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08/02 FOCS (Phill)

Foundations of Programming

Identifiers, variables, data and functions

Learning outcomes/key ideas

Describe (and change) the type of data in Python
Store data in variables using assignment statements
Describe the difference between printing and returning values
Define and call functions in Python

No official textbook, but some handy references:

In case you want to dig deeper into any of these topics:

W3Schools: https://www.w3schools.com/python/default.asp
A textbook co-authored by Niema: https://stepik.org/course/56730/promo
A book that also includes some material on Picobot: https://www.cs.hmc.edu/twiki/bin/view/CSforAll/

Later on, after we’ve had a chance to learn some more Python, this resource (suggested by Mentor Nola) is also helpful for checking your understanding. It doesn’t explain things—rather it just gives you practice problems:

https://codingbat.com/python

What is an identifier?

The name of something.
For example, the name of a variable:
```
my_number = 123
```
Later, we’ll learn of other things that we can give names: functions, classes, modules; the same naming rules apply.
Composed of letters, numbers and/or underscores starting with a letter or underscore.
Case matters: ABC is different than abc

It’s important to choose names thoughtfully:

A well-chosen name can make a problem easier to solve
A poorly-chosen name can make a problem harder to solve

We’ll see examples of this as we go through our coding practice together.

What is a keyword?

A word in the language that has a defined meaning.

Keywords may not be used as identifiers

course = "CSE 8A"   # ok!
class = "CSE 8B"    # not ok; "class" is a keyword

Case matters:

By case, we mean uppercase vs. lowercase, i.e. capital letters vs. small letters.

if = 3    # not OK; if is a keyword in Python
IF = 4    # allowed, but weird; we typically don't use all upper case for python variables
iF = 5    # allowed, but also a bit weird
If  = 6   # allowed, but weird.

What is a statement

What is a statement?

A statement is one unit of code.

It is often (though not always) one line of code:

Here are examples of some simple one line statements in Python:

import math             # one statement; imports the functions from the math module
print("Hello, World!")  # one statement; a function call to print
my_number = 123         # one statement; assigns the value 123 to the variable my_number
print("square root of ", my_number, "is", math.sqrt(my_number))   # one statement; a function call to print

You can separate multiple statements on a single line using semicolons. (This is uncommon among Python programmers, but you will see it sometimes).

You can also sometimes have a single statement that spans multiple lines.

x = 3; y=4; z=5   # Three assignment statements

# The next three lines are one statement; a single function call to print
print("x=",x,
      "y=",y,
      "z=",z) 

What is a variable?

There are various definitions; each of these emphasizes different aspects of what it means to be a variable.

A name associated with a value
A name of a storage location that holds a value
An association of a name of a storage location that holds a certain type of data.

xyz = 123               # integer (abbreviated "int") variable xyz holding an initial value (initialization) of 123
abc = 2.3               # "float" variable abc holding an initial value of 2.3
is_mentor = True        # boolean variable (named for George Boole, abbreviated "bool") holding an initial value of True
message = “Hello World” # string variable (a string of characters, abbreviation "str") holding an initial value of “Hello World”

Order of statements matter

In Python, statements are carried out top to bottom, in order

unless there is something to change the order (we’ll talk about those things a bit later; things like if/else, functions, loops, etc.)

This is true in most common programming languages (there are a few exceptions).

But it’s different from how we may be used to working with variables in algebra class.

For instance, in an algebra class, if I write:

y = x + 1
x = 7
solve for y

You would all understand how to solve this:

Plug in the value of y for x on line one (y = 7 + 1)
Replace 7 + 1 with 8
Now we have y = 8

But that’s not how variables work in Python. The order of assignment statements matters:

This works in Python

x = 7
y = x + 1
print("x = ",x)

This doesn’t. (We’ll show this on the repl during class)

y = x + 1
x = 7
print("x = ",x)

This also doesn’t work:

print("x = ",x)
x = 7

while this does:

x = 7
print("x = ",x)

The value of a variable can be changed

In Python, the value of a variable can be changed.

This is true in most common programming languages (there are a few exceptions), but different from how we may be used to variables in algebra class.

So here, what do you think will print?

x = 7
print("x = ",x)
x = 9
print("x = ",x)

How about this one?

x = 7
print("x = ",x)
x = x + 1
print("x = ",x)

Functions

Examples:

Print vs. Return

We discussed the difference between print and return.

def squared (x):
   ''' returns the value of x times itself '''
   return x * x  

def print_squared (x):
   ''' prints the value of x times itself '''
   print(x * x) 

We also talked about the special value None.

Every function call in Python that doesn’t end in a return statement behaves as if it had this statement at the end of the function:
```
return None
```
The value None is different from zero; it’s the absence of any value at all.

Print vs. Return: More information

Two videos you can watch for more information about print vs. return

https://www.youtube.com/watch?v=IbhQRbOVmL8
https://www.youtube.com/watch?v=m9YVldEkXNM

Functions

What is a Function?

Here is a huge brain dump about functions.

I’ll try to explain each of these ideas during lecture with examples, but don’t worry if you don’t get all of this on the first try.

A series of statements grouped together as a unit.
Grouping has a purpose
Grouping is named for that purpose

We need to understand the difference between function definitions and function calls.

Code inside the function definition is called “the function body”
A function body defines a “scope”
Code inside function can be reused without retyping the code.
The function is “called” to execute those statements
The function often produces a result that is returned to the caller (via a return statement)
The caller often captures the return result and stores into a variable.

Functions typically have inputs:

Inputs are called: parameters (sometimes called arguments)
There is a difference between “formal parameters” and “actual parameters”
Some people use “parameters” vs. “arguments” to describe this difference

Functions attached to an object are sometimes called: methods

We won’t need that information until later.

Example function

Used as a way to organize execution of a related group of statements to act as a unit to perform a task.

Function call performs that task.
Function definition includes:
name, parameters, function body - code
Indenting defines a “scope”
“first” and “second” are only known within the “add” function body

def add (first, second):
   ''' adds parameters and send back result '''
   return first + second   # adds parameters and send back result

Strings

A string is any sequence of characters between “ “ or ‘ ‘ # use double or single quotes

Examples:

my_college = "Warren"
your_college = "Sixth"
his_college = "ERC"
her_college = "Thurgood Marshall"
their_college = "Revelle"

Note that a number, if it is put in quotes, is still a string:

meaning_of_life = "42"

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