 # Predefined functions

## Learning Intentions

• Understand the term 'predefined function'

• Identify and use appropriate predefined functions

## Success Criteria

• Be able to explain the term predefined function
• Be able to identify and describe the predefined functions specified at Higher level

## Predefined functions

• A pre-defined function carries out an operation or calculation using the data we give (pass) it and returns a single value.  The functions listed below are relevant to the course content for National 5:
• Round
• Length
• Random
• The SQA sometimes refers to built-in object functions as predefined functions, when in actual fact they are not. If you want to find out more about this, look up object methods in Python.

## National 5

• At National 5 you looked at the following predefined function:

• Random

• Round

• Length

• Let's take a quick look at them again

## Random

• The random predefined function generates a random number between a range in Python:

``````import random

#Generates a number between 10 and 100
print(random.randint(10, 100))``````

## Round

• The round function rounds a number to a specified number of decimal places.

``````x = 3.145

#Will give us 3.15
print(round(x, 2))

#Will give us 3.0
print(round(x, 0))``````

## Length

• The length function is used to calculate the length of an array.

``````fib = [1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
#Should print 10
print (len(fib))``````

In your jotter, write down an example of how each of the predefined function shown before could be used in Python

## Higher Predefined functions

• For Higher level, you need to know three more predefined functions:

• Substrings

• Convert ASCII characters to integers and vice versa

• Casting function to convert a floating point to integer

• Modulus

## Substrings

``````name = "John Smith"

print(name)
print(name[0:3])
print(name[5:8])``````
• A substring is a part of string. For example, the word “Hello world” has substrings such as:
• Hello
• World
• Hell
• llo
• To obtain a substring in Python, you need to use the subscript operator syntax (substring syntax):

## Substrings

``````name = "John Smith"

print(name)
print(name[0:3])
print(name[5:8])``````

Python's index selection here, just like with a normal array, is from the start character (non-inclusive) index to the last character index (inclusive).

For each of the following, write what you think the output from the substring predefined function would be when:

name = "Hello Class"

print(name[0:5])

print(name[7:11])

print(name[0:4])

## Character to ASCII

``````char1 = "A"
print(ord(char1))``````
• As you may remember from National 5, ASCII is the encoding system used by computers to represent characters (both special characters and characters from the keyboard)
• In ASCII, the letter A is represented by the integer 65.
• The Python `ord` function can convert a character to it's ASCII integer representation:

## aSCII to Character

``````ascii_no = 65
print(chr(ascii_no))``````
• To convert an integer representation of an ASCII value back into a character, we use the `chr` function:

Using Python, convert the following characters to their ASCII integer representation:

1. P
2. !
3. \$

## Warm up

Using Python, convert the integers to characters

1. 93
2. 45
3. 71

## Converting an floating point to an integer value

• Another predefined function is the conversion of a floating point number to an integer.
• This is done with the floor function
• Floor simply rounds down at all times.
``````import math
x = 5.31
print(math.floor(x))
``````

## Modular arithmetic

• Modular arithmetic is one of the most useful kinds of mathematics in existence today, as such most programming languages exploit a method of returning a remainder value of a sum.
• Performing a mod operation results in the remainder being returned.
• In Python, the modulus operator (modulo) is represented by a `%` sign.
• For example:
• `5 % 2 = 1`
• `10 % 7 = 3`
• `21 % 7 = 0`

## Modular arithmetic

• What's really handy about modulus is how easy it is to figure out if a number is even or not by simply performing a `% 2` operation on the number:
• `0 % 2 = 0`
• `1 % 2 = 1`
• `2 % 2 = 0`
• `3 % 2 = 1`
• `4 % 2 = 0`
• `5 % 2 = 1`
• Any time that the return value is 1 the number is negative.

Using modular arithmetic, write a program in Python to display all numbers which are factors of 20 up to 200.

Presentation Overview
Predefined Functions
14:44 | 07-12-2023
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