Constructs

• Describe, exemplify and implement high-level constructs in Python

Success Criteria

• Be able to use and describe several different computational constructs in Python

Programming Languages

• All programming languages are a series of instructions that follow a order.

• Programming languages are compiled from a high-level language (such as Python) to machine code (made up of binary 1s and 0s).

• Some computer languages, such as PHP and Python, have a conversion step in between called op-code or byte-code.

Programming Languages

• Programming languages such as Python are made of constructs which are made up of expressions (you'll see this as we go on)

Assignment

• Assignment is one of the simplest constructs. In Python, variables do not have to be declared before they are assigned:

``````variable1 = "Hello class"
variable2 = 231``````

Mathematical Expressions

• Computers are very good at mathematical calculations.
• Programming languages take advantage of this.
• To do this, we simply write the mathematical expression using the correct operators:
Symbol Name Purpose Example
+ Plus Addition 5 + 6
- Minus Subtraction x - 10
* Times Multiplication 5 * var
/ Divide Division var / 4
``````var1 = int(input())
print(var1 * 5)``````

mATHEMATICAL EXPRESSIONS

• Computers are very good at mathematical calculations.
• Programming languages take advantage of this.
• To do this, we simply write the mathematical expression using the correct operators:
Symbol Name Purpose Example
+ Plus Addition 5 + 6
- Minus Subtraction x - 10
* Times Multiplication 5 * var
/ Divide Division var / 4
``````var1 = int(input())
print(var1 * 5)``````

Relational operators

• Computers can also very quickly perform comparisons.
• By comparison, we are talking about checking if something is equal to, greater than or not equal to.
Actual symbol Python symbol Name Example
= == Equal to 5 == 5
> > Greater than x > 10
< < Less than y < 6
!= Not equal to var ≠ 4
>= Greater than or equal to 6 >= 6
6 >= 3
<= Less than or equal to 6 <= 6
4 <= 6

String concatenation

• As mentioned before strings can be concatenated (joined) together in Python:

``````firstName = "Joe"
surName = "Balfour"
output = firstName + surName
print(output)``````

Flow Control

Flow Control

• Flow control is used to control the flow of the program.
• At National 5 you need to know about the most important flow control structures:
• Sequence
• Selection
• Repetition

Sequence

• Sequence is the order in which instructions are executed. For example, in programming terms, if you were to try and access the value in a variable before declaring the variable, the program would crash.
``````number1 = int(input())
number2 = int(input())``````

Selection

• Selection is another flow control construct that is very common.
• Selection is all about making decisions based some condition.
• A condition is either True or False.
• The most common type of selection construct is the if statement.
• If is a type of conditional statement.
• If a condition evaluates to False, an Else statement will run (if specified).

Conditions

• A condition is a expression that evaluates to True or False.
• For example, `5 > 10,` `6 == x`
• A complex condition is a condition that contains And/Or:
• For example, `5 > 10 And 6 > 3`, gives us True

Truth Tables

• Truth tables make it easy to figure out the result of a condition and what path a selection statement will take.
• For this you have two variables, A and B, which each have their own values.
A B A And B A Or B
True True True True
True False False True
False True False True
False False False False

Truth Tables

• Not is another keyword and logical operator available in most programming languages.
• If not is applied to a Boolean value, it flips it, so True becomes False and False becomes True.
A Not (A)
True False
False True

Truth Tables

• When using the not keyword in Python, you can use it as a function by writing not(x):
``print(not(5 > 10 And 10 < 4))``
• And you can also check if something is not equal to using != sign:
``print(5 != 10)``

For each of the following complex conditions, state whether it is true or false:

1. True And False
2. False Or True
3. False And False
4. 5 > 10 Or 3 < 10
5. 5 == 10 And 50 > 40
6. not(5 > 10 Or 3 != 10)
7. 5 != 9 And 5 >= 5

Selection

``````number1 = int(input())

if number1 > 10:
print("Greater than 10!")
else:
print("Not greater than 10")``````
1. What would be displayed if the number 5 is entered?
2. What would be displayed if the number 12 is entered?
3. What would be displayed if the number 10 is entered?

Repetition

• Repetition or iteration is perhaps the most useful construct available to programmers.
• Computers are very quick at repeating something several times over and much faster than humans.
• Repetition or iteration is also known a looping.
• A loop can be either a fixed loop or a conditional loop.

Fixed loops

• A fixed loop is a loop which repeats a specified number of times. In Scratch, this is called the repeat block.
• In Python, this is known as a for loop:

Fixed loops

``````max = 10
for i in range(0, max):
print(i)``````
• A fixed loop is a loop which repeats a specified number of times. In Scratch, this is called the repeat block.
• In Python this is written as shown below:

Open Python and create the following programs:

1. A program to show all numbers up to and including 1000
2. A program to show all multiples of two up to and including 2000
3. A program to ask the user for number and then loop until it reaches that number

Conditional loops

• Conditional loops work similarly to a selection statement mixed with a fixed loop.
• They are used to repeat whilst a condition is True (or in some cases, False).
• In Python this is known as a while loop:

Over the next few periods, we will work with the Python booklet, particularly pages 19 to 32.

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© 2020 - 2024 J Balfour
11:02 | 25-07-2024
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