Week 1 - Welcome to 61A!

Installing Python

• Follow the instructions over at Lab 00.

Intro to Python

A programming language is more than just a means for instructing a computer to perform tasks. The language also serves as a framework within which we organize our ideas about computational processes. Programs serve to communicate those ideas among the members of a programming community. Thus, programs must be written for people to read, and only incidentally for machines to execute.

We will introduce expressions, which are statements that Python can execute. Then, we will go over how Python executes them, and how they can be stored in variables. Lastly, we will discuss the print function.

A brief aside

Generally, in 61A your assignments (labs, homework, and projects) will be given in a Python file with the extension .py. You can edit these files, and the Python interpreter will execute it line by line.

That being said, Python is a great introductory programming language. There are many features that make it friendly, one of which is the Python interpreter, which can be started with python3 -i (try it out in your terminal!).

Throughout 61A, you will see expressions such as

>>> print("hi")
hi

The >>> represents statements executed directly in a Python interpreter (ie. not from a file), and the line after is the output we get from executing the >>> line.

Primitive Expressions

Primitive expressions represent the simplest building blocks that the language provides

Python defines many built in types of primitives, and the most commonly used types are int, float (floating point number), bool (boolean, or True/False type), str (string type), and other container types that we will get into later

>>> 3.14 # this is a float
3.14
>>> 'hello!' # this is a string
'hello!'
>>> "double quotes!" # this is another string
'double quotes!'

Call Expressions

Call expressions look like

>>> function_name(my, arguments, here)

and they apply a function to an argument, and give you a result back. Later in class, we will learn how to define our own functions to create more reusable building blocks.

>>> max(1, 3) # takes the max of 1 and 3
3
>>> pow(2, 3) # takes 2 to the 3rd power
8
>>> abs(-1) # returns the absolute value of -1
1

Names / Environment

Python also allows for assignment, meaning that you can store values inside of variables.

>>> a = 42
>>> a
42
>>> my_sum = a + 10
>>> my_sum
52
>>> ten = abs(-10)
>>> ten
10

The = binds the left name to the right value inside the current frame of execution. In the above case, we are executing these lines in the default global frame, which we can see below.

Evaluation

When Python evaluates a call expressions, it uses this order:

1. Evaluate operator and operand
2. Apply the operator to the operand.

Example #1: Simple Evaluation

>>> abs(-10)
10

1. Evaluate the operator and the operand
   • Here, the operator is abs and the operand is -10.
   • Looking up abs, we get that it’s the built-in function abs.
   • -10 is a primitive, so we are good to go.
2. Apply the operator to the operand
   • We apply the built-in function abs to -10, and we get 10 as our answer.

Example #2: Nested Evaluation

What happens when we have nested evaluation statements? (Example taken from composingprograms)

>>> mul(add(2, mul(4, 6)), add(3, 5)) # assume we have add and mul defined
208

When we have nested evaluation statements, Python will follow the same procedures. Let’s walk through a few steps:

1. Evaluate the operator and the operand
   • The operator in the outermost statement is mul. This is a valid name in our current environment. Then, our operands are add(2, mul(4, 6)) and add(3, 5). Since these are not primitive expressions, we must evaluate them to get a value.
   • For add(2, mul(4, 6)):
     1. Evaluate the operator and operand of add(2, mul(4, 6)). The operator is add and the operands are 2 and mul(4, 6). 2 is a primitive expression, so we can continue here. mul(4, 6) needs to be evaluated. Using the same process, we get 24.
     2. Apply the operator to the operand. Applying add to 2 and 24 returns 26.
   • For add(3, 5): Using the same process above, we get 8.
2. Apply the operator to the operand. Our operator is mul, and our operands are 26 and 8. Our final return value is 208.

Print

Print is a function that

1. Returns None (nothing), and
2. Prints a value out to the terminal.

>>> print("hi")
hi
>>> print(4.2)
4.2
>>> print(print("hi"))
hi
None

In the last example above, think about the evaluation order! (eval operator, operand, apply)