Class 42 — Monday, December 6

One more chance to be a class

Programming basics — Your semester achievement — Bask in the success

Dream big and work hard — Celebrate diversity — Always pay forward

Look both ways

• Back and ahead

Agenda

• Answer questions

Thanks again to the TAs for making the course successful

Amshala Bharathan	Kevin Luk	Sophia Rogers	Meghan Tracy
Katherine Brickley	Rachel McNamara	MaryJeanine Seaman
Bailey Greggs	Jimmy Njuguna	Shivania Surtio

Want to be a TA for CS 1112

• Departmental application

Test 3

• Form to indicate your intention to take test

• The default is you are not taking Test 3. Students who want to take test 3 must indicate their intention before the end of class on Monday, December 6.

Past homework

• Solutions are available

Test taking

• Students are expected to take the test in Rice 130 on Friday.

What to expect for the test

• Use of if input() statement

• Knowledge of if, for, and while statements; along with the ability to use them when problem solving.

• Ability to write functions using and producing numbers, strings, lists, dictionaries, datasets, and random values.

• Simple image manipulation

Built-in functions

• id( v ): returns indicator where in memory to find v.

• type( v ): returns the type of value stored in v.

• abs( v ): returns absolute value of v.

• len( s ): returns number of things making up s.

• min( s ): returns minimum value in s.

• max( s ): returns maximum value in s.

• sum( s ): returns the sum of s.

• int( s ): returns an integer constructed from s.

• float( s ): returns a decimal constructed from s.

• string( v ): returns a string constructed from s.

• sorted( s ): returns a sorted version of s.

Functions

• Functions can be invoked.

• The invocation identifies the function to be started up.

• The invocation includes the arguments. The arguments are evaluated. Copies of the values are used to initialize the parameters specified in the function definition.

• Functions must be defined. A function definition consists of a header followed by its body.

• Header: def name ( parameters ) :

• Body: remaining lines in function. How can you tell? They are indented. Body is also known as the statement list

• A return is a special statement in the body of function. When a return statement is reached in a function, the function stops executing its instructions; that is, the function immediately stop its execution.

• A return statement can optionally have a return expression following the keyword return. The expression is evaluated. A copy of that value is the return value of the function. The return value is said to be the value of the evaluation of the function invocation.

• All Python functions return values – if no return statement with a return expression is executed. The function returns None.

• A local variable is a variable initialized/defined inside a function definition.

• Local variables only exist inside the function definition (limited scope)

• The parameters of a function are the variables listed with the parentheses in the function header.

• Parameters are local variables

• The arguments to a function are the values given within the parentheses of an invocation of the function.

• The Python way of handling the arguments is pass by value; that is, the arguments are evaluated to determine their values. Copies of those values are used to initialize the parameters.

• Because Python uses pass by value, the value of an argument is unchanged by a function invocation; that is, the value of an argument is the same after the invocation as it was before the invocation.

• Implication: not possible to write a swapping function in Python

• Because a parameter has the same value as its argument, if that argument is a list, the contents of that list can be modified. Such a change is called a side effect of the function. Functions with side effects generally have a return value of None.

Sequence expression — adapted in part from docs.python.org

List patterns

• Suppose below alist, blist, and clist are lists and that value v is not a list

• To analyze a list use

result = ...

for element in alist :

  ...

return result

• To modify a list use

n = len( alist )

for i in range( 0, n ) :

  element = alist[ i ]

  ...

  alist[ i ] = element

• Important list operations

• alist.append( v )

• Appends v to end of alist.

• alist.remove( v )

• Removes first occurence of v from alist.

• alist.index( v )

• Index of first occurrence of v in alist.

• clist = alist + blist

• Concatenates alist and blist to make clist.

• v = alist.pop( )

• Removes the last element from alist. The value of that former element is the return value assigned to v.

• v = alist.pop( i )

• Removes the element at index i from alist. The value of that former element is the return value assigned to v.

• List gotchas

blist = alist + v # cannot add a list and a non-list

alist = alist.append( v ) # alist is no longer a list, it is None

Warnings

• A print() statement in a function is not supplying a return value. It is sending a message to the program user.

• An input() statement is wrong to include in a function body, information comes into a function through its parameters.

• No function you will be asked to implement should have a print() or input() statement.

FWIW

• Nested for-loops usually wrong unless using a list-of-lists (e.g., a dataset) or looking at one list in relation to another list or image manipulation

Dictionaries

• Unordered collection of key-value pairs

• Initialization examples

d = { 18 : "voting", 67 : "retirement", "CS 1112" : "course" }

d = {}

d[ 18 ] = "voting"

d[ 67 ] = "retirement"

d[ "eighteen" ] = 18

• Access values by key: d[ 18 ]

• Means the same thing as "voting"

• Access values by key: d[ 21 ]

• Will be an error because 21 is not a key

• Access values by key: d[ "voting" ]

• Will be an error because "voting" is not a key

• Access values by key: d.get( 18 )

• Means the same thing as "voting"

• Access values by key: d.get( 21 )

• Means the same thing as None (not an error)

• Access key by its value: not easy, probably need a loop

• Collection of all keys: d.keys()

• Collection of all values: d.values()

• Indices: there are none (just keys)

• List of all keys: list( d.keys() )

• How do we look at the keys in a dict

for key in d.keys():

  # do something with key

• How do we look at the values of a dict

for value in d.values():

  # do something with value

• Trying to find and change a particular entry

• By key; e.g., change key 18 to "adult"

d[ 18 ] = "adult"

• By value (find key for value 18)

found = None

for key in d.keys():

  value = d[ key ]

  if value == 18:

  found = key

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