week 5 - Principles of programming

week 5

Control flows
- 1. Selection (Conditional/If-Else)
- 1.1 If
- 1.2 If-else
- 1.3 If - elif(else +if) - else
- 1.4 Nested if

Control flows

control flows:

  • selection(if-else
  • repetition(loop)
  • Subprogram

1. Selection (Conditional/If-Else)

Controls which code executes based on conditions.

In flow charts, diamonds are used to represent a true/false condition.

### 1.1 If #### Syntax: ```Python if expression: statement statement ... ```
  • After the if expression, put a : (colon)
  • Each line inside the if-body is indented by the same amount
  • if body is executed only if the expression is True.
    • zero --> false
    • non-zero values --> True

Example:

amount = ... # 1000 , 500

if amount > 1000:
	print('You have a discount')
print(amount)
  • if-block: only executed if amount is greater than 1000
  • print(amount): outside the scope of the if block, so if block will end there

1.2 If-else

Syntax:

if expression:
	statement(s)
else:
	statement (s)
  • After the if expression and else, put a : (colon)
  • Each line inside the if-body and else-body is indented by the same amount
  • Only either if-body or else-body gets executed, not both

Examples:

amount = ... # 1000 , 500

if amount > 1000:
	discount = amount*0.10
else:
	disount = amount*0.05
print(discount)
  • if-block: only executed if amount is greater than 1000
  • else-block: only executed if amount is less than or equal to 1000 
name = ... # 'Perera'
gender = ... # 'male' , 'female'

if gender == 'male':
	print('Mr. ' + name) # string concatenation 
else:
	print('Ms. ' + name) # string concatenation

1.3 If - elif(else +if) - else

Syntax:

if expression_1:
	statement(s)
elif expression_2:
	statement (s)
elif expression_3:
	statement (s)

...
else:
	statement (s)
  • can use any number of elifs but at last there must be a else
  • conditions are executed from top to bottom, until one condition is satisfied.

Example:

amount = ... # 1000 , 500 , 2200

if amount > 2000:
	discount = amount*0.20
elif amount > 1000:
	discount = amount*0.10
else:
	disount = amount*0.05
print(discount)
  • if-block: only executed if amount is greater than 2000
  • elif-block: only executed if amount is greater than 1000 and less than or equal to 2000
  • else-block: only executed if amount is less than or equal to 1000

1.4 Nested if

if structure inside another if structure

Syntax:

if expression_1:
	statement(s)
	if expression_2: # start of inner if-elif-else
		statement (s)
	elif expression_3:
		statement (s)
	else:
		statement (s) # end of inner if-elif-else
else:
	statement (s)
  • Each line inside the if-body is indented by the same amount

Example:

gender = ... # 'male' , 'female'
name = ... # 'Perera'
married = ... # True , False

if gender == 'female':
	if married == True:
		print('Mrs. ' + name)
	else:
		print('Miss ' + name)
else:
	print('Mr. ' + name)
  • Outer if-block: executed if gender is 'female'
  • Inner if-block: executed if gender is 'female' and married is True
  • Inner else-block: executed if gender is 'female' and married is False
  • Outer else-block: executed if gender is not 'female'

Same example without nested if 

gender = ... # 'male' , 'female'
name = ... # 'Perera'
married = ... # True , False

if gender == 'female' and married == True:
	print('Mrs. ' + name)
elif gender == 'female' and married == False:
	print('Miss. ' + name)
else:
	print('Mr. ' + name)