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Chapter 5 Decision Control Instructions

CHAPTER 5

Decision Control Instructions

Keynote

5.1 What are Decision Control Instructions?

So far statements in all our programs got executed sequentially or one after the other. Sequence of execution of instructions in a program can be altered using: - Decision control instruction - Repetition control instruction

Decision Control Instruction

Three ways for taking decisions in a program:

if condition

if condition
if condition :
  statement1 
  statement2

if condition : else

if condition : else
if condition :
  statement1 
  statement2
else :
  statement3
  statement4

if condition1 : elif condition2 : elif condition3 : else :

if condition1 : elif condition2 : elif condition3 : else :
if condition1 :
  statement1 
  statement2
elif condition2 :
  statement3
elif condition3 :
  statement4
else : 
  statement5
  • The colon (:) after if, else, elif is compulsory.
  • Statements in if block, else, block, elif block have to be indented. Indented statements are treated as a block of statements.
  • Indentation is used to group statements. Use either 4 spaces or a tab for indentation. Don't mix tabs and spaces. They may appear ok on screen, but would be reported as error.
  • In the first form shown above else and elif are optional.
  • In the second form shown above, if condition is True all statements in if block get executed. If condition is False then statements in else block get executed.
  • In the third form shown above, if a condition fails, then condition in the following elif block is checked. The else block goes to work if all conditions fail.
  • if-else statements can be nested. Nesting can be as deep as the program logic demands.

5.2 Nuances of Conditions

Condition is built using relation operators <, >, <=, >=, ==, !=.

Condition is built using relation operators
10 < 20 # yields True
'Santosh' < 'Adi' # yields False, alphabetical order is checked
'gang' < 'God' # yields False, lowercase is > uppercase
a = b is assignment, a == b is comparison.

Ranges or multiple equalities can be checked more naturally.

Ranges or multiple equalities can be checked more naturally
if a < b < c # checks whether b falls between a and c
if a == b == c # checks whether all three are equal
if 10 != 20 != 10 # evaluates to True, even though 10 != 10 is False
Any non-zero number (positive, negative, integer, float) is treated as True, and 0 as False.
Any non-zero number (positive, negative, integer, float) is treated as True, and 0 as False
print(bool(3.14)) # prints True
print(bool(25)) # prints True
print(bool(0)) # prints False

5.3 Logical Operators

More complex decision making can be done using logical operators and, or and not. Conditions can be combined using and and or as shown below:

Conditions can be combined using and and or
cond1 and cond2 - returns True if both are True, otherwise False
cond1 or cond2 - returns True if one of them is True, otherwise False

Strictly speaking, we need not always combine only conditions with and/or. We can use any valid expression in place of conditions. Hence when used with expressions we may not get True/False.

and operator evaluates ALL expressions. It returns last expression if all expressions evaluate to True. Otherwise it returns first value that evaluates to False. 

a = 40 
b = 30
x = 75 and a >= 20 and b < 60 and 35 # assigns 35 to x
y = -30 and a >= 20 and b < 15 and 35 # assigns False to y
z = -30 and a >= 20 and 0 and 35 # assigns 0 to z

or operator evaluates ALL expressions and returns the first value that evaluates to True. Otherwise it returns last value that evaluates to False. 

a = 40 
b = 30
x = 75 or a >= 20 or 60 # assigns 75 to x 
y = a >= 20 or 75 or 60 # assigns True to y
z = a < 20 or 0 or 35 # assigns 35 to z

not Condition’s result can be negated using not. 

a = 10
b = 20
not (a <= b) # yields False. Same as a > b
not (a >= b) # yields True. Same as a < b
Shortcut for toggling values between 1 and 0: 
a = input('Enter 0 or 1')
a = not a # set a to 0 if it is 1, and set it to 1 if it is 0
a = not b does not change value of b.

If an operator needs only 1 operand it is known as Unary operator. If it needs two, then it is a binary operator.

  • not - needs only 1 operand, so unary operator
  • +, -, <, >, and, or, etc. - need 2 operands, so binary operators

5.4 Conditional Expressions

Python supports one additional decision-making entity called a conditional expression. <expr1> if <conditional expression> else <expr2> <conditional expression> is evaluated first. If it is true, the expression evaluates to <expr1>. If it is false, the expression evaluates to <expr2>.

Examples of condition expressions: 

age = 15
status = 'minor' if age < 18 else 'adult' # sets minor
sunny = False
print('Let's go to the', 'beach' if sunny else 'room')
humidity = 76.8
setting = 25 if humidity > 75 else 28 # sets 25
Conditional expressions can be nested. 
# assigns Prim
wt = 55
msg = 'Obese' if wt > 85 else 'Hefty' if wt > 60 else 'Prim'

5.5 all( ) and any( )

Instead of using the and and or logical operators, we can use the built-in functions all( ) and any( ) to get the same effect. Their usage is shown in the following program: 

a, b, c = 10, 20, 30
res = all((a > 5, b > 20, c > 15))
print(res) # prints False, as second condition is False
res = any((a > 5, b > 20, c > 15))
print(res) # prints True since one of the condition is True
Note that all( ) and any( ) both receive a single parameter of the type string, list, tuple, set or dictionary. We have passed a tuple of 3 conditions to them. If argument is a dictionary it is checked whether the keys are true or not. any( ) function returns True if at least one element of its parameter is True. all( ) function returns True if all elements of its parameter are True.

5.6 Receiving Input

The way print( ) function is used to output values on screen, input( ) built-in function can be used to receive input values from keyboard. input( ) function returns a string, i.e. if 23 is entered it returns '23'. So if we wish to perform arithmetic on the number entered, we need to convert the string into int or float as shown below. 

n = input('Enter your name: ')
age = int(input('Enter your age: '))
salary = float(input('Enter your salary: '))
print(name, age, salary)

5.7 Pass Statement

  • pass statement is intended to do nothing on execution. Hence it is often called a no-op instruction.
  • If we wish that on execution of a statement nothing should happen, we can achieve this using a pass statement. Its utility is shown in Problem 5.6. It is often used as a placeholder for unimplemented code in an if, loop, function or class. This is not a good use of pass. Instead you should use ... in its place.
  • If you use pass it might make one believe that you actually do not intend to do anything in the if/loop/function/class.

5.7.1 Ellipsis

Ellipsis is a built-in constant in Python. It is denoted by .... Ellipsis is used to represent an unknown value. It is often used as a placeholder for unimplemented code in an if, loop, function or class. Ellipsis is not the same as pass. pass is a statement, while ... is a constant. Ellipsis is often used as a placeholder for unimplemented code in an if, loop, function or class.

Problems

Problem 5.1

While purchasing certain items, a discount of 10% is offered if the quantity purchased is more than 1000. If quantity and price per item are input through the keyboard, write a program to calculate the total expenses.

Program 
qty = int(input('Enter value of quantity: ')) 
price = float(input('Enter value of price: ')) 
if qty > 1000 : 
dis = 10 
else : 
dis = 0 
totexp = qty * price - qty * price * dis / 100 
print('Total expenses = Rs. ' + str(totexp))
Output 
Enter value of quantity: 1200
Enter value of price: 15.50
Total expenses = Rs. 16740.0

Tips

  • input( ) returns a string, so it is necessary to convert it into int or float suitably. If we do not do the conversion, qty > 1000 will throw an error as a string cannot be compared with an int.
  • str( ) should be used to convert totexp to string before doing concatenation using +.

Problem 5.2

In a company an employee is paid as under: If his basic salary is less than Rs. 1500, then HRA = 10%of basic salary and DA = 90% of basic salary. If his salary is either equal to or above Rs. 1500, then HRA = Rs. 500 and DA = 98% of basic salary. If the employee's salary is input through the keyboard write a program to find his gross salary.

Program 
bs = int(input('Enter value of bs: '))
if bs > 1500 : 
hra = bs * 15 /100 
da = bs * 95 / 100 
ca = bs * 10 / 100 
else: 
hra = bs * 10 / 100 
da = bs * 90 / 100 
ca = bs * 5 / 100 
gs = bs + da + hra + ca 
print('Gross Salary = Rs. ' + str(gs))

Tips

  • if block and else block can contain multiple statements in them, suitably indented.

Problem 5.3

Percentage marks obtained by a student are input through the keyboard. The student gets a division as per the following rules: 

Percentage above or equal to 60 - First division
Percentage between 50 and 59 - Second division
Percentage between 40 and 49 - Third division
Percentage less than 40 - Fail
Write a program to calculate the division obtained by the student.

Program 
per = int(input('Enter value of percentage: '))
if per >= 60 : 
print('First Division')
elif per >= 50 : 
print('Second Division')
elif per >= 40 : 
print('Third Division')
else : 
print('Fail')
Output 
Enter value of percentage: 55
Second Division

Problem 5.4

A company insures its drivers in the following cases: - If the driver is married. - If the driver is unmarried, male & above 30 years of age. - If the driver is unmarried, female & above 25 years of age. In all other cases, the driver is not insured. If the marital status, sex and age of the driver are the inputs, write a program to determine whether the driver should be insured or not.

Program 
ms = input('Enter marital status: ')
s = input('Enter sex: ') 
age = int(input('Enter age: ')) 
if ( ms == 'm' ) or ( ms == 'u' and s == 'm' and age > 30 ) \
or ( ms == 'u' and s == 'f' and age > 25 ) : 
print('Insured')
else : 
print('Not Insured')
Output 
Enter marital status: u
Enter sex: m
Enter age: 23
Not Insured

Problem 5.5

Suppose there are four flag variables w, x, y, z. Write a program to check in multiple ways whether one of them is true.

Program 
# Different ways to test multiple flags
w, x, y, z = 0, 1, 0, 1

if w == 1 or x == 1 or y == 1 or z == 1 :
print('True')

if w or x or y or z :
print('True')

if any((w, x, y, z)):
print('True')

if 1 in (w, x, y, z) :
print('True')
Output 
True
True
True
True

Tips

  • any( ) is a built-in function that returns True if at least one of the element of its parameter is True.
  • We have to pass a string, list, tuple, set or dictionary to any( ).
  • There is another similar function called all( ), which returns True if all elements of its parameter are True. This function too should be passed a string, list, tuple, set or dictionary.

Problem 5.6

Given a number n we wish to do the following: 

If n is positive - print n * n, set a flag to true
If n is negative - print n * n * n, set a flag to true
if n is 0 - do nothing
Is the code given below correct for this logic? 
n = int(input('Enter a number: '))
if n > 0 :
    flag = True
    print(n * n)
elif n < 0 :
    flag = True
    print(n * n * n)

Answer This is misleading code. At a later date, anybody looking at this code may feel that flag = True should be written outside if and else. Better code will be as follows: 
n = int(input('Enter a number: '))
if n > 0 :
    flag = True
    print(n * n)
elif n < 0 :
    flag = True
    print(n * n * n)
else :
    pass # does nothing on execution

Exercise

[A] Answer the following questions:

a. Write conditional expressions for 

- If a < 10 b = 20, else b = 30
- Print 'Morning' if time < 12, otherwise print 'Afternoon'
- If marks >= 70, set remarks to True, otherwise False
b. Rewrite the following code snippet in 1 line: 
x = 3 
y = 3.0 
if x == y :
    print('x and y are equal') 
else :  
    print('x and y are not equal')
c. What happens when a pass statement is executed?

[B] What will be the output of the following programs:

a. 

i, j, k = 4, -1, 0
w = i or j or k 
x = i and j and k 
y = i or j and k 
z = i and j or k 
print(w, x, y, z)
b. 
a = 10
a = not not a
print(a)

c. 

x, y, z = 20, 40, 45 
if x > y and x > z :
print('biggest = ' + str(x))
elif y > x and y > z :
print('biggest = ' + str(y))
elif z > x and z > y :
print('biggest = ' + str(z))

d. 

num = 30 
k = 100 if num <= 10 else 500
print(k)
e. 
a = 10
b = 60
if a and b > 20 :
    print('Hello')
else :
    print('Hi')
f. 
a = 10
b = 60
if a > 20 and b > 20 :
    print('Hello')
else :
    print('Hi')
g. 
a = 10
if a = 30 or 40 or 60 :
    print('Hello')
else :
    print('Hi')
h. 
a = 10
if a = 30 or a == 40 or a == 60 :
    print('Hello')
else :
    print('Hi')
i. 
a = 10
if a in (30, 40, 50) :
    print('Hello')
else :
    print('Hi')

[C] Point out the errors, if any, in the following programs:

a. 

a = 12.25
b = 12.52
if a = b :
    print('a and b are equal')
b. 
if ord('X') < ord('x')
    print('Unicode value of X is smaller than that of x')
c. 
x = 10 
if x >= 2 then
    print('x')
d. 
x = 10 ; y = 15
if x % 2 = y % 3
    print('Carpathians\n')
e. 
x, y = 30, 40
if x == y :
    print('x is equal to y')
elseif x > y :
    print('x is greater than y')
elseif x < y :
    print('x is less than y')

[D] If a = 10, b = 12, c = 0, find the values of the following expressions:

a != 6 and b > 5 
a == 9 or b < 3
not ( a < 10 )
not ( a > 5 and c ) 
5 and c != 8 or !c

[E] Attempt the following questions:

  • a. Any integer is input through the keyboard. Write a program to find out whether it is an odd number or even number.
  • b. Any year is input through the keyboard. Write a program to determine whether the year is a leap year or not.
  • c. If ages of Ram, Shyam and Ajay are input through the keyboard, write a program to determine the youngest of the three.
  • d. Write a program to check whether a triangle is valid or not, when the three angles of the triangle are entered through the keyboard. A triangle is valid if the sum of all the three angles is equal to 180 degrees.
  • e. Write a program to find the absolute value of a number entered through the keyboard.
  • f. Given the length and breadth of a rectangle, write a program to find whether the area of the rectangle is greater than its perimeter. For example, the area of the rectangle with length = 5 and breadth = 4 is greater than its perimeter.
  • g. Given three points (x1, y1), (x2, y2) and (x3, y3), write a program to check if all the three points fall on one straight line.
  • h. Given the coordinates (x, y) of center of a circle and its radius, write a program that will determine whether a point lies inside the circle, on the circle or outside the circle. (Hint: Use sqrt( ) and pow( ) functions)
  • i. Given a point (x, y), write a program to find out if it lies on the X-axis, Y-axis or on the origin.
  • j. A year is entered through the keyboard, write a program to determine whether the year is leap or not. Use the logical operators and and or.
  • k. If the three sides of a triangle are entered through the keyboard, write a program to check whether the triangle is valid or not. The triangle is valid if the sum of two sides is greater than the largest of the three sides.
  • l. If the three sides of a triangle are entered through the keyboard, write a program to check whether the triangle is isosceles, equilateral, scalene or right angled triangle.

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