MIT OCW: Introduction to Computer Science and Programming in Python

Link to course

Goal

Review computer science foundations and python fundamentals as I already took Software 1 and 2 using Java at Ohio State. I anticipate being able to pick up on some nuances now that I’m not learning it for the first time and have professional programming experience.

Lesson 1 - What is Computation

Types of knowledge

• declarative knowledge: statements of facts (x = 16)
• imparative knowledge: recipe or how to do something (def sqrt(x): …)

Recipes have steps, flow of control, termination criteria, etc. and therefore are algorithms

Basic Machine Architecture:

• Memory: data & sequence of instructions
• Control unit: flow / interpreter
• Arithmetic logic unit: does primitive instructions
• Input / output

Basic primitives

• Turing showed you can compute anything using 6 primitives (move right, move left, read, write, scan, do nothing)
• therefore, anything computable in one programming language is computable in any other programming language

Creating recipes:

• a programming language provides a set of primitive operations (words : float, bool, etc)
• expressions are complex but legal combinations of primitives (phrases, meaningful sentences : expressions operand operator)
• expressions have values and meanings in a programming language

Languages

• static semantics: syntactically valid strings have meaning.
  • “I are hungry” is syntactically valid (noun + verb + adjective), but has static semantic error
• semantics: meaning associated with syntactically correct string of symbols without semantic errors
  • English sentences can have many meanings (a reading lamp vs a reading lamp) but programming languages have only one meaning and it might not be what the programmer intended.

Languages & Errors

• syntactic errors are easily caught by the interpreter because the program can’t run when it has them.
• static semantic errors are checked for by some programming languages before executing but may run and have unpredictable behavior.
• no semantic errors but poor logic could lead to crashes, won’t stop, unexpected answer

Python programs

• a program is a sequence of definitions and commands.
  • definitions are evaluated
  • commands are executed by the python interpreter in a shell
• commands (statements) instruct the interpreter to do something
• can be typed directly in a shell or stored in a file that is read into a shell

Objects:

• programs manipulate data objects
• objects have a type that defines the kinds of things the program can do to them
• objects can be…
  • scalar (cannot be subdivided)
    • In python, 4 scalar types: int, float, bool, NoneType
  • non-scalar (have internal structure that can be accessed) (list etc.)

Expressions:

• expressions evaluate to a value, which has a type
• combine objects and operators to form expressions
• syntax for a simple expression could be

Operators on INTs and FLOATs

• sum + (result is float if at least one object is float)
• difference -
• product *
• division / (result is always float)
• remainder %
• power **

Binding variables and values

• equal sign is an assignment of a value to a variable name
  • “variable = value”
• store in computer memory
• assignment binds name to value
• retrieve value by invoking the name

Abstracting expressions

• using names rather than values in expressions allows you to run the code with different values by simply reassigning the variable rather than editing your expressions

Programming vs Math

• in programming you need to provide expressions, you can’t “solve for x”
• an assignment always has the expressions on the right evaluated to a value and a variable name on the left

Changing bindings

• variable names can re-bind to new values by using a new assignment
• previous value may still by stored in memory but the handle for it is lost and left for the garbage collector

Lesson 2 - Branching and Iteration

Strings

• letters, special characters, digits
• indicated with “” or ‘’
• concatenation operator: +
• allows some operations like ‘*’

Input/Output

• print()
  • adding ‘,’ between arguments in print() will automatically add spaces between.
  • using ‘+’ between args does not add spaces but all args must be str type
• input(“”)
  • arg is what will be printed and then it waits for user to provide an input.
  • input will be str type so may need to cast

Comparison operators on int, float, string

• comparisons below evaluate to a Boolean
  • i > j
  • i < j
  • i >= j
  • i <= j
  • i == j
  • i != j

Comparison operators on bools

• comparisons below evaluate to a Boolean
  • not a –> true if a is false
  • a and b –> true if both are true
  • a or b –> true if at least one is true

Control Flow - Branching

• if : / else:
• if : / elif : / else:
  • where conditions evaluate to a bool

Indentation

• Matters in python and is used to denote blocks of code

Control Loops while/for

• while :
  • repeat expression as long as condition is true
  • if you use a counter, initialize it outside the loop and change it inside
  • has unbounded number of iterations
• for in :
• range(5) == [0, 1, 2, 3, 4]
• range(7, 10) == [7, 8, 9]
• range(5, 11, 2) == [5, 7, 9]
• break
  • immeditately exits loop whatever loop it is in
  • skips the remaining expression in code block
  • exits only the innermost loop