Computing Scotland

Variables

Variables are a way of storing information in the program. The value they hold is able to change throughout the programs run time. They are made up of two components;

1. Name
2. Value

The name of the variable is important - think of it like you are packing up your belongings to move house. In each box there will be items, usually of the same kind, so for example, we are packing away all of your shoes. The box will contain all of your shoes and you would write on the box "shoes" so that you know in that box there are all of your shoes.

The name must be descriptive of what is inside of the variable. If my variable is storing a players score, I would call my variable score, or player1score.

There are a few rules to take note of when deciding on a variable name;

1. Be descriptive
2. Start with a letter
3. Can not contain spaces (player Score)
4. Can not be a reserved word

So, to use the players score as a worked example, the variable would look something like this:

player1score = 250

CSV Files

CSV stands for Comma Separated Values. It is a file that contains data which are, as by its name, separated by commas.

Here is an example of a CSV file containing the top 250 films on IMDb. Each row (ignoring the 1st) has the same type of data in each position as the next line; TitleID, Title, Year, Rating and Rating Count

TitleID,Title,Year,Rating,Rating Count
tt0111161,The Shawshank Redemption,1994,9.2,"2,089,382"
tt0068646,The Godfather,1972,9.2,"1,433,884"
tt0071562,The Godfather: Part II,1974,9.0,"995,997"
tt0468569,The Dark Knight,2008,9.0,"2,055,819"
tt0050083,12 Angry Men,1957,8.9,"591,688"
tt0108052,Schindler's List,1993,8.9,"1,082,960"
tt0167260,The Lord of the Rings: The Return of the King,2003,8.9,"1,487,166"
tt0110912,Pulp Fiction,1994,8.9,"1,633,178"
tt0060196,"The Good, the Bad and the Ugly",1966,8.8,"620,260"
tt0137523,Fight Club,1999,8.8,"1,670,480"

We can import this data into our Python program and store the data in parallel arrays or in an array of records. Once the data is imported and stored, we can then perform calculations, run algorithms and add to the data.

Reading a CSV file in Python

Below is Python code to import a csv file, read it's contents into an object and then loop through each line of the file. As we loop through each line, we can then access the individual pieces of data (separated by commas) and then store them in an appropriate data structure.

import csv

with open ("IMDbTop250.csv", 'r') as open_file:
	open_file_read = csv.reader(open_file)
	for row in open_file_read:
		#Store data 
open_file.close()

Let's break down the code;

with open ("IMDbTop250.csv", 'r') as open_file:

This line will open the file called IMDbTop250.csv and copy the raw contents into a data structure called open_file. We can choose what mode we open the file using;

• 'r' open for reading (default)
• 'w' open for writing, truncating the file first
• 'x' open for exclusive creation, failing if the file already exists
• 'a' open for writing, appending to the end of the file if it exists
• '+' open for updating (reading and writing)

We then use open_file to actually start reading through the data;

open_file_read = csv.reader(open_file)

Python has a built in function to read CSV files, so we pass into this function the open_file object and it will return back to us a formatted data structure that we can then start to read through using a loop.

for row in open_file_read:

This line will create a fixed loop for the amount of rows in the csv file (no longer using the csv file, but instead the open_file_read object). With each loop, we can access the individual pieces of data in the row, which are separated by commas using the same priciple of accessing each item in an array, so row[0] would access the first item in the row etc.

Finally, it is important that we close the file once we have extracted all of the content and stored a copy into our program

Writing to a CSV file in Python

To write to a CSV file we setup our code in a similar way to reading a csv file; first we need to open an exising csv file or create one, then we will use a built in function to write the data to the file and then close the file.

with open("IMDbTop250.csv","w+") as open_file:
	open_file.write(___________)

Loops

Loops allow us to repeat code either a set number of times or an unknown number of times. Using loops is incredibly important for reducing the amount of code needed and therefore improves the efficiency of our code.

For Loop

First we will look at a For loop which is used for when we know how many times we want the code to loop for.

for i in range(1,6):
print("Hello")

The print statement will run and print Hello 5 times.

>> Hello
>> Hello
>> Hello
>> Hello
>> Hello

You might be wondering, why does it say 1,6 but only prints 5? Well, the numbers you specify will have an impact on the number of loops, the first number will be the starting number, the second number is the ending number, but once the loop reaches 6, it will not execute a 6th time.

So an easy rule of thumb is to know your loop number, for example 10 and then add 1 to it. So, n+1 where n = number of iterations (loops)

While Loop

There will be situations where you don't know the amount of times you need to loop code, this is where a while loop comes in use.

A while loop will repeat code while a condition is True.

For example;

num = 0
while num != 10:
	print(num)
	num = num + 1

The outcome of this code would be;

>> 0
>> 1
>> 2
>> 3
>> 4
>> 5
>> 6
>> 7
>> 8
>> 9

It will print 0 - 9, as once num has a value of 10, it will stop the loop.

Selection

Selection is where we give the computer a condition and it must work out if the condition is true or false, from this outcome we can tell the computer to do x or y.

Let's consider the scenario where a teacher is entering grades into the computer and it must decide whether the person has passed or failed.

If a students achieves 40% or more on their test, they pass. If they achieve less than 40% then they fail.

score = 67
if score > 40:
	print("Fail")
else:
	print("Pass")

The outcome of the code above would be;

>> Pass

We can visualise the decision the computer needs to make by using a flowchart - the computer has two paths that it can go down but can only go down one.

Sometimes there is a situation where there is more than one True or False situation. Consider the teacher entering grades secnario; we have grades A, B, C, D, E, F and then Fail. We can ask the computer to choose a grade based on the inputted percentage.

This is where an ELIF condition is used. We start off with an IF, and then if that condition is False, it will move onto the next condition. And then if that condition is False, it will move onto the next condition and so on.

score = 56 
if score >=90:
	print ("A Grade") 
	elif score >= 80:
	print ("B Grade") 
	elif score >= 70:
	print ("C Grade") 
	elif score >= 60:
	print ("D Grade") 
	elif score >= 50:
	print ("E Grade")
	elif score >= 40:
	print("F Grade")
else:
	print("Fail")

Arrays

In Python we can store multiple values under one variable name, this is called an array. They work by assigning each item in the array an index number, this allows use to choose specific pieces of data without selecting them all.

An array will always start at index 0. This means that the first item in the array is always at position 0.

To setup an array we start as we would with creating a variable; the name. Same rules apply for array naming as does the variable naming. Next, we need to assign ( = ) the data to the array. Then we use square brackets to tell Python that the following data is to be stored as an array. Each item is separated by a comma.

numbers = [1,2,3,4,5]
prices = [2.50, 11.99, 285.75]
names = ["John", "Sarah", "Steve"]

Accessing the individual data inside the array is relatively straight forward. We call the array using its name and then using square brackets, give a index number of the value we wish to retrieve. For example, I want to print Thursday and Saturday from the array below, therefore I will choose 3 and 5 as my index values

days = ["Monday", "Tuesday", "Wednesday", 
"Thursday", "Friday", "Saturday",
"Sunday"]

print(days[3])
print(days[5])

The above code will output the following;

>> Thursday
>> Saturday

It is therefore safe to assume that the last index position of an array is n-1, where n is the number of items it holds.

Parallel Arrays

At higher you will be expected to understand and be prepared to use Parallel Arrays

Parallel arrays are just 1D arrays that we covered above, but they are related to one another in terms of the data they hold.

names = ["Max","Anna","Alisha","Callum"]
score = [67,87,56,90]

Take the example above, we have 2 arrays; names and scores. The arrays contain different types of information, but they hold related information. Max has a score of 67, Anna has a score of 87, Alisha has a score of 56 and Callum has a score of 90.

This makes the arrays parallel as we are linking (not through programming, but as we understand) the data stored in them. Whatever is at position 0 in one array is related to the data stored at position 0 in the other array(s).

names = ["Max","Anna","Alisha","Callum"]
score = [67,87,56,90]

for i in range(0,len(names)):
	print(names[i], "scored", score[i])

The above code will display the following:

>> Max score 67
>> Anna scored 87
>> Alisha scored 56
>> Callum scored 90

Records

At Higher we are introduced to another data storage concept; Records

Creating Records

Records are a way to store multiple pieces of data within one structure. Let's consider the scenario where we are storing information about cars that we have for sale at a car garage.

We know cars have the following information about them;

• Make
• Model
• Colour
• Engine Size
• Registration

We could use parallel arrays to store each piece of information about a car, but managing these multiple arrays becomes time consuming, difficult and error prone. Instead we will opt for the record structure.

class car():
    def __init__(self, reg, make, model, engine, colour):
        self.carReg = reg
        self.carMake = make
        self.carModel = model
		self.engineSize = engine
        self.carColour = colour

The above code will setup a class blueprint of which we can make copies (instances) of when we want to start storing cars information for each of the cars we have.

class car():
    def __init__(self, reg, make, model, engine, colour):
        self.carReg = reg
        self.carMake = make
        self.carModel = model
		self.engineSize = engine
        self.carColour = colour
		
car1 = car("SP20 RFL","VW","Polo","1.4","Red")
car2 = car("RF67 LLP","Peugeot","208","1.2","Silver")

We have setup 2 cars currently with some information about each one. We use the class we outlined at the beginning as the structure for which we will store the cars information in.

Retrieving Records Data

Now we have created and stored the information relating to each car, we can retrieve the data from each instance in a simple way. We call the instance of the class and then use a . to indicate access to an instances variable(s)

print(car2.carReg)
print(car1.carMake)

>> RF67 LLP
>> VW

SET min TO array[0]
FOR counter FROM 1 TO array LENGTH DO
	IF item < min THEN
		SET min TO item
	END IF
END FOR
RETURN min

min = f_array[0]
for i in range(1, len(f_array)):
	if min > f_array[i]:
		min = f_array[i]		

SET found TO false
SET counter TO 0
GET target FROM KEYBOARD (STRING)
REPEAT
	IF target == names[counter] THEN
		SET found TO True
		SEND "Target was found at position" & counter TO display
	ELSE
		SET counter TO counter + 1
	END IF
UNTIL counter = 10 OR found == True
IF found == False THEN
	SEND "The target " & target & " was not found" TO DISPLAY
END IF

names = ["Mary","Alice","Steven","Frank"]
counter = 0
found = False
target = input("Enter the target: ")
while found == False and counter != 4:
    if names[counter] == target:
        found = True
        print(target + " was found at position " + str(counter+1))
    else:
        counter = counter + 1
if found == False:
    print(target + " was not found")

SET array TO ["Red", "Blue", "Red", "Blue", "Red", "Red", "Blue"]
SET count TO 0
GET target FROM KEYBOARD
FOR loop FROM 0 TO array LENGTH
	IF target = array[loop] THEN
		SET count TO count + 1
	END IF
END FOR

array = ["Red", "Blue", "Red", "Blue", "Red", "Red", "Blue"]
count = 0
target = input("Enter data to search: ")
for i in range(0, len(array)):
	if target == array[i]:
		count = count + 1