Lecture 20 – Test revision COMPSCI 101 Principles of Programming

Lecture 20 – Test revision

COMPSCI 101Principles of Programming

2CompSci 101 - Principles of Programming

Naming conventionsPython typesVariables StringsOperatorsAlgebraic expressionsThe math module, the random moduleGetting input from the user FunctionsBoolean expressions, selection statements Loops – while and for…in Lists

Test topics


Variable namesMust begin with a letter or underscore, e.g., number1 is OK, but

1number is notMay contain letters, digits, and underscores, e.g.,

this_is_an_identifier_123May be of any lengthUpper and lower case letters are different, e.g., Rope_Length

is not the same variable as rope_lengthThe standard way for most things named in python is lower case

with separate words joined by an underline, e.g., my_list

xageage_of_childbox1box_2_ageage_

3

age of child

age-child

1st

2_box

valid variable names:

NOT valid variable names:


Python types encountered this semester Information in a program is categorised into different types. A type

in Python defines two things:the internal structure of the type (the information being stored)the kinds of operations you can perform with this type of object

intfloatstring booleanlist


Converting between types Some types can be converted into different types:

result1 = int("2.1")result2 = float("2.1 ")input = "2.1"result3 = "$" + float(input)

result1 = float("2.1")result2 = int("24")result3 = float("24")input = "2.1"result4 = "$" + str(float(input) * 2)

NO!

YES!


Doing CalculationsThe following mathematical operators can be used with integers

and with floating point numbers: Addition + Subtraction - Multiplication * Division / and // Exponentiation ** Modulus %

result1 = 25 / 4 + 4 * (10 % 3)

result2 = 25 - 7 * (3 + 12) / 3

result3 = 17 % 5 ** 2 - 12 + 15

result4 = 4 ** 2 // 5 / 2

print(result1, result2, result3, result4)

B - Brackets

Exponents (**)

Multiplication, Division, Modulus, Integer division

Addition, Subtraction


Assignment operatorVariables can only store one value. Any value assigned to a

variable replaces the value previously stored in the variable.

a = 3

b = 12

c = 6

d = 1

d = d * a

c = c + 2 * a

d = d - b // c

c = c * b % c

b = b // 2

print (a, b, c, d)


math moduleThe math module contains lots of useful functions:

import math

def main():

area = 221.67

radius = math.sqrt(area / math.pi)

print("Radius of circle", radius)

main()


Manipulating string objects

strings: the len() function, string slicing, string indices, repeating a string

words = "Cheeky one"

letter1 = words[0]

position = len(words) - 2

letter2 = words[position]print(letter1, letter2)

first_part = words[0:4]

second_part = words[5:8]print(second_part, first_part)

first_part = words[:3]

second_part = words[7:]print(second_part, first_part)

encouragement = "Go! "

encouragement = encouragement * 4print(encouragement)


Dot notation with string objects

String instances have many methods which can be applied to them such as upper(), lower(), find(), strip(), rfind()

words = "Bim Bam"

words_lower = words.lower()

words_upper = words.upper()print(words, words_lower, words_upper)

words = "Sweety pie"

postion1 = words.find("e")

postion2 = words.rfind("e")

postion3 = words.rfind("ty")

position4 = words.rfind("typ")print(postion1, postion2, postion3,

position4)

letters1 = " Sugar pie "

letters2 = letters1.strip()print(len(letters2))


Common Python inbuilt functionsInbuilt:

len(), min(), max(), round(), abs()

number = min(32.7, max(16.4, 3, -1.1),

min(56.99, 32.2))print(number)

num1 = 32.657123

num2 = -16.48926

print(round(num1))

print(round(num2))

print(round(num1, 2))

print(round(num2, 3))

num1 = 10

num2 = 20

num3 = abs(num1 – num2)print(num3)


Getting input from the userThe input() function is used to get input from the user. The input()

function returns a string.

age = input("Enter age: ")print(age * 2, age * 3, age * 4)

number = input("Enter number: ")number = int(number)print(number * 2, number * 3, number * 4)

Enter age: 5

Enter number: 5


Random numbersWhat is the biggest value which can be printed?What is the smallest value which can be printed?

import random

def main(): num1 = random.randrange(2, 15, 3) num2 = random.randrange(24, 27, 2) num3 = num1 + num2

print(num3)

main()


Syntax of a Python function Format of a function

def function_name(comma_separated_parameters):

statements in the function

return value_to_be_returned

Function name Function parameterscolon'def'

Indentation (either 1 tab or 4 spaces)

Return value'return'Statements in

the body of the function.


Python functions

def display_welcome(name): message = "Welcome **" + name + " **" print(message)

def main(): display_welcome("Sam")

def display_welcome(name):message = "Welcome **" + name + " **"print(message)

return

def main(): display_welcome("Sam")

Both functions do the same thing. The return is optional.


Defining functionsComplete the definition of the check_sum() function, which

returns True if the sum of the first three parameter values is equal to the fourth parameter value, otherwise the method returns False. def check_sum( ):

def main(): print(check_sum(15, 2, 7, 24)) print(check_sum(10, 2, 3, 24))

main()True

False


Defining functionsComplete the definition of the get_next_even() function which

returns the first even number greater than the parameter number.

def get_next_even( ):

def main(): print(get_next_even(15)) print(get_next_even(10))

main()

16

12


Code traceShow the code trace and the output

for the program

def main():

years = 5

result = test1(years)

print("A", result)

result = test2(years + 5)

print("B", years)

def test1(num):

print("C", num)

num = test2(num) + 4

return num

def test2(num1):

num1 = max(num1 + 1, 10)

print("D", num1)

return num1

main()years 5

main() function

19COMPSCI 101 - Principles of Programming

Booleans represent "truth" values, i.e., True or FalseRelational operators compare two different things, which

evaluates to Boolean values

Boolean Operators are: and, or and not, e.g., (age=24, gender='F') (age > 18) and (gender == 'F') is True (age > 34) or (gender == 'M') is Falsenot (age > 18) is False

Boolean expression

Description Operator Example

Equals == age == 10

Less than < age < 10

Less than or equal <= age <= 10

Greater than > age > 10

Greater than or equal >= age >= 10


If statements x = 10y = 5

if x < 10: if y != 5: print("A") else: print("B")elif y > 10: print("C")else: if y != 5: print("D") else: print("E")

a = 4b = 12c = 37d = 51

if a < b: print("A")if a > b: print("B")if d <= c: print("C")if c != d: print("D")

Give the output


Repetition – while loops

Give the possible output:

import randomdef main(): sum = 0

while sum < 10: number = random.randrange(1, 7) sum += number print(sum, number)

final_sum = sum – number print("Finally", final_sum)main()

while boolean_expression:

indented_code_block


range(start, stop, step) functionThe range() function defines a sequence of values within the

boundaries (exludes the last value). The step parameter is optional. A for...in loop can be used to repeat code for each element in the specified range. Give the output.

number = 50for num in range(10, 1, -4):number = number – num

print("number:", number)

sum = 0for number in range(10, 16, 2):sum += number

print("sum:", sum) number:

sum:


Repetition – for loops

Give the possible output:

def count_vowels(words): vowels = "aeiouAEIOU" length = len(words) count = 0 for i in range(length): if vowels.find(words[i]) != -1: count = count + 1 return count

def main(): word = input("Enter the word(s): ") print("Output: " + str(count_vowels(word)) + ".")

main()

for var_name in range(start, stop, step):

indented_code_block


ListsA list is a sequence of comma separated elements within square

brackets.Elements of a list can be accessed using the index.The '+' operator can be used to add a new element to the end of a

list

my_list = [1, 2, 3]

my_list[1] = my_list[0] + my_list[2]



print(my_list)

Give the output


ListsA list is a sequence of comma separated elements within square

brackets. The elements of a list can be traversed using a for…in loop. The elements of a list can be changed using the index value.my_list = [2, 6, 3]b = 0print("NOT changing list elements")for element in my_list: element = element + 2 b += elementprint("b:", b)print("my_list:", my_list)

print("Changing list elements")for index in range(len(my_list)): my_list[index] = my_list[index] + indexprint("my_list:", my_list)

NOT changing list elements

b:17

my_list:[2,6,3]

Changing list elements

my_list:[2,7,5]

Give the output


Sequences and for loopsA string is a sequence. Each element of the string sequence (i.e.,

each character) can be accessed using a for loop:

phrase = "Give it a go"count = 0

for letter in phrase:if letter in "aeiouAEIOU":

count += 1

print("count", count)

for item in sequence:

indented_code_block


Split a string into a list of wordsThe split() method splits a string into its parts. Default split (the

delimiter) is white space (e.g., words.split())Can use any other delimiters (e.g., words.split(", "), lines.split("\

n"))lots_of_words = "Whether you think you can or you think you

can't, you are right"my_list = lots_of_words.split()count = 0

for word in my_list:if len(word) == 3:

count += 1print("count: ", count)

Count: 7


The test is worth 15% of your final mark, which covers the topics up to Lecture 15 (included).

Date and Time: Tuesday 5th May 6:30pm - 7:45pm.Please bring your Student Id card, a pencil and an eraser.The test is a closed book test, so you cannot refer to any

material during the test. Calculators are not permitted.Please arrive by 6:15pm as you will be given 5 minutes'

reading time.The first part of the test is Multi-Choice Questions (2/3)

and the second part of test is written questions (1/3).Please notify Angela Chang if you have a test clash.

Test Next Week - Tuesday (5th May)

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Lecture 20 – Test revision COMPSCI 101 Principles of Programming