Python Notes (2) - Conditionals and Control Flow

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The note summaries conditionals, control flow and a game implemented by Python.

Python notes of open courses @Codecademy.

Conditionals & Control Flow

• Control flow: gives us this ability to choose among outcomes based off what else is happening in the program.
• Comparators:
  
  • ==: Equal to
  • !=: Not equal to
  • <: Less than
  • <=: Less than or equal to
  • >: Greater than
  • >=: Greater than or equal to
• Boolean operators: compare statements and result in boolean values.
  
  • and: which checks if both the statements are True;
  • or: which checks if at least one of the statements is True;
  • not: which gives the opposite of the statement.
  • Order of operations: not > and > or.
  • Parentheses (): ensure expressions are evaluated in the desired order. Anything in parentheses is evaluated as its own unit.

• Conditional Statement Syntax: If, Else and Elif

  • if: is a conditional statement that executes some specified code after checking if its expression is True.
  • if some_function():
    
    • In the event that some_function() returns True, then the indented block of code after it will be executed.
    • In the event that it returns False, then the indented block will be skipped.
    • Note the colons at the end of the if statement.
  • else: complements the if statement. An if/else pair says: “If this expression is true, run this indented code block; otherwise, run this code after the else statement.”

  • Elif: is short for “else if.” It means that “otherwise, if the following expression is true, do this!”

    
    # Example for `If`, `Else` and `Elif`
    
    if this_might_be_true():
        print "This really is true."
    elif that_might_be_true():
        print "That is true."
    else:
        print "None of the above."

PygLatin

Pig Latin is a language game, where you move the first letter of the word to the end and add “ay.” So “Python” becomes “ythonpay.” To write a Pig Latin translator in Python, here are the steps we’ll need to take:

• Ask the user to input a word in English.
• Make sure the user entered a valid word.
• Convert the word from English to Pig Latin.
• Display the translation result.

• Input
  
  • raw_input(): accepts a string, prints it, and then waits for the user to type something and press Enter (or Return). This string can also be stored in a variable, e.g., name = raw_input("What's your name?").
  • isalpha(): which returns False since the string contains non-letter characters.

• Output

  • String[1:4]: accesses a slice of “String”, i.e., returns everything from the letter at position 1 up till position 4 (note not including the position 4).

    
    # Example for PygLatin
    
    print 'Welcome to the Pig Latin Translator!'
    
    pyg = 'ay'
    
    original = raw_input('Enter a word:')
    
    if len(original) > 0 and original.isalpha():
        word = original.lower()
        first = word[0]
        new_word = word + first + pyg
        new_word = new_word[1:len(new_word)]
        print new_word
    else:
        print 'empty'

External Resources

• Conditionals and Loops
• Conditional statements in Python