Lecture 7: More on Functions + Excel

Craig Zilles (Computer Science)

March 9, 2020

https://go.illinois.edu/cs105sp20

CS 105

Today1. nested loops (indefinite loops)2. break and continue3. Dynamic Typing4. Scope5. Excel• Cell ranges• Functions: SUM, COUNTIF, COUNTIFS• INDEX/MATCH, *LOOKUP

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Loop nesting• I think nested loops were the most confusing part of the

text. I find them incredibly difficult to read and understand what is happening in them. The challenge problems based around them were pretty hard for me.

• Muddiest point is nesting loops and how to implement continue and break commands

• I would love to go over more how to follow the code in nested loops because sometimes I do not understand what gets read in what order.

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Nested Loops• Print out the intersection of two lists

list1 = [‘lemon’, ‘orange’, ‘lime’]

list2 = [‘banana’, ‘lemon’]

for thing1 in list1:

for thing2 in list2:

if thing1 == thing2:

print(thing1)

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Break leaves a loop earlyHow many chars are printed?

for c in "sleepy":if c == "e":

breakprint(c)

A) 0 B) 1 C) 2 D) 3 E) 6

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Break vs. Returndef func(a_list):

for item in a_list:

if item == "":

break

print(item)

print("done")

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def func(a_list):

for item in a_list:

if item == "":

return

print(item)

print("done")

• How are these different if we pass in: ["a", "b", "", "c"] ?

Continue• Used to skip over iterations (but not leave loop)mixed_list = [

'hi', 3, math.pi, 'there', ['CS', 105]]

for item in mixed_list:

if type(item) != str:

continue

print(item)

How many items are printed?A) 0 B) 1 C) 2 D) 3 E) 5

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Docstrings = Documentation Strings• I found docstrings to be confusing because I don't really

understand their function and why we would use them

• Help a user of your function (could be you) understand how to use your function • Read using the help() function (q to quit help)

def my_function():"Docstrings are a string literal

that are first thing in the function"return 32

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Python is a dynamically typed lang.• The + operator does (at least) three different things:• Integer addition (when given two integers)• String concatenation (when given two strings)• Floating point addition (when given 1 float,1 number)

• How does Python know which to do?• At run time it looks at the types of its operands

• This means that a single piece of code can do many different things at different times (polymorphism)• Means you can write less code• Also means that it can be hard to find bugs

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String 6 001000011 001010011

TypeNumber ofcharacters

Characters(Stored using Unicode encoding)

000100000 000110001 000110000 000110101‘CS 105’

C S 1 0 5

A polymorphic functionfor print_all(collection):

for item in collection:print(item)

print_all([1, 2, 3, 4])print_all({ 'key': 'val', 'CS': 105' })print_all(7)print_all('a string')

A) Error B) No error10

Functions• better understanding formal and actual parameters• What happens if we do not set any parameters when

defining a function?• How to return a value from a function to a variable.

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Scope• I am confused about scope of variables and functions• Is all that you have to do to allow global modification

type "global 'variable name' "? I don't understand global vs local variables and how they are used.

• Scope prevents functions from messing each other up

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Scopes in actionmy_var = 11

def my_print(my_var):print(my_var)

my_print(22)print(my_var)

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What does it print?

A)

B)

C)

D)

E) Error

1111

1122

2211

2222

Scope, cont.1. Every function is given a clean slate

• Scope: a mapping names to objects

2. Any variables written in a function are defined in the function's scope

• Can be overridden with global statement

3. The scope is destroyed when the function returns

4. If a name is read that doesn't exist in the function's scope, it tries the scope the function was defined in.

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Functions• better understanding formal and actual parameters

• What happens if we do not set any parameters when defining a function?

• How to return a value from a function to a variable.

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Announcements

• Quiz 2 this week (Thursday-Sunday)• Practice exams up by tomorrow noon-ish

• I will compute mid-term grades tonight• We'll have Informal Early Feedback in Labs this week

• We've now seen all of the basics of programming:• Variables, expressions, conditionals, loops, functions• Reading assignments will be shorter in general• We'll focus more on solving problems w/code

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Excel: Cell Ranges• Colon-separated pair: e.g., upper-left:lower-right• E.g., A6:A12 is part of column, B2:H2 is part of a row

• D4:H12 is a two-dimensional rectangle of cells• Can use relative or absolute cell references• E.g., $D$4:$H$12

• A 3D reference in Excel includes the Sheet name• <sheetname>!<row><column>• E.g., statements!B2

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How many cells in region A1:B10

A) 9B) 10C) 18D) 20E) Some other value

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Excel: Functions

• Functions generate values for assignment expressions• =COUNTIF(A1:A30, MAX(A1:A30))

• Excel is case-insensitive: • =MAX(A1:A30) is the same as =max(a1:a30)

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Excel Fns: SUM(range)• Equivalent in Python:total = 0for cell in range:

total += sum

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Excel Fns: COUNTIF(range, criteria)• Equivalent in Python:count = 0for cell in range:

if cell == criteria: # sort ofcount += 1

• Criteria could be:• A single value: 7 or "Illinois"• A comparison: ">7" or ">" & B2• A string with wildcards: "*BADM*" or "CS 10?"• * matches any # of things, ? is any one character

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Excel Fns: COUNTIFS

• COUNTIFS(range1, criteria1, range2, criteria2, …)

• Ranges 1 and 2 must have equal # of rows and columns

• Counts each time the nth value of each range meets that range's criteria• E.g., =COUNTIFS(A0:A6,">10", B0:B6,"empty")

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INDEX – get value at known position• 1-dimensional lookup: • =INDEX(cellrange, index)• cellrange is 1-dimensional range• index is 1-based, not 0-based like in Python

• =INDEX(B3:B11, 4) ---> B6

• 2-dimensional lookup:• =INDEX(cellrange, index1, index2)• cellrange is 2-dimensional range• index1 specifies the row, index2 the column

• =INDEX(B3:D11, 4, 2) ---> C6

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INDEX• Which cell does =INDEX(C10:F20, 3, 2) read from?

A) D12B) E11C) E12D) E13E) F12

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MATCH – find a position in a list• =MATCH(value, cell-range, match-type)• value is the value we're looking for• cell-range is a row or a column (not 2D)• match-type is:• 0 = Exact match• 1 = Approximate (largest less than or equal, values

must be sorted in increasing order)• -1 = Approximate (smallest value greater than or

equal, values must be sorted in descending order)• returns 1-based index into the cell range for a cell

containing the value• Return #N/A if no match

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Using INDEX with MATCH

• =INDEX(B1:B20, MATCH('Craig', A1:A20, 0))• Search A1:A20 for a cell holding 'Craig'• Read the corresponding element of the B column

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VLOOKUP – INDEX + MATCH together• Handles a common case of INDEX + MATCH• Less flexible, but only 1 function

• =VLOOKUP (value, table, col_index, [range_lookup])• value - value to look for in the first column of table.• table - table from which to retrieve a value.• col_index - The column in the table from which to

retrieve a value.• range_lookup - [optional] TRUE = approximate match

(default). FALSE = exact match.• Value searched for must be to the left of value to return• HLOOKUP does for rows, what VLOOKUP does for cols

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Next week's reading

• More on Strings! Building and manipulating text data:• More control with formatting• Replace X with Y• Splitting strings into pieces

• Files! (How data is stored persistently (e.g., on disks)• Paths to files (directory hierarchies)• Opening files• Reading and writing files

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