Chapter 6: Arrays

Example 6.1, page no. 126

In [1]:
a = [0, 0, 0]
a[2] = 55.55
a[0] = 11.11
a[1] = 33.33
print "a[0] = " , a[0] 
print "a[1] = " , a[1] 
print "a[2] = " , a[2] 

a[0] =  11.11
a[1] =  33.33
a[2] =  55.55

Example 6.2, page no. 127

In [1]:
SIZE=5 # defines the size N for 5 elements
a = []
# declares the array's elements as type double
print "Enter " , SIZE , " numbers:\t"
for i in range(SIZE):
    a.append(float(raw_input()))
    
print "In reverse order: "
for i in range(SIZE-1,-1,-1):
    print "\t" , a[i]

Enter  5  numbers:	
11.11
33.33
55.55
77.77
99.99
In reverse order: 
	99.99
	77.77
	55.55
	33.33
	11.11

Example 6.3, page no. 128

In [3]:
a = [ 22.2, 44.4, 66.6 ]

size = len(a)
for i in range(size):
    print "\ta[" , i , "] = " , a[i]

	a[ 0 ] =  22.2
	a[ 1 ] =  44.4
	a[ 2 ] =  66.6

Example 6.4, page no. 128

In [4]:
a = [ 22.2, 44.4, 66.6 , 0 ,0,0,0]
size = len(a)
for i in range(size):
    print "\ta[" , i , "] = " , a[i] 

	a[ 0 ] =  22.2
	a[ 1 ] =  44.4
	a[ 2 ] =  66.6
	a[ 3 ] =  0
	a[ 4 ] =  0
	a[ 5 ] =  0
	a[ 6 ] =  0

Example 6.5, page no. 129

In [5]:
import numpy
SIZE = 4
a = numpy.zeros(4)
# declares the array's elements as type float
for i in range(SIZE):
    print "\ta[" , i , "] = " , a[i]

	a[ 0 ] =  0.0
	a[ 1 ] =  0.0
	a[ 2 ] =  0.0
	a[ 3 ] =  0.0

Example 6.6, page no. 129

In [1]:
SIZE=4
a = [ 33.3, 44.4, 55.5, 66.6 ]
for i in range(7): # ERROR: index is out of bounds!
    print "\ta[" , i , "] = " , a[i] 

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-1-b39022f1d3ab> in <module>()
      2 a = [ 33.3, 44.4, 55.5, 66.6 ]
      3 for i in range(7): # ERROR: index is out of bounds!
----> 4     print "\ta[" , i , "] = " , a[i]
      5 

IndexError: list index out of range
	a[ 0 ] =  33.3
	a[ 1 ] =  44.4
	a[ 2 ] =  55.5
	a[ 3 ] =  66.6
	a[ 4 ] =

Example 6.7, page no. 130

In [7]:
a = [ 22.2, 44.4, 66.6 ]
x=11.1
print "x = " , x 
a.append(88.8) # ERROR: index is out of bounds!
print "x = " , x 

 x =  11.1
x =  11.1

Example 6.8, page no. 130

In [8]:
a = [ 22.2, 44.4, 66.6 ]
x=11.1
print "x = " , x 
a[3333] = 88.8 # ERROR: index is out of bounds!
print "x = " , x 

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-8-63b6c8e60294> in <module>()
      6 x=11.1
      7 print "x = " , x
----> 8 a[3333] = 88.8 # ERROR: index is out of bounds!
      9 print "x = " , x

IndexError: list assignment index out of range
x =  11.1

Example 6.9, page no. 131

In [10]:
def sum_(a):
    s = 0
    for i in a:
        s += i
    return s
    
a = [ 11, 33, 55, 77 ]
print "sum(a) = " , sum_(a) 

sum(a) =  176

Example 6.10, page no. 132

In [12]:
def read(a):
    print "Enter integers. Terminate with 0:\n"
    n = 1
    while True:
        n = int(raw_input("a[" + str(len(a)) + "]: "))
        if n == 0:
            break
        a.append(n)
        

def print_(a):
    for i in a:
        print i ,


a = []
read(a)
print "The array has " , len(a) , " elements: "
print_(a)

Enter integers. Terminate with 0:

a[0]: 11
a[1]: 22
a[2]: 33
a[3]: 44
a[4]: 0
The array has  4  elements: 
11 22 33 44

Example 6.11, page no. 133

In [13]:
import sys
a = [ 22, 44, 66, 88 ]
print "a = " , id(a)  # the address of a[0]

a =  169156908

Example 6.12, page no. 133

In [14]:
def index(x,a,n):
    for i in range(len(a)):
        if (a[i] == x):
            return i
    return n # x not found

a = [ 22, 44, 66, 88, 44, 66, 55 ]
print "index(44,a,7) = " , index(44,a,7)
print "index(50,a,7) = " , index(50,a,7) 

index(44,a,7) =  1
index(50,a,7) =  7

Example 6.13, page no. 134

In [15]:
def sort(a,n):
    # bubble sort:
    n = len(a)
    for i in range(n):
        # bubble up max{a[0..n-i]}:
        for j in range(n-i-1):
            if (a[j] > a[j+1]):
                a[j],a[j+1] = a[j+1],a[j]

def print_(a):
    for i in range(len(a)):
        print a[i],
    print ''
    
a = [55.5, 22.5, 99.9, 66.6, 44.4, 88.8, 33.3, 77.7]

print_(a)
sort(a,8)
print_(a)

55.5 22.5 99.9 66.6 44.4 88.8 33.3 77.7 
22.5 33.3 44.4 55.5 66.6 77.7 88.8 99.9

Example 6.14, page no. 135

In [16]:
def index(x,a,n):
    # PRECONDITION: a[0] <= a[1] <= ... <= a[n-1];
    # binary search:
    lo=0
    hi=n-1
    while (lo <= hi):
        i = (lo + hi)/2 # the average of lo and hi
        if (a[i] == x):
            return i
        if (a[i] < x):
            lo = i+1 # continue search in a[i+1..hi]
        else:
            hi = i-1 # continue search in a[lo..i-1]
    return n # x was not found in a[0..n-1]

a = [ 22, 33, 44, 55, 66, 77, 88 ]
print "index(44,a,7) = " , index(44,a,7)
print "index(60,a,7) = " , index(60,a,7) 

index(44,a,7) =  2
index(60,a,7) =  7

Example 6.15, page no. 136

In [17]:
def isNondecreasing(a,n):
    # returns true iff a[0] <= a[1] <= ... <= a[n-1]:
    for i in range(1,n):
        if (a[i]<a[i-1]):
            return False
    return True

a = [ 22, 44, 66, 88, 44, 66, 55 ]
print "isNondecreasing(a,4) = " , isNondecreasing(a,4)
print "isNondecreasing(a,7) = " , isNondecreasing(a,7)

isNondecreasing(a,4) =  True
isNondecreasing(a,7) =  False

Example 6.16, page no. 137

In [5]:
def isNondecreasing(a,n):
    # returns true iff a[0] <= a[1] <= ... <= a[n-1]:
    for i in range(1,n):
        if (a[i]<a[i-1]):
            return False
    return True


def index(x,a,n):
    # PRECONDITION: a[0] <= a[1] <= ... <= a[n-1];
    # binary search:
    assert(isNondecreasing(a,n))      
        
    lo=0
    hi=n-1
    while (lo <= hi):
        i = (lo + hi)/2
        if (a[i] == x):
            return i
        if (a[i] < x):
            lo = i+1  # continue search in a[i+1..hi]
        else:
            hi = i-1  # continue search in a[lo..i-1]
    return n  # x was not found in a[0..n-1]

a = [ 22, 33, 44, 55, 66, 77, 88, 60 ]
print "index(44,a,7) = " , index(44,a,7) 
print "index(60,a,7) = " , index(60,a,7)

 index(44,a,7) =  2
index(60,a,8) =  7

Example 6.17, page no. 137

In [19]:
Day =  [ 0, 1, 2, 3, 4, 5, 6 ]
high = [ 88.3, 95.0, 91.2, 89.9, 91.4, 92.5, 86.7]

for i in Day:
    print "The high temperature for day " , i , " was " , high[i] 

 The high temperature for day  0  was  88.3
The high temperature for day  1  was  95.0
The high temperature for day  2  was  91.2
The high temperature for day  3  was  89.9
The high temperature for day  4  was  91.4
The high temperature for day  5  was  92.5
The high temperature for day  6  was  86.7

Example 6.18, page no. 139

In [20]:
def sort(a,n):
    a.sort()

def print_(a,n):
    for i in a:
        print i,
    print ''
a = [55.5, 22.5, 99.9, 66.6, 44.4, 88.8, 33.3, 77.7]
print_(a,8);
sort(a,8)
print_(a,8)

55.5 22.5 99.9 66.6 44.4 88.8 33.3 77.7 
22.5 33.3 44.4 55.5 66.6 77.7 88.8 99.9

Example 6.19, page no. 139

In [21]:
def read(a):
    print "Enter 15 integers, 5 per row:\n"
    for  i in range(3):
        ar = []
        print "Row " , i , ": ",
        for j in range(5):
            ar.append(int(raw_input()))
        a.append(ar)

def print_(a):
    for  i in range(3):
        for j in range(5):
            print a[i][j],
        print ''
a = []
read(a)
print_(a)

Enter 15 integers, 5 per row:

Row  0 : 44
77
33
11
44
 Row  1 : 60
50
30
90
70
 Row  2 : 85
25
45
45
55
 44 77 33 11 44 
60 50 30 90 70 
85 25 45 45 55

Example 6.20, page no. 140

In [24]:
def read(score):
    for s in range(3):
        print  "Student " , s , ": ",
        st = []
        for q in range(5):
            st.append(int(raw_input()))
        score.append(st)

def printQuizAverages(score):
    for s in range(3):
        sm = 0
        for q in range(5):
            sm += score[s][q]
        print "\tStudent " ,  s , ": " , sm/5.0

def printClassAverages(score):
    for q in range(5):
        sm = 0
        for s in range(3):
            sm += score[s][q]
        print "\tQuiz " , q , ": " , sm/3.0



NUM_STUDENTS = 3
NUM_QUIZZES = 5


score = []
print "Enter " , NUM_QUIZZES , " scores for each student: "
read(score)
print "The quiz averages are:"
printQuizAverages(score)
print "The class averages are: "
printClassAverages(score)

Enter  5  scores for each student: 
Student  0 : 8
7
9
8
9
 Student  1 : 9
9
9
9
8
 Student  2 : 5
6
7
8
9
 The quiz averages are:
	Student  0 :  8.2
	Student  1 :  8.8
	Student  2 :  7.0
The class averages are: 
	Quiz  0 :  7.33333333333
	Quiz  1 :  7.33333333333
	Quiz  2 :  8.33333333333
	Quiz  3 :  8.33333333333
	Quiz  4 :  8.66666666667

Example 6.21, page no. 141

In [25]:
def numZeros(a,n1,n2,n3):
    count = 0
    for i in range(n1):
        for j in range(n2):
            for k in range(n3):
                if (a[i][j][k] == 0):
                    count += 1
    return count


a = [ [ [5,0,2], [0,0,9], [4,1,0], [7,7,7] ],[ [3,0,0], [8,5,0], [0,0,0], [2,0,9] ]]
print "This array has " , numZeros(a,2,4,3) , " zeros"

This array has  11  zeros