Today - function call, return, start String data type

Foreshadow: Big Picture

Program made of functions
Data flows between functions
How do functions exchange data?
Function: black box model
data in: parameters
data out: return
Today 1 function at a time, later build whole picture

Theme: "Black Box" Function Model

Black-box model of a function: function runs with parameters as input, returns a value. All the complexity of its code is sealed inside. Calling it is relatively simple - provide parameters, get back an answer.

function is black box with parameters in and result out

4 Functions Today

(had a tech problem in lecture, now fixed)

> Black Box Functions

1. Parameters = Input

Parameters are the inputs of a function
Each parameter is like a variable with a value already in it
Where does the value come from...
(a) (non-answer) Not our concern how, but there's a value in there
(b) (real answer) The caller code places a value between the parens when calling the function, and this is the value of each parameter

2. `return` yields the result of a function

Inside the body of a function, like this:

def foo(n):
    ...
    return n * 10
    ...

Return exits the run of foo() immediately
The run continues at the caller code
The value, e.g. n * 10 is the return value of the foo() call

Suppose the caller code looks like

   # in caller function
   a = foo(6)
   b = foo(7)

The return line within foo() causes 60 to be returned by foo(6)
essentially a = 60
This is called the "return value" of the function
The return value of foo(7) is 70, assigned to b

Example 1. "Winnings1" Function

Say we work for the CA lottery
Writing a function for a game
Rules: input is an int score 0..10 inclusive
Output:
score 5 or more, winnings = score * 12
score 4 or less, winnings = score * 10
We could make a table showing output for each input
Writing code, we need to be organized thinking about the cases and their outputs
Code needs to work for all cases, i.e. to be general
Later: we'll see how to put in formal tests that the code is getting the right output for each input

Winnings1 code

This code works
score parameter comes in
if/logic to check for score >= 5
return is used twice
"pick off" of cases
So past the first if.. what is true of score?
We know score < 5

def winnings1(score):
    """Compute winnings1 described above."""
    if score >= 5:
        return score * 12
    
    # Run gets here: we know score < 5
    return score * 10

Winnings if-return Case Strategy

The function needs to handle all input cases
If-logic detects one case .. returns the answer
The return exits the function immediately with a return value
Lines below have if-return logic for other cases
Code is "picking off" the cases one by one
As we get past if-checks .. input must be a remaining case
Analogy: coin sorting machine
coin rolls past holes, each a bit larger
dime, penny, nickel, quarter
a dime never makes it to the quarter-hole
The dime is picked-off earlier
Bug demo: change <= to <

Return None Default

Demo: cut out the code, just have a "pass"
If a function does not run a return, None is default function result
If you see None in your output - maybe you forgot the return
Or your if-logic for some case doesn't return

Example 2. Winnings2 Example

Now there's 3 cases

Move the cutoffs around a little.

Winnings2: Input score is in the range 0..10 inclusive. Bonus! if score is 10 exactly, winnings is score * 15. If score is between 4 and 9 inclusive, winnings is score * 12. if score is 3 or less, winnings is score * 10.

Winnings2 Example

Now there's 3 cases; the cutoff numbers are different. Write a series of if-return to pick off the cases.

Solution:

def winnings2(score):
    """Compute winnings2 described above."""
    if score == 10:
        return score * 15
  
    if score >= 4:  # score <= 9 is implicit
        return score * 12
    
    # All score >= 4 cases have been picked off.
    # so score < 4 here.
    return score * 10

Example 3. Lottery Scratcher Example

An extra example for practice.

Lottery scratcher game: Have three icons called a, b, and c. Each is in the range 0..10 inclusive. If all three are the same, winnings is 100. Otherwise if 2 are the same, winnings is 50. Otherwise winnings is 0. Use and/or/== to make the tests.

def scratcher(a, b, c):
    """Return scratcher winnings described above."""
    pass

Solution:

def scratcher(a, b, c):
    """Return scratcher winnings described above."""
    # 1. All 3 the same
    if a == b and b == c:
        return 100
    
    # 2. Pair the same (3 same picked off above)
    if a == b or b == c or a == c:
        return 50
    
    # 3. Run gets to here, nothing matched
    return 0

Interpreter

Python interpreter - runs the python code
You can type at it directly - super handy
Try out little ideas, see what they do
Interpreter has >>> prompt
You type an expression
It evaluates, prints result
How to get the interpreter:
1. Use "console" tab in PyCharm lower left
2. Use "Interpreter" button at bottom of parlante.org pages

>>> 1 + 1  # you type this
2          # Python prints this

Strings

Super common data type
Sequence of 'characters' or 'chars'
data e.g. urls, paragraphs
In Python code a string "literal" is written on one line within single quote marks ' (prefer)
Double quote also works "
'hello'
"hello"

len() function

Returns number of chars in string
Works on other collections too
Not noun.verb style

>>> len('Hello')
5
>>> len('x')
1
>>> len('')
0

Empty String ''

The "empty" string is just the string with 0 chars
Written like: '' or ""
This is a valid string, len 0
A common case to think about with string algorithms
Does the code need to work on the empty string?
Probably yes.

Zero Based Indexing - Super Common

Everything in computers uses this zero-based indexing scheme
Characters in strings are "indexed" from 0
e.g. 'Python'

Python
012345

Index number addresses each char in the string
Foreshadow: we'll use index numbers, [ ], and len()...
For many many linear type data arrangements
But for today strings

the string 'Python' with 6 index numbers 0..5

String Square Bracket Index

Use square bracket to access a char by index number
Valid index numbers: 0..len-1
Get out of bounds error

>>> s = 'Python'
>>> s[0]
'P'
>>> s[1]
'y'
>>> s[4]
'o'
>>> s[5]
'n'
>>> s[6]
IndexError: string index out of range

String Immutable

A string in memory is not editable
"Immutable" is official the official term
e.g. square bracket cannot change

>>> s = 'Python'
>>> s[0]        # read ok
'P'
>>> s[0] = 'X'  # write not ok
TypeError: 'str' object does not support item assignment

String +

Plus operator + combines 2 strings to make a bigger string
aka "concatenate"
Uses new memory
Does not change original string

>>> a = 'Hello'
>>> b = 'there'
>>> a + b
'Hellothere'
>>> a
'Hello'

String + Add At End Pattern

Use + to add at the end of a string, yielding a new bigger string
Use like this to grow the string s: s = s + 'xxx'
The += operator works here too
The individual strings are still immutable -
We are constructing a new, bigger string with each step

>>> a = 'Hi'
>>> a = a + '!'
>>> a
'Hi!'
>>> a = a + '!'
>>> a
'Hi!!'
>>> a = a + '!'
>>> a
'Hi!!!'

Loop Over All Chars in a String

How to loop over all chars in string s
Use index numbers: 0 .. len-1
Use range function to make the numbers, 0, 1, 2, ... len(s)-1
Typical to use "i" as variable in loop

for i in range(len(s)):
  # access s[i] in here

Example 4, double_char

Start result as empty string
Loop over each char in string
For each char: use + to add it at the end of the result

Solution code:

def doublechar(s):
    """Return doublechar as described above."""
    result = ''
    for i in range(len(s)):
        result = result + s[i] + s[i]
    return result