SICP-Notes-Lecture 05 Environment Diagrams

Lecture 05 Environment Diagrams

These are my notes for SICP(Structure and Interpretation of Computer Programs). Hope they’ll be of some help to you.

Environment Diagrams

What ?

• A visual tool to keep track of bindings and state of a computer program
• It can be applied to similar languages aside of python

Why?

• conceptual

  • understand why programs work the way they do
  • confidently predict how a program will behave

• helpful for debugging

  • when you’re really stuck,

    diagramming code > staring at lines of cod

What have we seen so far

• Assignment Statement
• Def Statement
• Call Expressions

Terminology: Frames

• A frame keeps track of variable-to-value bindings
  • Every call expression has a corresponding frame
• Global, a.k.a. the global frame, is the starting frame
  • It doesn’t correspond to a specific call expression
• Parent frames
  • The parent of a function is the frame in which it was defined
  • If you can’t find a variable in the current frame, you check it’s parent, and so on. If you can’t find the variable, NameError

Review: Evaluation Order

Remember to evaluate the operator, then the operand(s), then apply the operator onto the operand(s)

The environment diagram should reflect Python’s evaluation.

Variable Lookup

• Lookup name in the current frame
• Lookup name in the parent frame, its parent frame, etc…
• Stop at the global frame
• If not found, an error is thrown

Important: There was no lookup done in f1 since the parent of f2 was Global

Evaluation vs Apply

A new frame is created only when an application happens. Evaluation will not create a new frame.

Lambda Expressions

Expressions that evaluate to functions!

>>> square = lambda x: x * x # A function with parameter x that returns the value of x * x
>>> square
<function <lambda> ... >
>>> square(4)
16
>>> x = square(5)
>>> x
25

Lambda Expressions vs def Statements

square = lambda x: x * x

def square(x):
return x * x

• Both create a function with the same behavior
• The parent frame of each function is the frame in which they were defined
• Both bind the function to the same name
• Only the def statement gives the function an intrinsic name
  • you need to use an assignment statement to bind the function created by lambda expression to a name in order to refer to it later

Summary

• Environment Diagrams formalize the evaluation procedure for Python

  • Understanding them will help you think deeply about how the code that you are writing actually works

• Lambda functions are similar to functions defined with def, but are nameless

• A Higher Order Function is a function that either takes in functions as an argument (shown earlier) and/or returns a function as a return value (will see soon)

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