Hello World! An Invitation to Computer Science

Lecture Slides

Introduction to Programming in Python
Thursday April 3, 2008

Key ideas from last time

• high-level languages v. assembly languages
• there are 1000s of 3rd generation languages
• the import of C
• programming paradigms

How are high-level programs executed?

• parsing, mapping to discrete fragments,
    eventual conversion to machine code
• interpreters and translators ("compilers")
• analogies with natural language

What is Python?

• imperative scripting language, with O-O features
    (and some functional features)
• interpreted (executed via virtual machine)
• indentation effects computation!
• designed for readable and to be similar to pseudocode
• many features which we will not discuss in this course

Identifiers

• case sensitivity
• readability, consistency, convention
• reserved words

Expressions

• literals: integers, floats, True/False, strings
• identifiers (must be assigned values first!)
• combinations of simpler expressions
    using operators (and parentheses)
• order of operations: what you might expect (PEMDAS)
    (parentheses can make more readable)
• spaces optional in expressions
    (but can also make more readable)
• 9.0 / 5.0 v. 9 / 5

Statements

• output using print in its various forms
• simple assignment
• input assignment
• while loops

Accumulator pattern

def factorial():
    n = input("Enter value for n: ")
    product = 1
    i = 2
    while i <= n:
        product = product * i
        i = i + 1
    print "The factorial of", n, "is", product

Real computers are finite

• numbers usually represented in one fixed-width register
• advantage: arithmetic in hardware - very fast!
• disadvantage: overflow errors
• Python and Scheme use mixed-representation:
    hardware for small values
    software for larger values

Another example

def sumDigits():
    n = input("Enter value for n: ")
    sum = 0
    x = n
    while x > 0:
        sum = sum + (x % 10)
        x = x / 10
    print "Sum of digits in ", n, "is", sum