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====== Python first steps ======
Mithat Konar\\ 
based on Dierbach's //[[http://www.wiley.com/WileyCDA/WileyTitle/productCd-EHEP002046.html|Introduction to Computer Science using Python]]//

===== The Python language =====

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

===== IDLE =====

  * [[https://docs.python.org/3/library/idle.html%7C|IDLE]] is Python’s ‘standard’ IDE.
  * Good for starting to learn Python.
  * Many [[https://wiki.python.org/moin/IntegratedDevelopmentEnvironments|other IDEs]].

===== IDLE in interactive mode =====

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

<code python>
>>> 4 + 6
10
>>> 3 * 7
21
</code>
===== Basic math =====

  * ''%%+%%'', ''%%-%%'', ''%%*%%'', ''%%/%%''
  * ''%%**%%'' is exponentiation
  * Note the use of parenthesis in multiplication.

<code python>
>>> 4 + 6
10
>>> 3 * 7
21
>>> 5 / 2
2.5
>>> 2 * (7 + 3)
20
>>> 2 ** 3
8
</code>
===== The standard library and import =====

  * Python’s core is limited.
  * Add functionality by importing modules.
  * Lots of modules in the [[https://docs.python.org/3/library/|standard library]].

<code python>
>>> import math
>>> 4 * math.pi
12.566370614359172
>>> math.factorial(4)
24
</code>
===== Variables =====

  * **variable**: a name associated with a value.

<code python>
>>> r = 5
>>> 2 * r
10
</code>
  * Variables can store character **string** values as well.

<code python>
>>> city = 'Jakarata'
</code>
===== Basic input and output =====

  * Use ''%%print()%%'' to generate output.

<code python>
>>> print('Hello, there.')
Hello there.
>>> n = 5
>>> print(n)
5
</code>
  * Use ''%%input()%%'' to get input.

<code python>
>>> name = input('What do I call you? ')
What do I call you? Brian
>>> print('Hi,', name)
Hi, Brian
</code>
===== Using IDLE to develop a program =====

  * Create a new program file with //File > New File// or //Ctrl+N// (Linux and Windows).
  * Write code:

<code python>
name = input('What do I call you? ')
print('Hi,', name)
print('Eat more vegetables.')
</code>
  * 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 =====

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

<code python>
# 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.')
</code>
===== More about strings =====

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

<code python>
>>> name = 'Carol ' + 'Cleveland'
>>> print(name)
Carol Cleveland
</code>
===== Converting values =====

  * Convert a number to a string with ''%%str()%%''.

<code python>
>>> x = 99
>>> message = 'Number of red balloons: ' + str(x)
>>> print(message)
Number of red balloons: 99
</code>
  * Convert a string to an integer with ''%%int()%%''.

<code python>
>>> x = '99'
>>> x + 1 # error because x is not a number but 1 is.
>>> int(x) + 1
100
</code>
===== More about input() =====

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

<code python>
num_balloons = input('How many balloons? ')
num_balloons_converted = int(num_balloons) + 1
</code>
  * Combining getting input and converting:

<code python>
num_balloons = int(input('How many balloons? '))
</code>
===== More about print() =====

  * You can print more than one thing with ''%%print()%%''.
  * Separate each thing with a comma.

<code python>
>>> num = 99
>>> print("There are", num, "balloons.")
There are 99 balloons
</code>
  * Each “thing” will be converted into a string.

===== A slightly more substantial program =====

  - Define the problem.
  - Design a solution.
  - Code the solution.
  - Test the solution.
  - Document the solution.

===== Define the problem =====

> 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.

===== Define the problem =====

  * 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:

<code>
What base? 10
What power of 10? 4 
10 to the power of 4 is 10000
</code>
===== Design a solution =====

  - Get the base from the user.
  - Get the exponent from the user.
  - Calculate //base// to the //exponent// power.
  - Print “//base// to the power of //exponent// is //result//.”

===== Code the solution =====

<code python>
# 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)
</code>
===== Test the solution =====

<code python>
>>> 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'
</code>
Oops. Back to step 3.

===== Code the solution (again) =====

<code python>
# 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)
</code>
===== Test the solution =====

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

===== Code the solution (last time?) =====

<code python>
# 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))
</code>
===== Test the solution =====

<code python>
>>> Enter the base: 10
>>> Enter the exponent: 4
>>> 10 to the power of 4 is 10000
</code>
Success. But we can do better…

===== Code the solution (last time!) =====

<code python>
# Exponent Calculator
# 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)
</code>
===== Test the solution =====

<code python>
>>> Enter the base: 10
>>> Enter the exponent: 4
>>> 10 to the power of 4 is 10000
</code>
The code is easier to read and it still works.

===== Document the solution =====

  * 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.**