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cs_lec_2

Warren Hall can be used for CS 61a help Types of expressions Expression descrbes a computation and evaluates to a value

18+69
6/23
sqrt(3493161)

The way the call expression is done is as so: ===function call notation Call notation===

Anatomy of call expression

THIS IS THE CALL EXPRESSION add (2,3)) Operator(the function name) ( Operand, Operand )

Evaluation procedure for call exressions:

Evaluating nested expression: This is all just one call expression mul(add(4,mul(4,6)),add(3,5)) The operator is mul, the operands are add(4,mul(4,6)) and add(3,5) Call procedure is the operands.

All arguments are values Argument is just a number

Execution rule for assignment statements:

  1. evaluate all expressions to the right of = from left to right.

Defining Functions ssignment is a simple means of abtraction: binds names to values

Function definition is a more powerful means of abstraction: binds names to expression

Function definition is a more powerful means of abstraction: binds names to expressions Function signature indicates how many arguments a function takes

def <name> <formal Paramenters>:
    return <return expressions>

Calling user-defined functions Procedure for calling/applying user-defined functions (version 1):

  1. Add a local frame, forming a new environemnt
  2. Bind the functions' formal paramenters to its arguments in that frame
  3. Execute the body of the function in that new environment
from operator import mul
def square(x):
    return mul(x,x)
square(-2)

Global frame 

mul → func mul(...)
square → func square(x)

Original name of fucntion called square

    x | -2
    return value 4

A function

Looking Up Names In Environment​

Every expression is evaluated in the context of an environment.

So far, the ccurrent environment is either:

  • The globl frame alone, or
  • A local frame, followed by the global frame

Most important two things I'll say all day: An environment is a sequence of frames.

A name evaluates to the value bound to that name in the earliest frame of the curent environemnt in which that name is found.