1. Arrays (Lists) in Python
one thing after another

2. Problem Given 5 numbers, read them in and calculate their average
THEN print out the ones that were above average

3. Data Structure Needed
Need some way to hold onto all the individual data items after processing them
making individual identifiers x1, x2, x3,... is not practical or flexible
the answer is to use an ARRAY
a data structure - bigger than an individual variable or constant

4. An Array (a List)
You need a way to have many variables all with the same name but distinguishable!
In math they do it by subscripts or indexes
x1, x2, x3 and so on
In programming languages, hard to use smaller fonts, so use a different syntax
x [1], x[0], table[3], point[i]

5. Semantics numbered from 0 to n-1 where n is the number of elements

6. Properties of an array (list)
Heterogeneous (any data type!)
Contiguous
Have random access to any element
Ordered (numbered from 0 to n-1)
Number of elements can change very easily (use method .append)
Python lists are mutable sequences of arbitrary objects

7. Syntax Use [] to give initial value to, like x = [1,3,5]
refer to individual elements
uses [ ] with index in the brackets
most of the time you don’t refer to the whole array as one thing, or just by the array name (one time you can is when passing a whole array to a function as an argument)

8. List Operations you know
Operator
Meaning
<seq> + <seq>
Concatenation
<seq> * <int-expr>
Repetition
<seq>[]
Indexing
len(<seq>)
Length
<seq>[:]
Slicing
for <var> in <seq>:
Iteration
<expr> in <seq>
Membership (Boolean)
Python Programming, 2/e

9. Indexing an Array The index is also called the subscript
In Python, the first array element always has subscript 0, the second array element has subscript 1, etc.
Subscripts can be variables – they have to have integer values
k = 4
items = [3,9,’a’,True, 3.92]
items[k] = 3.92
items[k-2] = items[2] = ‘a’

10. List Operations
Lists are often built up one piece at a time using append. nums = [] x = float(input('Enter a number: ')) while x >= 0: nums.append(x) x = float(input('Enter a number: '))
Here, nums is being used as an accumulator, starting out empty, and each time through the loop a new value is tacked on.
Python Programming, 2/e

11. List Operations Method Meaning <list>.append(x)
Add element x to end of list.
<list>.sort()
Sort (order) the list. A comparison function may be passed as a parameter.
<list>.reverse()
Reverse the list.
<list>.index(x)
Returns index of first occurrence of x.
<list>.insert(i, x)
Insert x into list at index i.
<list>.count(x)
Returns the number of occurrences of x in list.
<list>.remove(x)
Deletes the first occurrence of x in list.
<list>.pop(i)
Deletes the ith element of the list and returns its value.
Python Programming, 2/e

12. Using a variable for the size
It is very common to use a variable to store the size of an array
SIZE = 15
arr = []
for i in range(SIZE):
arr.append(i)
Makes it easy to change if size of array needs to be changed

13. Solution to starting problem
SIZE = 5
n = [0]*SIZE
total = 0
for ct in range(SIZE):
n[ct] = float(input("enter a number “))
total = total + n[ct]
cont'd on next slide

14. Solution to problem - cont'd
average = total / SIZE
for ct in range(5):
if n[ct] > average:
print (n[ct])

15. Scope of counter in a for loop
The counter variable has usual scope (body of the function it’s in)
for i in range(5):
counter does exist after for loop finishes
what‘s its value after the loop?

16. Initialization of arrays
a = [1, 2, 9, 10] # has 4 elements
a = [0] * 5 # all are zero

17. Watch out index out of range!
Subscripts range from 0 to n-1
Interpreter WILL tell you if an index goes out of that range
BUT the negative subscripts work as they do with strings (which are, after all, arrays of characters)
x = [5]*5
x[-1] = 4 # x is [5,5,5,5,4]

18. Assigning Values to Individual Array Elements
temps = [0.0] * 5
m = 4
temps[2] = 98.6;
temps[3] = 101.2;
temps[0] = 99.4;
temps[m] = temps[3] / 2.0;
temps[1] = temps[3] - 1.2;
// What value is assigned? ?
temps[0] temps[1] temps[2] temps[3] temps[4]

19. What values are assigned?
SIZE =5
temps = [0.0]* SIZE
for m in range(SIZE):
temps[m] = m * 0.2
for m in range(SIZE-1, -1, -1):
print(temps[m])
? ? ? ? ?
temps[0] temps[1] temps[2] temps[3] temps[4]

20. Indexes Subscripts can be constants or variables or expressions
If i is 5, a[i-1] refers to a[4] and a[i*2] refers to a[10]
you can use i as a subscript at one point in the program and j as a subscript for the same array later - only the value of the variable matters

21. Variable Subscripts What is temps[m + 1] ? What is temps[m] + 1 ?
What is temps[m + 1] ?
What is temps[m] + 1 ?
temps[0] temps[1] temps[2] temps[3] temps[4]

22. Random access of elements
Problem : read in numbers from a file, only single digits - and count them - report how many of each there were
Use an array as a set of counters
ctr [0] is how many zero's, ctr[1] is how many ones, etc.
ctr[num] +=1 is the crucial statement

23. Parallel arrays
Sometimes you have data of different types that are associated with each other
like name (string) and GPA (float)
You CAN store them in the same array
ar = [“John”, 3.24, “Mary”, 3.9, “Bob”, 2.7]
You can also use two different arrays "side by side"

24. Parallel arrays, cont'd for i in range(SIZE):
name[i], gpa[i] = float(input(“Enter”))
Logically the name in position i corresponds to the gpa in position i
Nothing in the syntax forces this to be true, you just have to program it to be so.

25. Parallel Arrays EXAMPLE
Parallel arrays are two or more arrays that have the same index range and whose elements contain related information, possibly of different data types
EXAMPLE
SIZE = 50
idNumber = [“ “]*SIZE
hourlyWage = [0.0] *SIZE parallel arrays

26. SIZE = 50 idNumber = [“ “] *SIZE // Parallel arrays hold hourlyWage =[0.0] *SIZE // Related information
idNumber[0] hourlyWage[0]
idNumber[1] hourlyWage[1]
idNumber[2] hourlyWage[2]
idNumber[48] hourlyWage[48]
idNumber[49] hourlyWage[49]

27. Selection sort - 1-d array
Algorithm for the sort
1. find the maximum in the list
2. put it in the highest numbered element by swapping it with the data that was at that location
3. repeat 1 and 2 for shorter unsorted list - not including highest numbered location
4. repeat 1-3 until list goes down to one

28. Find the maximum in the list
# n is number of elements
max = a[0] # value of largest element # seen so far
for i in range(1, n): # note start at 1, not 0
if max < a[i]:
max = a[i]
# now max is value of largest element in list

29. Find the location of the max
max = 0 # max is now location of the # largest seen so far
for i in range(1,n):
if a[max] < a[i]:
max = i
# now max is location of the largest in
# array

30. Swap with highest numbered
Remember element at right end of list is
numbered n-1
temp = a[max]
a[max] = a[n-1]
a[n-1] = temp
# there is a shorter way in Python!

31. The Python way!
The previous code of finding the max and its location will work in ANY high-level language.
Python has some nice functions and methods to make it easier!
Let’s try that.

32. The Python Way To find the max of the whole list mx = max(a)
loc = a.index(mx)
Is using index SAFE here? If it doesn’t find mx in a, it will crash!
But you just got mx from the list using the max function, so it IS in the list a.

33. The Python Way The swap then becomes a[loc], a[n-1] = a[n-1],a[loc]
Python “hides” the temporary third variable

34. Find next largest element and swap (generic way)
max = 0;
for i in range(1,n-1): # note n-1, not n
if a[max] < a[i]:
max = i
temp = a[max]
a[max] = a[n-2]
a[n-2] = temp

35. put a loop around the general code to repeat for n-1 passes
for pss in range(n, 1, -1):
max = 0
for i in range(1,pss):
if a[max] <= a[i]:
max = i
temp = a[max]
a[max] = a[pss-1]
a[pss-1] = temp

36. The whole thing the Python way
for pss in range(n, 1, -1): # n-1 passes mx = max(a[0:pss]) loc = a.index(mx) a[loc], a[pss-1] = a[pss-1], a[loc]

37. 2-dimensional arrays
Data sometimes has more structure to it than just "a list"
It has rows and columns
You use two subscripts to locate an item
The first subscript called “row”, second called “column”

38. 2-dimensional arrays syntax a = [[0]*5 for i in range(4)]
# 5 columns, 4 rows
Twenty elements, numbered from [0][0] to [4][3]
a = [[0]*COLS for i in range(ROWS)]
Which has ROWS rows and COLS columns in each row (use of variables to make it easy to change the size of the array without having to edit every line of the program)

39. EXAMPLE -- Array for monthly high temperatures for all 50 states
NUM_STATES = 50
NUM_MONTHS = 12
stateHighs = [[0]*NUM_MONTHS for i in range(NUM_STATES)]
[0]
[1]
[2] .
stateHighs[2][7]
[48]
[49]
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10][11]
row 2,
col 7
might be
Arizona’s
high for
August

40. Processing a 2-d array by rows
finding the total for the first row
for i in range(NUM_MONTHS):
total = total + a[0][i]
finding the total for the second row
total = total + a[1][i]

41. Processing a 2-d array by rows
total for ALL elements by adding first row, then second row, etc.
for i in range(NUM_STATES):
for j in range(NUM_MONTHS):
total = total + a[i][j]

42. Processing a 2-d array by columns
total for ALL elements by adding first column, second column, etc.
for j in range(NUM_MONTHS):
for i in range(NUM_STATES):
total = total + a[i][j]

43. Finding the average high temperature for Arizona
total = 0
for month in range(NUM_MONTHS):
total = total + stateHighs[2][month]
average = round (total / NUM_MONTHS)
average 85

44. Passing an array as an argument
Arrays (lists) are passed by reference = they CAN be changed permanently by the function
Definition def fun1 (arr):
Call the function as
x = fun1 (myarr)

45. Arrays versus Files Arrays are usually smaller than files
Arrays are faster than files
Arrays are temporary, in RAM - files are permanent on secondary storage
Arrays can do random or sequential, files we have seen are only sequential

46. Using Multidimensional Arrays
Example of three-dimensional array

47. NUM_DEPTS = 5 # mens, womens, childrens, electronics, furniture
NUM_MONTHS = 12
NUM_STORES = 3 # White Marsh, Owings Mills, Towson
monthlySales = [[[0]*NUM_MONTHS for i in range(NUM_DEPTS)] for j in range(NUM_STORES)]
monthlySales[3][7][0]
sales for electronics in August at White Marsh
3 STORES
sheets
5 DEPTS
rows
12 MONTHS columns

48. Example of filling a 3-d array
def main(): NUM_DEPTS = 5 # mens, womens, childrens, electronics, furniture NUM_MONTHS = 12 NUM_STORES = 3 # White Marsh, Owings Mills, Towson monthlySales = [[[0]*NUM_MONTHS for i in range(NUM_DEPTS)] for j in range(NUM_STORES)] storeNames = ["White Marsh", "Owings Mills", "Towson"] deptNames = ["mens", "womens", "childrens", "electronics", "furniture"] for store in range(NUM_STORES): print (storeNames[store], end=" ") for dept in range(NUM_DEPTS): print (deptNames[dept], end = " ") for month in range(NUM_MONTHS): print("for month number ", month+1) monthlySales[store][dept] [month] = float(input("Enter the sales ")) print() print (monthlySales)

49. Find the average of monthly_sales
total = 0
for m in range(NUM_MONTHS):
for d in range(NUM_DEPTS):
for s in range(NUM_STORES):
total += monthlySales
[s][d][m]
average = total /
(NUM_MONTHS * NUM_DEPTS * NUM_STORES)

50. Problem: student data in a file
The data is laid out as
Name, section, gpa
John Smith, 15, 3.2
Ralph Johnson, 12, 3.9
Bob Brown, 9, 2.5
Etc.

51. Read in the data inf = open(“students”,”r”) studs = []
for line in inf:
data = line.split(“,”)
studs.append(data)
inf.close()
#studs looks like [[“John Smith”,15,3.2], #[“Ralph Johnson”,12,3.9],[“Bob Brown”…]]

52. Find the student with highest GPA
max = 0 for j in range(1, len(studs)): if studs[max][2] < studs[j][2]: max = j #max is now location of highest gpa studs[max][0] is the name of the student studs[max][1] is the student’s section