Computer Programming using Python

In Lesson 18 you planned solutions as algorithms and flowcharts, and in Lesson 19 you met the languages that turn plans into programs.

Now you will write real programs in Python: programs that read data from the keyboard, calculate, decide and repeat, and print their results on the screen.

Python reads almost like plain English, it tells you clearly where a mistake is, and it is the language that grades 9 and 10 of the school curriculum carry on with, so what you learn here is the foundation of everything that follows.

Every program in this lesson was run in Python 3.12, and the output shown under each program is exactly what appeared on the screen. Type them in, run them, and then change them: that is how programming is learnt.

20.1 Python: features, installing Python and IDLE

Features of Python

1. Easy to read and learn. Its keywords are plain English words — print, input, if, else, for, while — and it needs very little punctuation.
2. Interpreted. A program runs at once, without a separate compiling step, and stops at the first error with a message that names the line, so mistakes are easy to find.
3. Indentation is part of the language. The statements that belong inside an if or a loop are shown by starting them further to the right. This forces every Python program to be neatly laid out.
4. Dynamically typed. You do not declare what kind of value a variable will hold; Python works it out from the value you store.
5. A large library of ready-made functions for mathematics, text, dates, graphics and much more.
6. Free and open source, and it runs on Windows, Linux, macOS and even on a phone.
7. Widely used. Python is used to build websites, analyse data, control robots and train artificial intelligence, so it is a language you can keep using for a long time.
8. Programs are saved as plain text files with the extension .py.

Installing Python and starting IDLE

Download the latest Python 3 installer for your computer from the official website, python.org/downloads, and run it. On Windows, tick Add python.exe to PATH on the first screen of the installer, then choose Install Now.

On most Linux computers and on macOS a Python 3 is already present. The installer also installs IDLE (Integrated Development and Learning Environment), a simple editor made for beginners. Start it from the Start menu by typing IDLE.

>>> print("Namaste, Nepal!")
Namaste, Nepal!
>>> 7 + 5
12
>>> 2 ** 10
1024
>>> "Nepal" + " " + "Python"
'Nepal Python'

The lines that begin with >>> are typed by you; the others are Python's answers. In interactive mode the value of an expression is shown at once, which is why 7 + 5 printed 12 without a print.

# Program 20.1: my first program
print("Shree Janata Secondary School")
print("Biratnagar-7, Morang")
print()
print("Class 8 Computer Science")

Shree Janata Secondary School
Biratnagar-7, Morang

Class 8 Computer Science

20.2 Character set, keywords, identifiers, variables and data types

Character set

The character set is the set of characters a language understands.

Python uses the letters (A–Z, a–z), the digits (0–9), the special characters (such as + - * / % = < > ( ) [ ] : , . # " ' and the underscore _) and white space (space, tab and the end of a line).

Because Python uses Unicode, text inside quotation marks may also contain Nepali, as in print("नमस्ते").

Keywords

Keywords are the words that have a fixed meaning in Python. They are used for the language's own purposes, so they cannot be used as the name of a variable. Python 3.12 has 35 keywords:

At this level you will use only a few: if, elif, else, for, while, in, and, or, not, True, False, break and import. Note that True, False and None begin with a capital letter, but all the others are in lower case.

Identifiers

An identifier is the name a programmer gives to a variable (and, later, to a function). The rules are:

1. It may contain letters, digits and the underscore _.
2. It must not start with a digit.
3. It must not contain spaces or special characters such as - + * $ @ %.
4. It must not be a keyword.
5. It is case-sensitive.

Choose names that explain themselves: price and area are better than p and a in a long program.

Comments

A comment begins with # and runs to the end of the line. Python ignores it; it is written for the human reader. Use comments to say what a program does and to explain any step that is not obvious.

Variables and constants

The = sign in Python is not the equals of mathematics. It means store the value on the right in the name on the left. So age = age + 1 is a perfectly good statement: it takes the old value of age, adds 1, and stores the result back in age.

A constant is a value that does not change while the program runs, such as π = 3.14159 or the VAT rate. Python has no special constant type; by custom, the name of a constant is written in capital letters (PI = 3.14159) to tell the reader do not change this.

Data types

The data type of a value tells Python what kind of data it is and what can be done with it. The four basic types are:

The function type() tells you the type of a value. Notice that "2025" in quotation marks is a string, not a number: you can join it to other text but you cannot do arithmetic with it until it is converted (Section 20.4).

# Program 20.2: variables and data types
name = "Sunita"
age = 13
height = 1.52
is_monitor = True

print(name, type(name))
print(age, type(age))
print(height, type(height))
print(is_monitor, type(is_monitor))

age = age + 1
print("Next year", name, "will be", age)

Sunita <class 'str'>
13 <class 'int'>
1.52 <class 'float'>
True <class 'bool'>
Next year Sunita will be 14

Several variables can be given values in one statement. This also gives the neatest way to swap two values, because the right-hand side is worked out completely before anything is stored.

# Program 20.3: several variables at once, and swapping
a, b = 10, 25
print("Before:", a, b)
a, b = b, a
print("After: ", a, b)

Before: 10 25
After:  25 10

20.3 Operators and expressions

An operator is a symbol that tells Python to do an operation; the values it works on are the operands. An expression is a combination of values, variables and operators that Python works out to a single value, such as a * b + 5.

# Program 20.4: the arithmetic operators on two numbers
a = 17
b = 5
print("a + b  =", a + b)
print("a - b  =", a - b)
print("a * b  =", a * b)
print("a / b  =", a / b)
print("a // b =", a // b)
print("a % b  =", a % b)
print("a ** b =", a ** b)

a + b  = 22
a - b  = 12
a * b  = 85
a / b  = 3.4
a // b = 3
a % b  = 2
a ** b = 1419857

The % operator is the most useful of these in programs. A number n is even if n % 2 == 0, and a number is divisible by 5 if n % 5 == 0. The last digit of a number is n % 10, and n // 10 removes the last digit.

Relational operators

A relational (comparison) operator compares two values and gives a Boolean result, True or False.

x = 12
print(x > 10, x == 12, x != 12)
print(x > 10 and x < 20)
print(x < 5 or x % 2 == 0)
print(not x > 10)

True True False
True
True
False

Assignment operators

Besides =, Python has short forms that change a variable using its own value:

The operator + also joins two strings ("Nepal" + "Python" gives "NepalPython"), and * repeats a string ("ab" * 3 gives "ababab").

Order of operations

When an expression has several operators, Python works them out in this order. Operators in the same row are taken from left to right (except **, which goes from right to left).

# Program 20.5: algebra written as Python expressions
a = 3
b = 4
c = 5
print((a + b) / c)
print(a ** 2 + 2 * a * b)
print((a ** 2 + b ** 2) ** 0.5)
print(10 + 8 / 2 ** 2 * 3)

1.4
33
5.0
16.0

20.4 Input and output

Output: print()

The print() function displays values on the screen. Several items separated by commas are printed with a space between them, and each print() ends by moving to a new line. Two optional settings change this:

• sep= sets what goes between the items (a space is the default);
• end= sets what goes at the end (a new line is the default).

Inside a string, an escape sequence is a backslash followed by a letter: \n starts a new line and \t inserts a tab.

# Program 20.6: print() with sep, end and escape characters
print("Nepal", "India", "Bhutan")
print("Nepal", "India", "Bhutan", sep=", ")
print("Loading", end="...")
print("done")
print("Name\tClass\nRita\t8")

Nepal India Bhutan
Nepal, India, Bhutan
Loading...done
Name    Class
Rita    8

Input: input()

The input() function shows a prompt, waits for the user to type something and press Enter, and returns what was typed. It always returns text (a string), even if the user types digits. To use the value as a number it must be converted:

So a number is read with age = int(input("Enter your age: ")), or float(...) if it may have a decimal part, such as a price or a length. If the user types something that is not a number, such as thirteen, the conversion fails with a ValueError (Section 20.8).

x = input("First number: ")
y = input("Second number: ")
print("As text   :", x + y)
print("As numbers:", int(x) + int(y))

First number: 25
Second number: 17
As text   : 2517
As numbers: 42

The first line shows what goes wrong without conversion: "25" + "17" joins the two pieces of text into 2517. After int(), the same + adds.

Formatting output with f-strings

An f-string is a string with the letter f before the opening quotation mark. Anything inside curly brackets { } is replaced by its value. After a colon you can say how the value is to look: {area:.2f} shows a number with exactly two digits after the decimal point.

# Program 20.7: simple interest and amount
p = float(input("Enter the principal (Rs): "))
t = float(input("Enter the time (years): "))
r = float(input("Enter the rate (% per year): "))
si = p * t * r / 100
print("Simple interest = Rs", si)
print("Amount          = Rs", p + si)

Enter the principal (Rs): 25000
Enter the time (years): 3
Enter the rate (% per year): 8
Simple interest = Rs 6000.0
Amount          = Rs 31000.0

# Program 20.8: Celsius to Fahrenheit
c = float(input("Temperature in Celsius: "))
f = c * 9 / 5 + 32
print(c, "degrees C =", f, "degrees F")

Temperature in Celsius: 37
37.0 degrees C = 98.6 degrees F

# Program 20.9: area and circumference of a circle
PI = 3.14159
r = float(input("Radius in cm: "))
area = PI * r ** 2
circumference = 2 * PI * r
print(f"Area          = {area:.2f} cm2")
print(f"Circumference = {circumference:.2f} cm")

Radius in cm: 7
Area          = 153.94 cm2
Circumference = 43.98 cm

20.5 Built-in functions and the math module

A function is a ready-made, named set of instructions that does one job: you give it a value (the argument) in brackets and it gives back a result. You have already used print(), input(), int(), float() and type().

Python has many more built in, and more still in modules, collections of extra functions that you bring in with the import statement.

The functions that begin with math. live in the math module. Write import math once, at the top of the program, before using them.

# Program 20.10: built-in and math functions
import math

print(abs(-7.5))
print(max(14, 9, 31), min(14, 9, 31))
print(round(3.14159, 2))
print(math.sqrt(144))
print(math.floor(-4.5), math.ceil(-4.5))
print(math.pi)

7.5
31 9
3.14
12.0
-5 -4
3.141592653589793

String functions

A string is a sequence of characters, and each character has a position called its index, counted from 0 at the left. A negative index counts from the right: -1 is the last character. A slice s[a:b] takes the characters from index a up to, but not including, index b.

# Program 20.11: string functions
word = "Bhaktapur"
print(len(word))
print(word.upper())
print(word.lower())
print(word[0])
print(word[0:4])
print(word[-3:])
print(word.count("a"))

9
BHAKTAPUR
bhaktapur
B
Bhak
pur
2

20.6 Selection: if, if … else and if … elif … else

A program that always does the same thing is not very useful. Selection lets it choose what to do, depending on a condition. In Python the choice is written with if.

if condition:
    statements that run when the condition is True
elif another_condition:
    statements that run when this one is True
else:
    statements that run when none of the above is True

Three rules must never be forgotten:

1. The if, elif and else lines end with a colon (:).
2. The statements that belong to a branch are indented by the same amount, normally four spaces. The indentation is how Python knows where the branch ends.
3. elif and else are optional. Python checks the conditions from the top, runs the first branch whose condition is True and skips all the rest.

# Program 20.12: pass or fail
marks = int(input("Enter the marks (out of 100): "))
if marks >= 40:
    print("Pass")
else:
    print("Fail")

Enter the marks (out of 100): 72
Pass

Enter the marks (out of 100): 28
Fail

# Program 20.13: odd or even
n = int(input("Enter a number: "))
if n % 2 == 0:
    print(n, "is even.")
else:
    print(n, "is odd.")

Enter a number: 14
14 is even.

Enter a number: 35
35 is odd.

# Program 20.14: the greatest of three numbers
a = int(input("First number: "))
b = int(input("Second number: "))
c = int(input("Third number: "))
if a >= b and a >= c:
    print(a, "is the greatest.")
elif b >= a and b >= c:
    print(b, "is the greatest.")
else:
    print(c, "is the greatest.")

First number: 42
Second number: 17
Third number: 58
58 is the greatest.

# Program 20.15: SEE grade from percentage
p = float(input("Enter the percentage: "))
if p >= 90:
    grade = "A+"
elif p >= 80:
    grade = "A"
elif p >= 70:
    grade = "B+"
elif p >= 60:
    grade = "B"
elif p >= 50:
    grade = "C+"
elif p >= 40:
    grade = "C"
elif p >= 35:
    grade = "D"
else:
    grade = "NG"
print("Grade:", grade)

Enter the percentage: 92.5
Grade: A+

Enter the percentage: 64
Grade: B

Enter the percentage: 38
Grade: D

Enter the percentage: 30
Grade: NG

# Program 20.16: leap year
year = int(input("Enter a year: "))
if (year % 4 == 0 and year % 100 != 0) or year % 400 == 0:
    print(year, "is a leap year.")
else:
    print(year, "is not a leap year.")

Enter a year: 2024
2024 is a leap year.

Enter a year: 2100
2100 is not a leap year.

Enter a year: 2000
2000 is a leap year.

# Program 20.17: vowel or consonant
ch = input("Enter a letter: ").lower()
if ch in "aeiou":
    print("Vowel")
else:
    print("Consonant")

Enter a letter: E
Vowel

Enter a letter: m
Consonant

Nested if

An if may be placed inside another if. The inner statement is indented one more step, and it is asked only when the outer condition is True.

# Program 20.18: positive, negative or zero (nested if)
n = float(input("Enter a number: "))
if n >= 0:
    if n == 0:
        print("Zero")
    else:
        print("Positive")
else:
    print("Negative")

Enter a number: -3.5
Negative

Enter a number: 0
Zero

20.7 Loops: for, while and nested loops

A loop repeats a group of statements. Without loops, printing a table of ten lines would take ten print() statements. Python has two loops. The statements that are repeated are the loop body, and like the body of an if they are indented.

The for loop and range()

A for loop is used when you know how many times to repeat, or when you want to go through a series of values. The function range() makes the series:

print(list(range(5)))
print(list(range(1, 6)))
print(list(range(1, 11, 2)))
print(list(range(10, 0, -3)))

[0, 1, 2, 3, 4]
[1, 2, 3, 4, 5]
[1, 3, 5, 7, 9]
[10, 7, 4, 1]

for variable in range(start, stop, step):
    statements to repeat

The loop variable takes each value of the series in turn, and the body runs once for each. The stop value is never reached, so to print 1 to 10 you write range(1, 11).

The while loop

A while loop repeats as long as a condition is True. The condition is tested before every pass, so if it is False at the start the body is never run. The body must change something that the condition looks at, or the loop will never end.

while condition:
    statements to repeat

Counting loops

# Program 20.19: multiplication table
n = int(input("Which table? "))
for i in range(1, 11):
    print(n, "x", i, "=", n * i)

Which table? 7
7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

# Program 20.20: counting up with a step, and counting down
for i in range(5, 51, 5):
    print(i, end=" ")
print()
for i in range(5, 0, -1):
    print(i, end=" ")
print("Lift off!")

5 10 15 20 25 30 35 40 45 50
5 4 3 2 1 Lift off!

# Program 20.21: factorial
n = int(input("Enter a number: "))
f = 1
for i in range(1, n + 1):
    f = f * i
print(f"{n}! = {f}")

Enter a number: 6
6! = 720

Enter a number: 0
0! = 1

# Program 20.22: sum and average of n numbers
n = int(input("How many numbers? "))
total = 0
for i in range(n):
    x = float(input("Enter a number: "))
    total = total + x
print("Sum     =", total)
print("Average =", total / n)

How many numbers? 4
Enter a number: 12
Enter a number: 15
Enter a number: 9
Enter a number: 20
Sum     = 56.0
Average = 14.0

Loops that depend on a condition

# Program 20.23: sum of digits, reverse and palindrome
n = int(input("Enter a number: "))
original = n
total = 0
rev = 0
while n > 0:
    d = n % 10
    total = total + d
    rev = rev * 10 + d
    n = n // 10
print("Sum of digits =", total)
print("Reverse       =", rev)
if rev == original:
    print("It is a palindrome.")
else:
    print("It is not a palindrome.")

Enter a number: 2582
Sum of digits = 17
Reverse       = 2852
It is not a palindrome.

Enter a number: 1331
Sum of digits = 8
Reverse       = 1331
It is a palindrome.

# Program 20.24: Fibonacci series
n = int(input("How many terms? "))
a, b = 0, 1
count = 0
while count < n:
    print(a, end=" ")
    a, b = b, a + b
    count = count + 1
print()

How many terms? 10
0 1 1 2 3 5 8 13 21 34

break

The break statement leaves a loop at once. With while True:, which would otherwise never end, it makes a loop that runs until the user says stop.

# Program 20.25: add numbers until 0 is entered
total = 0
while True:
    x = int(input("Enter a number (0 to stop): "))
    if x == 0:
        break
    total = total + x
print("Total =", total)

Enter a number (0 to stop): 15
Enter a number (0 to stop): 30
Enter a number (0 to stop): 5
Enter a number (0 to stop): 0
Total = 50

Nested loops

A loop inside a loop is a nested loop. For every pass of the outer loop, the inner loop runs completely. Nested loops are used for tables and patterns.

# Program 20.26: a pattern with nested loops
for row in range(1, 5):
    for col in range(row):
        print("*", end=" ")
    print()

*
* *
* * *
* * * *

# Program 20.27: prime or composite
n = int(input("Enter a number greater than 1: "))
factors = 0
for i in range(1, n + 1):
    if n % i == 0:
        factors = factors + 1
if factors == 2:
    print(n, "is a prime number.")
else:
    print(n, "is a composite number.")

Enter a number greater than 1: 29
29 is a prime number.

Enter a number greater than 1: 21
21 is a composite number.

20.8 Writing, running and debugging simple programs

From problem to program

1. Plan the solution: write the algorithm or draw the flowchart (Lesson 18) and list the input, process and output.
2. Open IDLE and choose File → New File. A blank editor window opens.
3. Type the program, one statement per line. Use four spaces for each indentation step; IDLE indents for you after a colon.
4. Save it with File → Save, giving a name such as marks.py. IDLE will ask for a name before it runs an unsaved program.
5. Run it with F5 (Run → Run Module). The input prompts and the output appear in the Shell window.
6. Read any error message, correct the program and run it again.
7. Test with data whose answers you know, including unusual values such as 0 or a negative number.

Finding and fixing errors

An error in a program is called a bug, and finding and removing it is debugging. There are three kinds.

1. Syntax errors break the rules of the language. Python finds them before the program starts, shows the line and points to the place with an arrow ^. Nothing runs until they are corrected.

marks = 45
if marks >= 40
    print("Pass")

  File "main.py", line 2
    if marks >= 40
                  ^
SyntaxError: expected ':'

A missing bracket is reported at the place where Python noticed the problem, and a wrongly indented line gives an IndentationError:

print("Welcome to Python"

  File "main.py", line 1
    print("Welcome to Python"
         ^
SyntaxError: '(' was never closed

n = 8
if n > 5:
print("big")

  File "main.py", line 3
    print("big")
    ^^^^^
IndentationError: expected an indented block after 'if' statement on line 2

2. Run-time errors stop a correct-looking program while it is running, and Python prints a traceback that ends with the kind of error and the line where it happened. Three common ones:

price = 120
print(prcie * 2)

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    print(prcie * 2)
          ^^^^^
NameError: name 'prcie' is not defined. Did you mean: 'price'?

a = 10
b = 0
print(a / b)

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    print(a / b)
          ~~^~~
ZeroDivisionError: division by zero

age = int(input("Enter your age: "))
print(age + 1)

Enter your age: thirteen
Traceback (most recent call last):
  File "main.py", line 1, in <module>
    age = int(input("Enter your age: "))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid literal for int() with base 10: 'thirteen'

The last line of a traceback is the one to read first: NameError (a name that was never given a value, here a misspelling of price), ZeroDivisionError (dividing by 0), ValueError (a value of the right type but wrong content).

A fourth very common one is the TypeError, which Python gives when you add a number to text, as happens if int() is forgotten:

age = input("Enter your age: ")
print(age + 1)

Enter your age: 13
Traceback (most recent call last):
  File "main.py", line 2, in <module>
    print(age + 1)
          ~~~~^~~
TypeError: can only concatenate str (not "int") to str

3. Logical errors give no message at all: the program runs to the end but gives a wrong answer.

They are found by testing with answers you know, by a trace table (writing down the value of each variable after every step) and by adding temporary print() statements to show the values while the program runs.

# Program 20.28 (first attempt): average of n numbers
n = int(input("How many numbers? "))
total = 1
for i in range(n)
    x = float(input("Enter a number: "))
    total = total + x
print("Average =", total / n)

  File "main.py", line 4
    for i in range(n)
                     ^
SyntaxError: expected ':'

How many numbers? 4
Enter a number: 12
Enter a number: 15
Enter a number: 9
Enter a number: 20
Average = 14.25

# Program 20.28 (corrected)
n = int(input("How many numbers? "))
if n <= 0:
    print("Nothing to average.")
else:
    total = 0
    for i in range(n):
        x = float(input("Enter a number: "))
        total = total + x
    print("Average =", total / n)

How many numbers? 4
Enter a number: 12
Enter a number: 15
Enter a number: 9
Enter a number: 20
Average = 14.0

How many numbers? 0
Nothing to average.

Practice questions

Group A — Objective (1 mark each)

1. Which function displays output on the screen? [1]ainput()bprint()cshow()ddisplay()
2. What is the value of 17 % 5? [1]a3b3.4c2d5
3. What is the value of len("NEPAL")? [1]a4b5c6dNEPAL
4. "COMPUTER"[3:6] gives [1]aCOMbPUTcMPUdTER
5. Which of these is a valid variable name? [1]a2nd_termbtotal marksctotal_marksdclass
6. How many times does for i in range(1, 11, 3): repeat its body? [1]a3b4c10d30
7. What does input() always return? [1]aan intba floatca stringda Boolean
8. The value of math.floor(-4.5) is [1]a−4b−5c4d−4.5
9. In Python the expression 2 ** 3 * 2 gives [1]a64b12c16d10
10. Which of these is a string method? [1]aabs()bmax()cupper()drange()
11. The # symbol is used to [1]aremove a linebwrite a comment that is not executedcrepeat statementsdread input
12. A while loop whose condition is False at the start [1]aruns oncebruns forevercnever runs its bodydgives a syntax error

Group B — Short answer (2 marks each)

1. Write any four features of Python. [2]
2. Write Python expressions for(a) \frac{a+b}{2}(b) \pi r^2(c) \sqrt{b^2-4ac}(d) p \times t \times \frac{r}{100}. [2]
3. State any four rules for naming an identifier. [2]
4. Write the output of print(len("BHAKTAPUR")), print("BHAKTAPUR"[:4]) and print("BHAKTAPUR"[-3:]). [2]
5. Differentiate between the for loop and the while loop. [2]
6. Write a program to find the area of a triangle from its base and height. [2]
7. Write a program to check whether a number is divisible by 5. [2]
8. Write a program to display the squares of the numbers from 1 to 5. [2]
9. Write the output of this program and say how many times the loop runs. [2]

for i in range(1, 11, 3):
    print(i, end=" ")

10. What is the difference between /, // and %? Give the value of each for 17 and 5. [2]

b = float(input("Base of the triangle: "))
h = float(input("Height of the triangle: "))
area = 1 / 2 * b * h
print("Area of the triangle =", area)

Base of the triangle: 10
Height of the triangle: 6
Area of the triangle = 30.0

n = int(input("Enter a number: "))
if n % 5 == 0:
    print(n, "is divisible by 5.")
else:
    print(n, "is not divisible by 5.")

Enter a number: 35
35 is divisible by 5.

for i in range(1, 6):
    print(i ** 2, end=" ")

1 4 9 16 25

Group C — Long answer (4 marks each)

1. Write a program that reads the marks of a student in five subjects, then displays the total, the percentage and the SEE grade (A+ 90 and above, A 80, B+ 70, B 60, C+ 50, C 40, D 35, NG below 35). [4]
2. Write a program to display the series 5, 10, 15, …, 50 and the sum of its terms. [4]
3. Write a program to check whether a number is prime or composite. [4]
4. Write a program to display this pattern: [4]

5 4 3 2 1
5 4 3 2
5 4 3
5 4
5

5. Dry-run the program below in a trace table and write its output. [4]

n = 5
f = 1
while n > 1:
    f = f * n
    n = n - 2
print(f)

total = 0
for i in range(1, 6):
    m = float(input(f"Marks in subject {i}: "))
    total = total + m
percentage = total / 5
print("Total      =", total)
print("Percentage =", percentage)
if percentage >= 90:
    grade = "A+"
elif percentage >= 80:
    grade = "A"
elif percentage >= 70:
    grade = "B+"
elif percentage >= 60:
    grade = "B"
elif percentage >= 50:
    grade = "C+"
elif percentage >= 40:
    grade = "C"
elif percentage >= 35:
    grade = "D"
else:
    grade = "NG"
print("Grade      =", grade)

Marks in subject 1: 78
Marks in subject 2: 85
Marks in subject 3: 69
Marks in subject 4: 91
Marks in subject 5: 74
Total      = 397.0
Percentage = 79.4
Grade      = B+

total = 0
for i in range(5, 51, 5):
    print(i, end=" ")
    total = total + i
print()
print("Sum =", total)

5 10 15 20 25 30 35 40 45 50
Sum = 275

# Program 20.27: prime or composite
n = int(input("Enter a number greater than 1: "))
factors = 0
for i in range(1, n + 1):
    if n % i == 0:
        factors = factors + 1
if factors == 2:
    print(n, "is a prime number.")
else:
    print(n, "is a composite number.")

for i in range(1, 6):
    for j in range(5, i - 1, -1):
        print(j, end=" ")
    print()

5 4 3 2 1
5 4 3 2
5 4 3
5 4
5

15