Exam practice

A programming language is a language, with its own words and rules (syntax), used to write instructions that tell a computer what to do and how to do it. Programs, from operating systems to apps and games, are written in programming languages such as Python, Java and C. There are two main types:

• Low-level languages are close to the machine. Machine language is written in 0s and 1s, which the processor understands directly; assembly language uses short codes such as ADD and MOV, translated by an assembler. They run fast but are hard to write, read and correct, and each works only on one kind of processor.
• High-level languages are close to human language and maths, such as Python, Java, C, C++ and JavaScript. They are easy to learn, write, read and debug, and run on different computers, but must be translated into machine code by a compiler or an interpreter before they run.

An algorithm is a set of step-by-step instructions, written in simple language, for solving a particular problem or doing a particular task. It begins with Start and ends with Stop, and each step is clear and in order.
A flowchart is the pictorial (graphical) representation of an algorithm: its steps drawn with standard shapes (oval, parallelogram, rectangle, diamond…) joined by arrows that show the order and direction of flow.

(Any five of these.)

Algorithm:

Step 1: StartStep 2: Read a number nStep 3: Is n % 2 == 0? (Is the remainder of n ÷ 2 equal to 0?)        If Yes: display "Even"        If No:  display "Odd"Step 4: Stop

Flowchart:

Python is a high-level, general-purpose programming language created by Guido van Rossum and first released in 1991. It is interpreted, free and open source, and is used in web development, data science, artificial intelligence, automation, game development and cyber security. Its code uses indentation instead of braces, which makes it neat and readable. Its features:

• Easy to read and write: simple syntax close to English.
• Beginner-friendly: short programs do useful things, so it is a good first language.
• Versatile: suits small scripts as well as big web, data and AI projects.
• Extensive standard library: ready-made code for maths, dates, files and much more.
• Rich ecosystem: many free libraries and frameworks made by its community.
• Free, open source and portable: runs on Windows, macOS and Linux.

Input/output (I/O) statements let a program interact with its user.

• input() reads data from the user: it shows a prompt, waits for the user to type and press Enter, and gives back what was typed, as a string. For numbers it is used with int() or float(): age = int(input("Enter your age: ")).
• print() displays output on the screen: text, numbers, variables and results, several at once separated by commas: print("Your age is", age).

So input() brings data into the program and print() shows the results.

Formatted string literals (f-strings) are strings with an f before the opening quote, inside which variables or expressions are written in curly braces { }; Python puts each one’s value in its place. They are the newest and most used way of formatting strings in Python.

user_name = input("Enter your name: ")print(f"Hello {user_name}! Welcome to our program")

Enter your name: SonamHello Sonam! Welcome to our program

A data type is the kind of a value, which decides what can be done with it. Python’s basic data types:

• Integer (int): whole numbers, positive, negative or zero: age = 15, -3, 0.
• Float (float): numbers with a decimal point: height = 5.7, 3.14, -0.5.
• String (str): text in quotes, letters, digits and symbols: name = "Sanjog", "Python@".
• Boolean (bool): only True or False: is_student = True.

Python also has collection types, such as the list ([10, 20, 30]) and the dictionary ({"name": "Sanjog"}).

Type casting is converting a value from one data type to another, for example the string "25" from input() into the integer 25. It is of two types:

• Implicit type casting (automatic): Python converts by itself when an expression needs it, so no data is lost. In z = 10 + 5.5, the integer 10 is converted to a float and z is 15.5.
• Explicit type casting: the programmer converts on purpose with int(), float() or str(): int(4.5) gives 4, float(4) gives 4.0, str(11) gives "11", and int(input("Enter a number: ")) makes the user’s input a number.

Python’s main types of operators are arithmetic, relational (comparison), logical and assignment operators.
Arithmetic operators do mathematical calculations:

For example, print(17 % 5) prints 2, the remainder of 17 ÷ 5, and n % 2 == 0 checks whether n is even.
(Any one type, explained with examples, earns the marks; relational operators compare two values and give True or False, logical operators and, or, not combine conditions, and assignment operators such as = and += store values.)

A conditional statement is a decision-making statement: it checks a condition and runs a block of code only if the condition is True, so the program chooses what to do. Python’s conditional statements are if, if-else, if-elif-else and nested if. The block under each is indented.
For example, an if-else statement:

marks = int(input("Enter your marks: "))if marks >= 40:    print("You passed.")else:    print("You failed.")

Enter your marks: 58You passed.

If the condition marks >= 40 is True, the if block runs; otherwise the else block runs.

Iteration is the process of repeating a set of instructions again and again, a given number of times or until a condition is met. It is done with loops; Python has the for loop and the while loop.

cities = ["Kathmandu", "Biratnagar", "Nepalgunj", "Pokhara", "Butwal",          "Birendranagar", "Mahendranagar", "Balefi"]cities.append("Malangawa")      # a. add Malangawa at the enddel cities[7]                   # b. remove Balefi, the element at index 7for city in cities:             # c. print the items one by one    print(city)

KathmanduBiratnagarNepalgunjPokharaButwalBirendranagarMahendranagarMalangawa

The city names are strings, so they need quotes. Balefi is the eighth element, at index 7. cities.remove("Balefi") would also work: it removes an element by its value.

• a.program
• b.syntax
• c.interpreter
• d.flowchart
• e.comment
• f.Boolean (bool)
• g.relational (comparison) operator
• h.operands
• i.conditional statement
• j.iteration (a loop)

• i.LLL: Low-Level Language
• ii.HLL: High-Level Language
• iii.I/O: Input/Output
• iv.IDE: Integrated Development Environment

With sides a, b and c, Heron’s formula gives the area: first the half-perimeter s = (a + b + c) ÷ 2, then area = √(s(s − a)(s − b)(s − c)).

import matha = float(input("Enter side a: "))b = float(input("Enter side b: "))c = float(input("Enter side c: "))s = (a + b + c) / 2area = math.sqrt(s * (s - a) * (s - b) * (s - c))print("The area of the triangle is", area)

Enter side a: 3Enter side b: 4Enter side c: 5The area of the triangle is 6.0

Check: a 3-4-5 triangle has a right angle, so its area is ½ × 3 × 4 = 6. (The three sides must be able to make a triangle: each less than the other two added together.)

radius = float(input("Enter the radius: "))area = 3.14159 * radius ** 2circumference = 2 * 3.14159 * radiusprint("Area:", round(area, 2))print("Circumference:", round(circumference, 2))

Enter the radius: 7Area: 153.94Circumference: 43.98

Area = πr², circumference = 2πr. round(…, 2) keeps two decimal places.

diameter = float(input("Enter the diameter: "))radius = diameter / 2area = 3.14159 * radius ** 2circumference = 3.14159 * diameterprint("Area:", round(area, 2))print("Circumference:", round(circumference, 2))

Enter the diameter: 14Area: 153.94Circumference: 43.98

The radius is half the diameter, and the circumference is π × diameter.

A constant is a value that doesn’t change while the program runs. Python has no special keyword for it; by custom, a constant’s name is written in capitals.

PI = 3.14159                  # a constantradius = float(input("Enter the radius: "))print("Area:", round(PI * radius * radius, 2))print("Circumference:", round(2 * PI * radius, 2))

Enter the radius: 3.5Area: 38.48Circumference: 21.99

(Python’s math.pi is π to 15 decimal places: import math, then math.pi.)

x = 12y = 20print("x == y:", x == y)print("x <= y:", x <= y)print("x >= y:", x >= y)print("x != y:", x != y)

x == y: Falsex <= y: Truex >= y: Falsex != y: True

Each comparison gives a Boolean, True or False.

a = float(input("Enter the first number: "))b = float(input("Enter the second number: "))if a < b:    print("The smaller number is", a)else:    print("The smaller number is", b)

Enter the first number: 8.5Enter the second number: 3The smaller number is 3.0

If the two are equal, the else prints b, which is equally right.

a = float(input("Enter the first number: "))b = float(input("Enter the second number: "))c = float(input("Enter the third number: "))if a <= b and a <= c:    smallest = aelif b <= a and b <= c:    smallest = belse:    smallest = cprint("The smallest number is", smallest)

Enter the first number: 14Enter the second number: 6Enter the third number: 9The smallest number is 6.0

(min(a, b, c) does the same in one line.)

color = input("Enter the color of the light: ").lower()if color == "green":    print("Go")elif color == "red":    print("Stop")elif color == "orange":    print("Be ready")else:    print("Invalid color")

Enter the color of the light: OrangeBe ready

.lower() turns what the user typed into small letters, so Orange, ORANGE and orange all match "orange".

Take, as an example, theory out of 50 and practical out of 50, with a pass at 40% of each: 20 marks. (Change the numbers to your school’s.) A student must pass both.

theory = float(input("Enter the theory marks (out of 50): "))practical = float(input("Enter the practical marks (out of 50): "))if theory < 0 or theory > 50 or practical < 0 or practical > 50:    print("Invalid marks: each must be from 0 to 50.")elif theory >= 20 and practical >= 20:    print("Pass")else:    print("Fail")

Enter the theory marks (out of 50): 34Enter the practical marks (out of 50): 42Pass

The validity check comes first: marks such as 65 or −3 are reported as invalid, not judged pass or fail. Try 18 and 45: the student fails, because the theory is below 20.

name = input("Enter a name: ")for i in range(20):    print(name)

Enter a name: PasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasangPasang

range(20) gives 20 numbers (0 to 19), so the loop runs 20 times.

for n in range(1, 40, 2):    print(n)

13579111315171921232527293133353739

Odd numbers go up in twos from 1; the 20th is 2 × 20 − 1 = 39, so the range stops before 40.

count = 1while count <= 30:    print(count * 2)    count += 1

24681012141618202224262830323436384042444648505254565860

The counter counts the even numbers printed (1 to 30), and each one is twice the counter.

n = int(input("Enter a number: "))factorial = 1i = 1while i <= n:    factorial *= i    i += 1print("The factorial of", n, "is", factorial)

Enter a number: 6The factorial of 6 is 720

6! = 1 × 2 × 3 × 4 × 5 × 6 = 720. For 0 the loop doesn’t run at all and the answer is 1, which is right: 0! = 1.

total = 0count = 0odd = 1while count < 20:    total += odd    odd += 2    count += 1print("The sum of the first 20 odd numbers is", total)

The sum of the first 20 odd numbers is 400

A neat check: the sum of the first n odd numbers is always n²: 1 + 3 = 4, 1 + 3 + 5 = 9… and 20² = 400.

number = int(input("Enter a number: "))total = 0while number > 0:    total += number % 10    number //= 10print("The sum of the digits is", total)

Enter a number: 4096The sum of the digits is 19

4 + 0 + 9 + 6 = 19. % 10 takes the last digit and //= 10 removes it, as in practice tasks 27 and 28.

rivers = ["Koshi", "Gandaki", "Karnali", "Mahakali", "Bagmati"]print("Second:", rivers[1])print("Fourth:", rivers[3])

Second: GandakiFourth: Mahakali

Indexes start at 0, so the second element is at index 1 and the fourth at index 3.

Slicing takes a part of a list: list[start:stop] gives the elements from index start up to, but not including, index stop, just as range() stops before its end.

peaks = ["Everest", "Kangchenjunga", "Lhotse", "Makalu", "Cho Oyu"]print(peaks[0:3])

['Everest', 'Kangchenjunga', 'Lhotse']

peaks[:3] means the same: a missing start is 0.

A list comprehension builds a list in one line, from a for loop written inside the square brackets: [expression for item in sequence].

squares = [n ** 2 for n in range(1, 11)]print(squares)

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

It does the same as this longer loop:

squares = []for n in range(1, 11):    squares.append(n ** 2)print(squares)

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

capitals = {"Koshi": "Biratnagar", "Gandaki": "Pokhara", "Karnali": "Birendranagar"}print(capitals)

{'Koshi': 'Biratnagar', 'Gandaki': 'Pokhara', 'Karnali': 'Birendranagar'}

The keys are three of Nepal’s provinces, and the values their capitals.

A dictionary comprehension is like a list comprehension, in curly braces, with a key and a value: {key: value for item in sequence}.

squares = {n: n ** 2 for n in range(1, 6)}print(squares)

{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

The in operator checks whether a key is in a dictionary, and gives True or False.

capitals = {"Koshi": "Biratnagar", "Gandaki": "Pokhara", "Karnali": "Birendranagar"}key = input("Enter a province: ")if key in capitals:    print(key, "is in the dictionary. Its capital is", capitals[key])else:    print(key, "is not in the dictionary.")

Enter a province: BagmatiBagmati is not in the dictionary.

fees = {"Grade 9": 1500, "Grade 10": 1800}more_fees = {"Grade 11": 2500, "Grade 12": 2800}merged = fees.copy()       # a copy, so fees itself stays as it wasmerged.update(more_fees)   # add every pair of more_feesprint(merged)

{'Grade 9': 1500, 'Grade 10': 1800, 'Grade 11': 2500, 'Grade 12': 2800}

update() adds the second dictionary’s pairs to the first; a key in both takes the second’s value. In Python 3.9 and later, merged = fees | more_fees does the same in one step.