Goals of the course; what is computation; introduction to data types, operators, and variables.

Skills to develop:

• Computational thinking.
• Understand code.
• Understand abilities & limits of computation.
• Map problems into computations.

• What does it mean to think like a computer scientist?
• What is computation? (As something independent from the actual computer)
• What is knowledge?
• Declarative: statements of fact.
• Imperative: description of how to do something (recipe).

• Earliest computers: fixed program computers (fixed in the circuitry to do specific things: calculator, Atanasoff-Berry computer, Turing's bombe).
• Stored program computer: sequence of instructions describing the process to be applied by the computer.
• Memory.
• Control unit.
• Arithmetic Logic Unit.
• Input/Output.
• Series of instructions.

• Heart of the computer: instructions and flow.
• A program is a recipe.
• Recipes are based on a set of primitives that can be combined in a sequence to accomplish something.
• Fixed set of primitives can program anything.
• What re the right primitives? (Turing's six primitives).
• Programming language to describe the recipe.

• Dimensions of a programming language:
• High-level vs. low-level.
• General vs. targetted.
• Interpreted vs. compiled.
• Syntax: what are the legal expressions in this language?
• Semantics:
• Static semantics: what programs are meaningful?
• Full semantics: what does it mean?, what is going to happen when we run it?
• Interpreter or compiler checks the syntax.
• Interpreter or compiler also provides some help with static semantics.
• No help with full semantics (wrong answer, crash, apparently correct answer...): programming style helps here.

• Values:
• Numbers: integers, floats...
• Strings.
• Operators: addition, subtraction, multiplication, division...
• Variables: to store values.

Operators and operands; statements; branching, conditionals, and iteration.

Handout (PDF).

Elements needed when writing a program:

• Representation for fundamental data (numbers, strings):
• Value.
• Type.
• Instructions to manipulate that data (operands, operators):
• Entered at the interpreter: evalued & printed.
• From script: no print, unless explictly stated.

Overall concepts about operators and operands:

• Overloading operators:

      >>> 3*'ab'
      'ababab'
  
      >>> 3+'ab'
      Traceback (most recent call last):
        File "", line 1, in 
      TypeError: unsupported operand type(s) for +: 'int' and 'str'
      

• Type conversion:

      >>> str(3)+'ab'
      '3ab'
      

• Type checking (weak vs. strong):

      >>> 'a'<3
      False
  
      >>> 4<'3'
      True
  
      >>> '4'<'3'
      False
      

• Type discipline when programming is very important.
• Other peculiarities about operators (integer division in Python):

      >>> 9/5
      1
  
      >>> 9%5
      4
      

• Operator precedence (when in doubt, use parenthesis):

      >>> 3+4*5
      23
  
      >>> (3+4)*5
      35
      

• Ability to create variables via assignment statement:

      x=3*5
      

• Assigning a variable truly creates a pointer from the variable name to the location in memory that holds its value (link):

      >>> x=3*5
      >>> z=x
      >>> print x
      15
      >>> print z
      15    
      

• What is the type of a variable? Inherited from its value.
• Python does dynamic typing:

      >>> x=3
      >>> print x
      3
      >>> x='abc'
      >>> print x
      abc
      

• Good style: don't change types arbitrarily.
• Variables can be used any place it's legal to use the value.

• Statements: legal commands that Python can interpret (print, assignment, etc.).
• Advice for good programming style:
  • Commenting the code.
  • Good variable names (documentation of sorts).
  • Some variable names are excluded:

            >>> print='abc'
              File "", line 1
                print='abc'
                     ^
            SyntaxError: invalid syntax
            

• Branching programs: something that can change the order of instructions based on some test (value of a variable, usually).
• Conditional:

    x = 15
    if (x/2)*2 == x: print 'Even'
      print 'Even'
    else: print 'Odd'
    

• Boolean combinations (and, or, not): true or false.
• Iterations or loops:

      y=0
      x=3
      itersLeft = x
      while(itersLeft>0):
         y=y+x
         itersLeft = itersLeft - 1
         #print 'y =',y,',itersLeft=',itersLeft
      print y
      

• Loops could lead to infinite loops:
  • They have to involve a variable that is changing.
  • The variable needs to be initialized outside the loop.

>>> test = (1,2,3,4)
>>> test
(1, 2, 3, 4)
>>> test[0]
1
>>> test[1]        # selecting
2
>>> test[10]
Traceback (most recent call last):
  File "", line 1, in 
IndexError: list index out of range
>>> test [-1]
4
>>> test [-2]
3
>>> test [1:3]      # slicing
(2, 3)
>>> test [:3]
(1, 2, 3)
>>> test [2:]
(3, 4)

x = 100
divisors = ()
for i in range (1,x):
  if x%i == 0:
  divisors = divisors + (i)

>>> s1 = 'abcdefg'
>>> s2 = 'hijklmn'
>>> s1
'abcdefg'
>>> s1[0]
'a'
>>> s1+s2
'abcdefghijklmn'
>>> s1[2:4]
'cd'
>>> s1[:5]
'abcde'

sumDigits = 0
for c in str(1952):
  sumDigits += int (c)
print sumDigits