Arrays (Lists) in Python
one thing after another
Problem
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Given 5 numbers, read them in and
calculate their average
THEN print out the ones that were
above average
Data Structure Needed
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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)
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You need a way to have many variables
all with the same name but
distinguishable!
In math they do it by subscripts or
indexes
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x1, x2, x3 and so on
In programming languages, hard to use
smaller fonts, so use a different syntax
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x [1], x[0], table[3], point[i]
Semantics
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numbered from 0 to n-1 where n is the
number of elements
0
1
2
3
4
5
Properties of an array (list)
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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
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Use [] to give initial value to, like x =
[1,3,5]
refer to individual elements
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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)
List Operations you know
Operator
<seq> + <seq>
<seq> * <int-expr>
<seq>[]
len(<seq>)
<seq>[:]
for <var> in <seq>:
<expr> in <seq>
Meaning
Concatenation
Repetition
Indexing
Length
Slicing
Iteration
Membership (Boolean)
Python Programming, 2/e
8
Indexing an Array
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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’
9
List Operations
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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
10
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
11
Using a variable for the size
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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)
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Makes it easy to change if size of array
needs to be changed
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
Solution to problem - cont'd
average = total / SIZE
for ct in range(5):
if n[ct] > average:
print (n[ct])
Scope of counter in a for loop
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The counter variable has usual scope
(body of the function it’s in)
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for i in range(5):
counter does exist after for loop finishes
what‘s its value after the loop?
Initialization of arrays
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a = [1, 2, 9, 10] # has 4 elements
a = [0] * 5 # all are zero
Watch out index out of range!
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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]
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?
7000
99.4
temps[0]
7004
7008
?
98.6
temps[1]
temps[2]
7012
101.2
temps[3]
7016
50.6
temps[4]
18
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
?
?
?
?
?
temps[0]
temps[1]
temps[2]
temps[3]
temps[4]
19
Indexes
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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
Variable Subscripts
temps = [0.0]*5
m=3
. . . . . .
What is temps[m + 1] ?
What is temps[m] + 1 ?
7000
100.0
temps[0]
7004
7008
7012
7016
100.2
100.4
100.6
100.8
temps[1]
temps[2]
temps[3]
temps[4]
21
Random access of elements
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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
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ctr [0] is how many zero's, ctr[1] is how
many ones, etc.
ctr[num] +=1 is the crucial
statement
Parallel arrays
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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
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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 i
 Nothing in the syntax forces this to be
true, you just have to program it to be
so.
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
25
SIZE = 50
idNumber = [“ “] *SIZE
hourlyWage =[0.0] *SIZE
// Parallel arrays hold
// Related information
idNumber[0]
4562
hourlyWage[0]
9.68
idNumber[1]
1235
hourlyWage[1]
45.75
idNumber[2]
6278
hourlyWage[2]
12.71
.
.
.
.
.
.
.
.
.
.
.
.
idNumber[48]
8754
hourlyWage[48]
67.96
idNumber[49]
2460
hourlyWage[49]
8.97
26
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
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
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!
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The previous code of finding the max
and its location will work in ANY highlevel 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.
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The Python Way
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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
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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
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syntax
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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)]
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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)
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] [3] [4] [5] [6] [7] [8] [9] [10][11]
[0]
[1]
[2]
row 2,
.
col 7
.
might be
.
Arizona’s
high for [48]
August [49]
66 64 72 78 85 90 99 105 98 90 88 80
stateHighs[2][7]
39
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
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]
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
43
Passing an array as an
argument
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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
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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
Using Multidimensional Arrays
Example of three-dimensional array
46
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)]
5 DEPTS
rows
monthlySales[3][7][0]
sales for electronics in August at White Marsh
12 MONTHS columns
47
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()
print (monthlySales)
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)
Problem: student data in a file
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The data is laid out as
Name, section, gpa
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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 = 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