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

Data Types

Python supports lots of different data types, but the there are 4 data types that you will need to know about;

1. Integer
2. Float
3. String
4. Boolean

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Integers

Integers are whole numbers. This could be 1, 4, 50, 1000 etc - they have no decimal after.

player1score = 250

We can perform calculations using Integers and Floats. We can add +, subtract -, multiply *, and divide / and more.

The main thing to remember is that when you want to perform a calculation on a variable we have to do something with the answer. Let's say we want to add 50 to our player1score variable to take its value to 300

player1score = 250
player1score + 50

The above code will not work - we are adding 50 to the player1score, but the data stored in that variable is not being updated.

player1score = 250
player1score = player1score + 50
OR
player1score += 50

We have to say player1score equals itself + 50.

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Float

Since Integers can not have a decimal after, another data type is needed, this is where Float's come in. They can have a decimal value; 3.14, 100.1, 120.567 etc

price = 7.99

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String

A string is a way of storing text. The string data type allows use to store a string of characters, these characters can be any letter of the alphabet, a number, or a symbol. To store a string it must be enclosed by quotes - for example;

myText = "Hello"

Strings can be manipulated using a variety of functions built into Python. Here is a list.

Strings can be added, although not in the same sense as an integer or float can be added, but rather strings can be concatenated - which simply means to merge two things together.

text1 = "Hello"
text2 = "Friend"
text3 = text1 + text2
print(text3)

The outcome of the code above would be;

>> HelloFriend

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Boolean

A boolean is a data type that only has two possible values; True or False.

access = True

Python will automatically determine the datatype of the variable based on the data you assign it - if you create a variable x = 3, it will assume the data type is an integer, if you create a variable y = 0.4 then it will assume the data type is float.

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. There are two types of loops you will learn about; fixed and conditional.

Fixed 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. Hence the name, fixed loop.

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)

Conditional 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. We give the code a condition, and when that condition becomes false, the loop stops.

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.

Comparison	Meaning
==	Equal to
!=	Not equal to
<=	Less than or equal to
>=	Greater than or equal to
>	Greater than
<	Less than

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.

You can use multiple arrays in a single program to store different collections of data.

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.

Input/Output

Input and Output is paramount in programming. A program becomes more useful when it has data input and output. Python has two built in functions (pre-made code) to deal with inputting and outputting data; input() and print()

Output

Let's start with the easier of the two; output. There are many situations where you would want to output data; printing a score, name, answer, question, instruction. But how do we do it?

print("Hello")

This single line of code will output in the Shell the word Hello

>> Hello

The print() function will print anything that is put between the brackets. We can print strings, integers, floats, booleans, arrays, and even variables.

num = 55
print(num)

Note, to print a variable, or its contents, we put the name of the variable inside the brackets - no quotes are needed because then it becomes a string.

>> 55

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Input

Now for the slightly more challenging of the two; Input.

Input is used for situations where we want the user to enter some information into our program. They might be asked to enter their name, age, a score, a price of an item, or any other data that we request. Whatever they enter, we must store this data in a variable. So the first step is to create a variable and name it according to the data to be entered.

For example, if we want the user to enter their name, we would create a variable called name or user_name. Again, it depends on the situation as to what you name your variable.

name = input ()
John

When the above line of code is executed, the program will wait until the user has entered their name - now it is important to know that the user could actually enter anything they wish and the program (at this stage) does not know if that data they have entered is correct.

Once the user has entered their name, it will be stored in the variable name and we can use this data to print out a greeting;

print("Hello " + name)

The outcome of this small program would be;

>> Hello John

We can take this a step further by doing an output and input with only one function.

name = input("What is your name?")

As you can see from the code above, we have entered a string between the brackets for the input() function. In here we can give a message to the user explaining what data we want them to enter. The outcome from this could would be;

>> What is your name?

And the program would wait until the user types in their name.

Standard Algorithms

In programming there are some universally agreed standards on how to program a particular algorithm; Standard Algorithms. They set out the standard practice of writing the code in order to solve a problem.

• Running Total
• Traversing 1-D Array
• Input Validation

Traversing a 1D Array

Traversing a 1 dimensional array is simply using a loop to step through the array to access the data stored at each index position.

You should look back at Arrays if you are still unsure as to how they work.

We can use a fixed loop to step through each index position, due to a fixed loop using a variable as a counter, we can use this counter as the index position of the array. As a quick example we have an array containing the daily visitor numbers for a park in a week.

dailyVisitors = [23,32,45,26,28,66,78]
for i in range(0, len(dailyVisitors)):
	print(dailyVisitors[i])

The code above will produce the following output;

>> 23
>> 32
>> 45
>> 26
>> 28
>> 66
>> 78

As the for loop runs, i will be given the value 0, then 1, then 2, then 3 and so on until it's value is 8 at which point the fixed loop stops. We then use this value stored in the variable i to tell the computer to print whatever data is stored in the array at that position.

Running Total

Some programs you will need to keep a running total. This could be a total count, or a total sum of all inputted values.

An example would be your program is to keep a running total of the number of daily visitors to a park for a week;

weeklyVisitors = 0
for i in range(1, 8):
	dailyVisitors = int(input("Enter the number of visitors for day " + str(i)))
	weeklyVisitors = weeklyVisitors + dailyVisitors

This would output:

>> Enter the number of visitors for day 1

In the code above, we have a couple of things happening;

1. we set up a variable to store the weekly total visitors - this variable will constantly be updated with the new weekly total as we add each days values into the program.
2. we begin a loop for the number of days in a week
3. in this loop we get input from the user by asking them how many visitors to the park were observed in each day and store this value in dailyVisitors variable
4. we then use the weeklyVisitors variable to set its new value to equal itself plus dailyVisitors variable

Line 3 is where we keep a running total of the weekly visitors as the content of the variable is constantly changed when the program is run.

Another isntance you will expected to implement a running total is when using an array;

dailyVisitors = [23,32,45,26,28,66,78]
weeklyVisitors = 0
for i in range(0, len(dailyVisitors)):
	weeklyVisitors = weeklyVisitors + dailyVisitors[i]

print("The total weekly visitors was:", weeklyVisitors)

The code above would produce the following output;

>> The total weekly visitors was: 298 

When using an array we need to use a fixed loop to allow us to loop through the array and access each index position and therefore its value to add to our running total.

Input Validation

A program is not going to be robust if we allow the user to enter any data they want, especially if we ask for data that is to be between two boundaries but we allow the program to accept any input.

For example, a program is written to ask the user to enter test scores as a percentage - the user can type in anything, but the program should reject exceptional data such as 110% or -20% as these particular pieces of data in the context of this program are unacceptable.

score = int(input("Please enter the score:"))
while score < 0 or score > 100:
	print("Error - score must be between 0 and 100")
	score = int(input("Please enter the score:"))

If we test the above code, we should get rejected when we enter in exceptional data

>> Please enter the score: 298
>> Error - score must be between 0 and 100
>> Please enter the score:-10
>> Error - score must be between 0 and 100
>> Please enter the score: 56