Contents

• 4. Basic calculations
  • 4.1 Basic mathematical operations
  • 4.2 Simple loops (for)
  • 4.3 Conditional loops (while)
  • 4.4 More complex calculations (if)

4. Basic calculations

4.1 Basic mathematical operations

• Variables
  • A variable is a name identifying a storage location for data. It is called a variable because the stored data can change.
  • For readability, choose names that reflect the data type or purpose.
    • (Reading other people's source code reveals some naming conventions.)

# Good variable examples
pos_x = 10 
num_values = 5
_event_flag = False

# Bad variable examples
abc = 4
a1 = 50
xxxx = 3.4
temp1 = 4
temp2 = 5

• Data types
  • These include integer, floating-point number, bool (True/False), and string.
  • Python manages them automatically, so you generally do not need to handle them explicitly.
    • However, be careful: a data type can change against your intention.

# Integer
number = 5

# Floating-point number
value = 2.5

# bool
is_number = True

# String (string)
name = 'Ivan'

# When an integer becomes a floating-point number
number = number / 2
print('number =', number)

number = 2.5

• Basic operators
  • Arithmetic
    • Addition +, subtraction -, multiplication *, division /)
  • Other operations
    • Quotient //, remainder %, exponentiation **
  • Logical operations
    • Greater than >, greater than or equal to >=, less than <, less than or equal to <=, equal to ==, not equal to !=, and, or

a = 10
b = 2.5
print(a+b, a-b, a*b, a/b, a//b, a%b, a**b)
print(a>b, a>=b, a<b, a<=b, a==b, a!=b, a>b or a<=b, a>b and a<=b)

12.5 7.5 25.0 4.0 4.0 0.0 316.22776601683796
True True False False False True True False

• Common mathematical functions (using NumPy)
  • Sine sin, cosine cos, exponential exp, round round, ceiling ceil, floor floor

# We will cover use of the numpy library in more detail later.
import numpy as np
angle = np.pi/4
print(angle)
print(np.sin(angle))
print(np.cos(angle))
print(np.exp(angle))
print(np.round(angle))
print(np.ceil(angle))
print(np.floor(angle))

0.7853981633974483
0.7071067811865475
0.7071067811865476
2.1932800507380152
1.0
1.0
0.0

4.2 Simple loops (for)

• The for statement is a built-in feature commonly used to repeat a specified number of times.
• Format

    for variable in items:
        statement
        ...

+ Each list element is assigned to the variable in order during iteration, and the loop ends after all elements have been processed. 
+ The list can contain any data type.

for i in [4, 'text', 1.5 , True, {'name':'Charles', 'age':15}]:
    print(i)

4
text
1.5
True
{'name': 'Charles', 'age': 15}

• However, the following form is more common.

for i in range(5):
    print(i)

0
1
2
3
4

• The continue statement can skip one iteration. (The if statement is explained in more detail at the end of this section.)

for i in range(5):
    if i==3:
        continue
    print(i)

0
1
2
4

• The break statement can completely end a loop. (break exits only one loop at a time.)

for i in range(5):
    if i==3:
        break
    for j in range(5):
        if j==2:
            break
        print(i, j)

0 0
0 1
1 0
1 1
2 0
2 1

• (Simple example) Multiplication table (Print only tables 2–4 to keep the output short.)

for i in range(2,5):
    for j in range(1,10):
        print('%d * %d = %d' % (i, j, i*j))
    print('----------')

2 * 1 = 2
2 * 2 = 4
2 * 3 = 6
2 * 4 = 8
2 * 5 = 10
2 * 6 = 12
2 * 7 = 14
2 * 8 = 16
2 * 9 = 18
----------
3 * 1 = 3
3 * 2 = 6
3 * 3 = 9
3 * 4 = 12
3 * 5 = 15
3 * 6 = 18
3 * 7 = 21
3 * 8 = 24
3 * 9 = 27
----------
4 * 1 = 4
4 * 2 = 8
4 * 3 = 12
4 * 4 = 16
4 * 5 = 20
4 * 6 = 24
4 * 7 = 28
4 * 8 = 32
4 * 9 = 36
----------

4.3 Conditional loops (while)

• The while statement is used to repeat only when a condition is satisfied.
• Format

    while(condition):
        statement
        ...

+ The condition has type bool and can be expressed as `True` or `False`.
    - Conditions can be combined as (condition `and` condition), (condition `or` condition).

• When the condition is True, the statements below execute, then the condition is checked again.
• When the condition is False, the loop stops and execution continues with the next statement.
• When the condition is always True, the loop has no way to stop and is often called an infinite loop.
  • Unless an infinite loop is intentional, carefully check whether some conditions could cause one.
  • continue and break work the same way as in a for statement, and break can exit an infinite loop. (See the example below.)

i = [1,2,3,4,-1,5,6,7,8]
j = []
count=0
while(i[count]>0):
    j.append(i[count])
    count = count + 1
print(j)

[1, 2, 3, 4]

• (Simple example) Multiplication table (Accept input and print the requested multiplication table.)

# Provide information about how the program works.
print('원하는 단수를 입력하시고, 종료하려면 -1을 입력하세요.')

# Deliberately use an infinite loop.
while(True):
    
    # Assume the user enters an integer.
    i = int(input())
    
    # Exit the loop when -1 is entered.
    if i==-1:
        break
    
    # Output
    for j in range(1,10):
        print('%d x %d = %d' % (i, j, i*j))

원하는 단수를 입력하시고, 종료하려면 -1을 입력하세요.
4
4 x 1 = 4
4 x 2 = 8
4 x 3 = 12
4 x 4 = 16
4 x 5 = 20
4 x 6 = 24
4 x 7 = 28
4 x 8 = 32
4 x 9 = 36
5
5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
-1

4.4 More complex calculations (if)

• The if statement performs the desired action when a condition is satisfied. (Unlike while, it does not repeat.)
• Format

    if (condition):
        statement
        ...
    elif (condition):
        statement
        ...
    else:
        statement
        ...

• The statements below run when the condition is True.
• If another condition needs to be checked when the condition is False, use the elif statement.
• Place any statements to run when the condition is False under the else statement.
• Examples of if statements are omitted because many have already been shown above.