing Concepts, Little Man Computer (LMC)

LMC simulators are based on the Little Man Computer (LMC) model of a computer, created by Dr. Stuart Madnick in 1965. The LMC simulator is generally used to for educational purposes, because it models a simple Von Neumann architecture computer which has all of the basic features of a modern computer. It is programmed using assembly code.

The LMC simulator demonstrates how assembly code is processed by the CPU using the Fetch, Decode and Execute (FDE) Cycle. It shows the impact of the FDE cycles on the main registers found inside the CPU:

The PC (Program Counter),
The CIR (Current Instruction Register),
The MAR (Memory Address Register),
The MDR (Memory Data Register),
The Accumulator.

The LMC simulator also demonstrates how the CPU interacts with the Random Access Memory (RAM) and how both data and instructions are stored in the RAM. On our LMC simulator the RAM consists of 100 memory cells (also called mailboxes). Each cell has a unique memory location (a number between 00 and 99).

Most assembly instructions consist of an opcode and an operand.
When coding in assembly code we use mnemonics (e.g. INP, ADD, SUB, LDA, STA, etc.) for the opcode of the instruction. A lookup table (See at the end of this blog post) is used to check the corresponding opcode matching a mnemonic. With the LMC simulator, the operand is a memory location (Direct Addressing) of where the data used by the instruction can be fetched from (e.g. LDA, ADD, SUB instructions) or needs to be stored (e.g. STA instruction).

Labels can also be used instead of hard coding memory locations in the code. Labels are extremely useful when using branching instructions (BRA, BRZ, BRP), as they can be used to locate a specific instruction in the code/RAM. They can also be used to name memory cells in the RAM used to temporary store values. (Similar to the use of variables in a high level code).

LMC simulators can be used to implement simple programs to perform basic mathematical operations such as adding or subtracting numbers, comparing or sorting numbers, etc.

Launch our online LMC SimulatorOpen in new tab/window

You will find on this blog a range of LMC challenges that you can complete using our online LMC simulator:

LMC Instruction Set / Lookup Table

Note that in the following table “xx” refers to a memory address (aka mailbox) in the RAM. The online LMC simulator has 100 different mailboxes in the RAM ranging from 00 to 99.

Mnemonic	Name	Description	Op Code
INP	INPUT	Retrieve user input and stores it in the accumulator.	901
OUT	OUTPUT	Output the value stored in the accumulator.	902
LDA	LOAD	Load the Accumulator with the contents of the memory address given.	5xx
STA	STORE	Store the value in the Accumulator in the memory address given.	3xx
ADD	ADD	Add the contents of the memory address to the Accumulator	1xx
SUB	SUBTRACT	Subtract the contents of the memory address from the Accumulator	2xx
BRP	BRANCH IF POSITIVE	Branch/Jump to the address given if the Accumulator is zero or positive.	8xx
BRZ	BRANCH IF ZERO	Branch/Jump to the address given if the Accumulator is zero.	7xx
BRA	BRANCH ALWAYS	Branch/Jump to the address given.	6xx
HLT	HALT	Stop the code	000
DAT	DATA LOCATION	Used to associate a label to a free memory address. An optional value can also be used to be stored at the memory address.

Tagged with: Little Man Computer, LMC

The missing key

Posted on September 12, 2019 by Administrator Posted in Computer Science, Python - Beginner, Python Challenges

The aim of this challenge is to write a computer program that will take two whole numbers (integers) as input and output the result of multiplying these two numbers together. However, to make this task more challenging, you have to assume the * key of your keyboard is broken or missing and hence you are not allowed to use the * operator in your code.

You can complete your code in the trinket below:

Hint?

View Hint #1

5 * 4 = 5 + 5 + 5 + 5
7 * 6 = 7 + 7 + 7 + 7 + 7 +7
8 * 3 = 8 + 8 +8

View Hint #2

Use a loop!

Tagged with: iteration

School Trip Bus Quote

Posted on September 7, 2019 by Administrator Posted in Computer Science, Python - Intermediate, Python Challenges, Solved Challenges

A school teacher is organising a school trip for the whole year group. He expects between 250 and 350 students to attend this trip. To estimate the cost of the trip, the school teacher has contacted a coach company to hire several coaches for a day.

The coach company has two categories of buses:

Bus	Number of seats	Cost per day
Large Bus	46 Seats	£360
Small Bus	16 Seats	£140

The school teacher would like a computer program that will:

Ask for the number of students taking part in the trip.
Ask for the number of teachers taking part in the trip.
Calculate the total number of participants (by adding the number of students and the number of teachers).
Find out and output the number of large coaches that will be required.
Find out and output the number of small coaches that will be required.
Calculate and output the total cost of hiring these coaches for a day.

Flowchart

Here is the flowchart for this algorithm:

Python Code

Your task is to use the above flowchart to complete the Python code below:

Testing

Once your code is done, complete the following tests to check that your code is working as it should:

Test #	Input Values	Expected Output	Pass/Fail?
#1	Number of students: 250 Number of teachers: 7	Number of large buses: 5 Number of small buses: 2 Total cost: £2080
#2	Number of students: 300 Number of teachers: 10	Number of large buses: 6 Number of small buses: 3 Total cost: £2580
#3	Number of students: 350 Number of teachers: 12	Number of large buses: 7 Number of small buses: 3 Total cost: £2940
#4	Number of students: 0 Number of teachers: 0	Number of large buses: 0 Number of small buses: 0 Total cost: £0

Extension Task #1

You have noticed that it is more cost effective to hire a large bus (£360) instead of three small buses (3 * £140 = £420) even if the large bus is not full.
So your task is to adapt the above algorithm so that, when calculating the number of large buses, if the number of pupils left (remainder) is greater than 32, your program will hire one extra large bus instead of 3 small buses. It will hence output the most cost effective solution for test #2 and test #3.

The new test plan would be as follows:

Test #	Input Values	Expected Output	Pass/Fail?
#1	Number of students: 250 Number of teachers: 7	Number of large buses: 5 Number of small buses: 2 Total cost: £2080
#2	Number of students: 300 Number of teachers: 10	Number of large buses: 7 Number of small buses: 0 Total cost: £2520
#3	Number of students: 350 Number of teachers: 12	Number of large buses: 8 Number of small buses: 0 Total cost: £2880
#4	Number of students: 0 Number of teachers: 0	Number of large buses: 0 Number of small buses: 0 Total cost: £0

Extension Task #2

To make your program more robust, you can add some validation checks on both inputs. These validation checks will ensure that the user can only input whole numbers (integers). To do so, you should check the following blog post: number only validation in Python.

Solution...

The solution for this challenge is available to full members!
Find out how to become a member:
➤ Members' Area

Tagged with: Flowchart

Real-Time Asteroid Watch

Posted on August 9, 2019 by Administrator Posted in Computer Science, JSON, Python - Advanced, Python Challenges

Every day asteroids are passing close to planet Earth. Their trajectory is closely monitored by NASA as some of these could potentially pose a threat to our planet. In this python challenge we are going to use the NeoWs API to retrieve some real-time data from NASA about these near Earth asteroids.

You can read more about the NeoWs API on: https://api.nasa.gov/api.html#neows-feed

JSON Data

This API uses JSON to format the data. JSON (JavaScript Object Notation) is a popular lightweight data-interchange format. Its main benefit is that it is easy for humans to read and write and it is easy for machines to parse and generate as you can see in the code provided below. You can read more about JSON on https://www.json.org/

You can see what the JSON data returned by the API looks like:
https://api.nasa.gov/neo/rest/v1/feed?start_date=2015-09-07&end_date=2015-09-08&api_key=DEMO_KEY

Python Code

Check our code to see how we make a call to the API and how we retrieve and extract the requested JSON data.

Your Task

Update this code to show all the near Earth asteroids approaching Earth in the next 7 days.
The JSON data also contains links to web pages for each asteroid (e.g. https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=3745034). Update the Python code to also display the relevant web addresses.

Tagged with: JSON

Maritime Signal Flags Translator

Posted on August 7, 2019 by Administrator Posted in Computer Science, HTML, CSS & JavaScript, JavaScript

International maritime signal flags are various flags used to communicate with ships. The purpose of the International Code of Signals is to provide ways and means of communication in situations related essentially to safety of navigation. The code uses a set of 26 flags for each letter of the alphabet plus additional flags for the 0 to 9 digits. Combinations of these alphanumeric characters are assigned as codes for various standardised messages. For instance, the master of a ship may wish to communicate with another ship, where his own radio may not be working.

The aim of this challenge was to design a web-based app to convert any message using the relevant flags.

You can use our online translator below and investigate the HTML, CSS and JavaScript code used to implement this translator.

See the Pen
Maritime Signal Flags Translator by 101 Computing (@101Computing)
on CodePen.

Tagged with: cryptography, CSS

Flip Cards in HTML, CSS & Javascript

Posted on June 27, 2019 by Administrator Posted in Computer Science, HTML, CSS & JavaScript

Check the following code used to create an interactive flip card that can be added to any existing web page.

Your Task

Change the HTML code to create a collection of flip cards using Computer Science terminology. (e.g. Programming Terminology)
Change the CSS code to change the look and feel of your flip card (front and back).

See the Pen
Flip Card by 101 Computing (@101Computing)
on CodePen.

Tagged with: CSS

Space Invaders in Scratch

Posted on June 25, 2019 by Administrator Posted in Block Programming, Computer Science, Scratch

This step by step tutorial will demonstrate how to re-create “Space Invaders” using Scratch.

Step 1Step 2Step 3Step 4Step 5Step 6Step 7Extension

Setting up the stage

Start a new project in Scratch and change the stage backdrop to the backdrop called “Stars”.
scratch-add-backdrop

The Spaceship/Rocket Sprite

Next, we will delete the Cat sprite and create a new Rocket sprite:

We will also resize our Rocketship sprite to 35% of its original size:
space-invader-resize-rocketship

The rocketship sprite has 5 costumes but costume 5 is bigger than the other 4 costumes, so we will delete costume 5:
space-invader-remove-costume

Let’s start coding

Using the Rocketship sprite, let’s add some code.

First we will make sure the “Stars” backdrop is always displayed when the game starts:
scratch-switch-to-stars-backdrop

Now let’s animate the Rocketship sprite:
scratch-animate-rocket

Controlling the Rocket ship using the left and right arrow keys

The rocket ship is positioned at the bottom of the screen. The user should be able to slide the rocket ship horizontally to the left or the right using the arrow keys.
scratch-left-right-rocket

The Invader Sprite

Let’s add a new sprite using the “Gobo” sprite from the scratch library:

We will then resize our invader sprite to 40% of its original size:
scratch-resize-invader

Let’s add some code to our invader sprite to position and animate the sprite:
scratch-animate-gobo

Now let’s add some code to make the invader move across the screen. Everytime the sprite reach the edge, it should drop down closer to the rocket ship and change direction (left/right movement).
scratch-invader-movement

Adding a Game Over Screen

The games ends when an invader hits the rocket. To implement this we will first need design a Game Over screen/backdrop:
scratch-game-over-screen

Then we will add some code to our Invader sprite to detect when the invader is touching the rocket ship.
scratch-game-over-trigger

Adding a laser beam sprite

To add a laser beam, we will paint a new sprite:

Firing the laser beam

The following code will let the user fire a laser beam by pressing the space bar. The laser beam will always be launched from the rocket ship current position:
scratch-fire-laser-beam

Adding a score variable

Let’s make a new variable called score. We will use this variable to score 1 point when the laser beam hits an invader. To do so we will need to go to the variable section:

Then you will click on the “Make a Variable button”.
scratch-make-variable

Give a name to your variable: score and tick the option “For all sprites”
scratch-ball-bouncing-code-complete

Make sure your variable is visible (Tick the checkbox next to the variable itself):
scratch-make-variable-visible

Using our Rocket ship sprite we will edit our code as follows to reset the score to 0 when the game starts:
scratch-reset-score-variable

Then we will add some code to our invader sprite:
scratch-laser-collide-invader

Let’s add some more invaders

We will duplicate our invader sprite 10 times:

All we have to do now is re-position each invader sprite one at a time by changing their x and y coordinates in the code:
scratch-invaders-positions

We now have a fully working game!

space-invader-scratch

Extension Task

Add a “Level Complete!” screen that appears when the user scores 10 points (All invaders have been destroyed!).

Tagged with: Scratch

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Your Task

Algorithm #1: While Loop

Trace Table

Algorithm #2: Factorial

Trace Table

Algorithm #3: Fizz-Buzz Sequence

Trace Table

Algorithm #4: Max Function

Trace Table

Did you know?

Little Man Computer

LMC Instruction Set

Solution...

LMC Instruction Set / Lookup Table

Hint?

Flowchart

Python Code

Testing

Extension Task #1

Extension Task #2

Solution...

JSON Data

Python Code

Your Task

Your Task

Setting up the stage

The Spaceship/Rocket Sprite

Let’s start coding

Controlling the Rocket ship using the left and right arrow keys

The Invader Sprite

Adding a Game Over Screen

Adding a laser beam sprite

Firing the laser beam

Adding a score variable

Let’s add some more invaders

Extension Task

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