Mithat Konar
from Dierbach's “Introduction to Computer Science using Python”

• Syntax is easy to read.
• Interpreted and can be executed interactively.
• Huge community.
  • Lots of support.
  • Lots of open source modules.

• IDLE is Python's 'standard' IDE.
• Good for starting to learn Python.
• Many other IDEs.

• Often called REPL (read-evaluate-print-loop) mode.
• >>> is the shell prompt

>>> 4 + 6
10
>>> 3 * 7
21

• +, -, *, /
• ** is exponentiation
• Note the use of parenthesis.

>>> 4 + 6
10
>>> 3 * 7
21
>>> 5 / 2
2.5
>>> 2 * (7 + 3)
20
>>> 2 ** 3
8

• Python's core is limited.
• Add functionality by importing *modules*.
• Lots of modules in the standard library.

>>> import math
>>> 4 * math.pi
12.566370614359172
>>> math.factorial(4)
24

• variable: a name associated with a value.

>>> r = 5
>>> 2 * r
10

• Variables can store character string values as well.

>>> city = 'Jakarata'

• Use print() to generate output.

>>> print('Hello, there.')
Hello there.
>>> n = 5
>>> print(n)
5

• Use input() to get input.

>>> name = input('What do I call you? ')
What do I call you? Brian
>>> print('Hi,', name)
Hi, Brian

• Create a new program file with File > New File or Ctrl+N on Linux and Windows.
• Write code:

name = input('What do I call you? ')
print('Hi,', name)
print('Eat more vegetables.')

• Save the file: File > Save (Ctrl+S) or File > Save As … (Ctrl+Shift+S)
  • File extension .py will be added automatically.
• Run: Run > Run Module (F5)

• Comments let you write notes to yourself or the reader of your code.
• Anything after a # will be ignored by the interpreter.

# This is my first Python program.
name = input('What do I call you? ')  # get user's name
print('Hi,', name)  # print name entered by user
print('Eat more vegetables.')

• The + operator can be used to concatenate (put together) two string values.

>>> name = 'Carol ' + 'Cleveland'
>>> print(name)
Carol Cleveland

• Convert a number to a string with str().

>>> x = 99
>>> message = 'Number of red balloons: ' + str(x)
>>> print(message)
Number of red balloons: 99

• Convert a string to an integer with int().

>>> x = '99'
>>> x + 1 # error because x is not a number but 1 is.
>>> int(x) + 1
100

• input() in Python 3 will always return a string.
• If you expect a number, convert it:

num_balloons = input('How many balloons? ')
num_balloons_converted = int(num_balloons) + 1

• Combining getting input and converting:

num_balloons = int(input('How many balloons? '))

1. Define the problem.
2. Design a solution.
3. Code the solution.
4. Test the solution.
5. Document the solution.

I need an exponent calculator.

Solution: Write a program that allows the user to enter any integer base and integer exponent, and displays the value of the base raised to that exponent.

Observations: The user will enter the values from the keyboard. It would be good if the user got feedback when the result is printed. I will use Python because it will be easy to implement the solution with it.

Proposed interaction:

What base? 10
What power of 10? 4 
10 to the power of 4 is 10000

1. Get the base from the user.
2. Get the exponent from the user.
3. Calculate _base_ to the _exponent_ power.
4. Print “base to the power of exponent is result.”

exponent_calculator.py

# Exponent Calculator
# Calculate a base to an exponent's power.
# Mithat Konar
 
base = input('Enter the base: ')
exponent = input('Enter the exponent: ')
result = base ** exponent
print(base + ' to the power of ' + exponent + ' is ' + result)

Enter the base: 10
Enter the exponent: 4
Traceback (most recent call last):
  File "/home/mithat/exponent_calculator.py", line 7, in <module>
    result = base ** exponent
TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'str'

Oops. Back to step 3.

exponent_calculator.py

# Exponent Calculator
# Calculate a base to an exponent's power.
# Mithat Konar
 
base = int(input('Enter the base: '))
exponent = int(input('Enter the exponent: '))
result = base ** exponent
print(base + ' to the power of ' + exponent + ' is ' + result)

Enter the base: 10
Enter the exponent: 4
Traceback (most recent call last):
  File "/home/mithat/exponent_calculator.py", line 8, in <module>
    print(base + ' to the ' + exponent + 'power is ' + result)
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Oops (again).

exponent_calculator.py

# Exponent Calculator
# Calculate a base to an exponent's power.
# Mithat Konar
 
base = int(input('Enter the base: '))
exponent = int(input('Enter the exponent: '))
result = base ** exponent
print(str(base) + ' to the power of ' + str(exponent) + ' is ' + str(result))

Enter the base: 10
Enter the exponent: 4
10 to the power of 4 is 10000

Success.

• In this case, we began documenting the program right from the start:
  • Writing out details of the problem.
  • Writing out the design of the solution.
  • Using comments in the source code.
• Do this.
• Every. Single. Time.