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Python Control Structures – 4th Day of Python Series

Hi friends, Today we will discuss about the python control structures.

In programming , typically the instructions are performed one by one or line by line. But there may be situations when all the statements in a program are not performed. Parts of the program which change the flow of instructions or in other word, change the flow of control are called as control structures.

python control structures
python control structure
control structure

Selection Control Structure in Python

control statements in python

Selection control is used for Decision making, branching between two or more paths.

Basically, selection control structure is used for braking the sequential flow.

Examples:

  • If – else
  • if – else and else if
  • Switch case(Not supported)

Python If – Else

if else statement
If-else control structure
if else in python
psuedo code for if – else

If statement

In Python, if statement evaluates the test expression inside the parenthesis. If the test expression is evaluated to true, then the statements inside the body of if is executed. If the test expression is evaluated to false then the statements inside the body of if is skipped and the control goes to else or elif block.

Else statement

If the If condition fails then the control goes to else statement and the body inside else indentation is executed.

if expression:
   statements
else:
   statements

If – else example:

x=10
y=20
if x > y :
  print(" X is bigger ")
else :
  print(" Y is bigger ")

output

Y is bigger

If with elif

if expression:
  statements
elif expression:
  statements
else:
  statements

if – else if example:

x=500
if x > 500 :
  print(" X is greater than 500 ")
elif x < 500 :
  print(" X is less than 500 ")
elif x == 500 :
  print(" X is 500 ")
else :
  print(" X is not a number ")

output


X is 500

Guess the output.

luggage_weight=30
weight_limit=30  #Weight limit for the airline
extra_luggage_charge=0
if(luggage_weight>0 and luggage_weight<=weight_limit):
    print("Check-in cleared")
elif(luggage_weight<=(weight_limit+10)):
    extra_luggage_charge=300*(luggage_weight-weight_limit)
else:
    extra_luggage_charge=500*(luggage_weight-weight_limit)

if(extra_luggage_charge>0):
    print("Extra luggage charge is Rs.", extra_luggage_charge)
    print("Please make the payment to clear check-in")

Nested if-else

In some conditions we have to place one condition inside another condition statement.

An if statement within another if statement is known as nested if statement. Similarly, any decision logic can be written within an else statement also.

if condition:
  if condition:
    statements
  else:
    statements
else:
  statements

Example:

mark = 72
      if mark > 50:
          if mark > = 80:
            print ("You got A Grade !!")
          elif mark > =60 and mark < 80 :
            print ("You got B Grade !!")
          else:
            print ("You got C Grade !!")
      else:
        print("You failed!!")

Output: You got B Grade!!

switch case is not supported by python.

Guess the output

code 1:

num1=100
num2=200
num3=6
if(5>=num3):
    if(num1>100 or num2>150):
        print("1")
elif(num1>=100 and num2>150):
    print("2")
else:
    print("3")

options : 1,2 or 3 ?

code 2:

if((num1/num2==5) and (num1+num2)>5):
    print("1")
elif((num1-num2)<=1 or (num1%num2)==0):
    print("2")
else:
    print("3")

Options:

num1=11, num2=2

num1=0, num2=5

num1=5, num2=1

num1=-10,num2=2

code 3:

a = -10
b = -200
c = 2000
d = 4000
if( a*b >=d):
    if(d>c):
        if(d%c!=0):
            print(11)
        else:
            print(22)
else:
    if(b/a >0):
        if(a<b or d%c!=0):
          print(33)
        else:
          print(44)

Python Iteration Control Structure(Loops)

In iteration control structure, a condition is given and the code inside the body executes until the certain condition is reached.

Python While loop

while loop in python

Syntax for while loop:

initialization;
while(condition)
{
  //Code block to execute something
}

Example:

x=0
while(x < =5):
  print(x)
  x+=1

Output:

0
1
2
3
4
5

Example:

baggage_count=100
no_of_baggage_picked=10
while(baggage_count>0):
    no_of_baggage_picked = (int)(input ("Number of baggage:"))
    baggage_count = baggage_count - no_of_baggage_picked
    print("No. of baggage remaining:",baggage_count)

For loops

In python, for-loop allows the loop to run over a specific sequence of values. In other words, for every value in the sequence, the loop runs once. Thus we can avoid infinite loops by using a for loop.

for loop in python

Example:

for number in 1,2,3,4,5:
    print("The current number is ",number)

Example:

for number in range(1,5):
    print ("The current number is ",number)

print("---------------------------")

for number in range(1,7,2):
    print ("The current number is ",number)

print("---------------------------")

for number in range(5,0,-1):
    print ("The current number is ",number)

Output:

The current number is  1
The current number is  2
The current number is  3
The current number is  4
---------------------------
The current number is  1
The current number is  3
The current number is  5
---------------------------
The current number is  5
The current number is  4
The current number is  3
The current number is  2
The current number is  1

Nested loops

Example:

number_of_passengers=5
number_of_baggage=2
security_check=True
for passenger_count in range(1, number_of_passengers+1):
    for baggage_count in range(1,number_of_baggage+1):
        if(security_check==True):
            print("Security check of passenger:", passenger_count, "-- baggage:", baggage_count,"baggage cleared")
        else:
            print("Security check of passenger:", passenger_count, "-- baggage:", baggage_count,"baggage not cleared")
number_of_passengers=5
number_of_baggage=2
security_check=True
for passenger_count in range(1, number_of_passengers+1):
    for baggage_count in range(1,number_of_baggage+1):
        if(security_check==True):
            print("Security check of passenger:", passenger_count, "-- baggage:", baggage_count,"baggage cleared")
        else:
            print("Security check of passenger:", passenger_count, "-- baggage:", baggage_count,"baggage not cleared")

When we want to stop a loop or break away from it we can use break statement.

Example:

for passenger in "A","A", "FC", "C", "FA",  "SP", "A", "A":
    if(passenger=="FC" or passenger=="FA"):
        print("No check required")
        continue

    if(passenger=="SP"):
        print("Declare emergency in the airport")
        break

    if(passenger=="A" or passenger=="C"):
        print("Proceed with normal security check")

    print("Check the person")
    print("Check for cabin baggage")

When we want to skip the remaining portion of loop statements and continue with the next iteration, we can use continue statement.

for passenger in "A","A", "FC", "C", "FA",  "SP", "A", "A":
    if(passenger=="FC" or passenger=="FA"):
        print("No check required")
        continue

    if(passenger=="SP"):
        print("Declare emergency in the airport")
        break

    if(passenger=="A" or passenger=="C"):
        print("Proceed with normal security check")

    print("Check the person")
    print("Check for cabin baggage")
if-else statement

Pass is used to pass the code body. pass transfers the control to the next statement.

Some best websites to practice

What are the 3 types of control structures?

1.sequential
2.selection
3.iteration

What are the two categories of Python loops?

What is Range function in Python?

range() function is a built in python function.IT is used when a user needs to perform a task between two ranges. Ex. range(3,8)

What does Len () do in Python?

len() function is used to return the length of the given item.

How to write while loop (syntax) in python?

syntax:
while(condition):
//code

what is the syntax of for loop in python?

syntax:
for condition:
//code

What are decision control structure?

selection control structure is also called decision control structure.
1. if- else
2. if – elif – else

Conclusion

In this chapter we have learnt about python control structures.

1.Selection control structure

2. Iteration control structure

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control structure in python
Python control structures

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