Arrays (Lists) in Python one thing after another.

Arrays (Lists) in Python

one thing after another

Problem

Given 5 numbers, read them in and calculate their average

THEN print out the ones that were above average

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

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]

Semantics

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

0 1 2 3 4 5

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

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)

Python Programming, 2/e 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)

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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’


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.


List OperationsMethod 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.

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

Solution to starting problem

SIZE = 5n = [0]*SIZEtotal = 0for ct in range(SIZE):

n[ct] = float(input("enter a number “))total = total + n[ct]

cont'd on next slide

Solution to problem - cont'd

average = total / SIZEfor ct in range(5):

if n[ct] > average: print (n[ct])

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?

Initialization of arrays

a = [1, 2, 9, 10] # has 4 elements a = [0] * 5 # all are zero

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]

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Assigning Values to Individual Array Elements

temps = [0.0] * 5m = 4temps[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]

7000 7004 7008 7012 7016

99.4 ? 98.6 101.2 50.6

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What values are assigned?SIZE =5

temps = [0.0]* SIZE for m in range(SIZE):

temps[m] = 100.0 + m * 0.2

for m in range(SIZE-1, -1, -1):print(temps[m])


7000 7004 7008 7012 7016

? ? ? ? ?

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

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Variable Subscriptstemps = [0.0]*5

m = 3 . . . . . .

What is temps[m + 1] ?

What is temps[m] + 1 ?


7000 7004 7008 7012 7016

100.0 100.2 100.4 100.6 100.8

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

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"

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 iNothing in the syntax forces this to be true, you just have to program it to be so.

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Parallel Arrays

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

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SIZE = 50

idNumber = [“ “] *SIZE // Parallel arrays hold

hourlyWage =[0.0] *SIZE // Related information

idNumber[0] 4562 hourlyWage[0] 9.68



. . . . . . . . . . . .



Selection sort - 1-d array

Algorithm for the sort1. find the maximum in the list2. 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

Find the maximum in the list

# n is number of elementsmax = a[0] # value of largest element

# seen so farfor 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

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

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!

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.

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.

The Python Way

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

variable

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

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

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]

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”

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)

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[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10][11]

66 64 72 78 85 90 99 105 98 90 88 80row 2,col 7might beArizona’shigh forAugust

EXAMPLE -- Array for monthly high temperatures for all 50 states

NUM_STATES = 50NUM_MONTHS = 12stateHighs = [[0]*NUM_MONTHS for i in range(NUM_STATES)]

[0] [1] [2] . .

stateHighs[2][7] . [48] [49]

Processing a 2-d array by rows

finding the total for the first rowfor i in range(NUM_MONTHS):

total = total + a[0][i]finding the total for the second row

for i in range(NUM_MONTHS):total = total + a[1][i]

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]

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]

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Finding the average high temperature for Arizona

total = 0for month in range(NUM_MONTHS): total = total + stateHighs[2][month]average = round (total / NUM_MONTHS)

average

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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)

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

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Using Multidimensional Arrays

Example of three-dimensional array

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NUM_DEPTS = 5 # mens, womens, childrens, electronics, furniture NUM_MONTHS = 12 NUM_STORES = 3 # White Marsh, Owings Mills, TowsonmonthlySales = [[[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

12 MONTHS columns

5 D

EP

TS

ro

ws

3 STORES

sheets

Example of filling a 3-d arraydef 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() print (monthlySales)

Find the average of monthly_sales

total = 0for 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)

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.

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”…]]

Find the student with highest GPA

max = 0for j in range(1, len(studs)):

if studs[max][2] < studs[j][2]:max = j

#max is now location of highest gpastuds[max][0] is the name of the studentstuds[max][1] is the student’s section

Arrays (Lists) in Python one thing after another.

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