Python Control Structures

Control Structures

A program statement that causes a jump of control from one part of the program to another is called control structure or control statement. As you have already learnt in C++, these control statements are compound statements used to alter the control flow of the process or program depending on the state of the process.

1. Sequential Statement

A sequential statement is composed of a sequence of statements which are executed one after another. A code to print your name, address and phone number is an example of sequential statement.

Example 6.1

#Program to print your name and address - example for sequential statement

print ("Hello! This is Shyam")

print ("43, Second Lane, North Car Street, TN")

Output

Hello! This is Shyam

43, Second Lane, North Car Street, TN

2. Alternative or Branching Statement

In our day-to-day life we need to take various decisions and choose an alternate path to achieve our goal. May be we would have taken an alternate route to reach our destination when we find the usual road by which we travel is blocked. This type of decision making is what we are to learn through alternative or branching statement. Checking whether the given number is positive or negative, even or odd can all be done using alternative or branching statement.

Python provides the following types of alternative or branching statements:

Simple if statement

if..else statement

if..elif statement

(i) Simple if statement

Simple if is the simplest of all decision making statements. Condition should be in the

Syntax:

if <condition>:

statements-block1form of relational or logical expression.

In the above syntax if the condition is true statements - block 1 will be executed.

Example

#Program to check the age and print whether eligible for voting

x=int (input("Enter your age :"))

if x>=18:

print ("You are eligible for voting")

Output 1:

Enter your age :34

You are eligible for voting

Output 2:

Enter your age :16

>>> 

As you can see in the second execution no output will be printed, only the Python prompt will be displayed because the program does not check the alternative process when the condition is failed.

(ii) if..else statement

The if .. else statement provides control to check the true block as well as the false block. Following is the syntax of ‘if..else’ statement.

Syntax:

if <condition>:

statements-block 1

else:

statements-block 2

if..else statement thus provides two possibilities and the condition determines which BLOCK is to be executed.

Example 6.3: #Program to check if the accepted number odd or even

a = int(input("Enter any number :"))

if a%2==0:

print (a, " is an even number")

else:

print (a, " is an odd number")

Output 1:

Enter any number :56

56 is an even number

Output 2:

Enter any number :67

67 is an odd number

An alternate method to rewrite the above program is also available in Python. The complete if..else can also written as:

Syntax:

variable = variable1 if condition else variable 2

Example 6.4: #Program to check if the accepted number is odd or even (using alternate method of if...else)

a = int (input("Enter any number :"))

x="even" if a%2==0 else "odd"

print (a, " is ",x)

Output 1:

Enter any number :3

3 is odd

Output 2:

Enter any number :22

22 is even

(iii) Nested if..elif...else statement:

When we need to construct a chain of if statement(s) then ‘elif’ clause can be used instead of ‘else’.

Syntax:

if <condition-1>:

statements-block 1

elif <condition-2>:

statements-block 2

else:

statements-block n

In the syntax of if..elif..else mentioned above, condition-1 is tested if it is true then statements-block1 is executed, otherwise the control checks condition-2, if it is true statements-block2 is executed and even if it fails statements-block n mentioned in else part is executed.

‘elif’ clause combines if..else-if..else statements to one if..elif…else. ‘elif’ can be considered to be abbreviation of ‘else if’. In an ‘if’ statement there is no limit of ‘elif’ clause that can be used, but an ‘else’ clause if used should be placed at the end.

if..elif..else statement is similar to nested if statement which you have learnt in C++.

Example 6.5: #Program to illustrate the use of nested if statement

Average               Grade

>=80 and above             A

>=70 and <80                B

>=60 and <70                C

>=50 and <60                D

Otherwise             E

m1=int (input(“Enter mark in first subject : ”))

m2=int (input(“Enter mark in second subject : ”))

avg= (m1+m2)/2

if avg>=80:

print (“Grade : A”)

elif avg>=70 and avg<80:

print (“Grade : B”)

elif avg>=60 and avg<70:

print (“Grade : C”)

elif avg>=50 and avg<60:

print (“Grade : D”)

else:

print (“Grade : E”)

Output 1:

Enter mark in first subject : 34

Enter mark in second subject : 78

Grade : D

Output 2 :

Enter mark in first subject : 67

Note

The two blocks of code in our example of if-statement are both indented four spaces, which is a typical amount of indentation for Python. In most other programming languages, indentation is used only to help make the code look pretty. But in Python, it is required to indicate to which block of code the statement belongs to.

Example 6.5a: #Program to illustrate the use of ‘in’ and ‘not in’ in if statement

ch=input (“Enter a character :”)

# to check if the letter is vowel

if ch in (‘a’, ‘A’, ‘e’, ‘E’, ‘i’, ‘I’, ‘o’, ‘O’, ‘u’, ‘U’):

print (ch,’ is a vowel’)

#to check if the letter typed is not ‘a’ or ‘b’ or ‘c’ if ch not in (‘a’, ’b’, ’c’):

print (ch,’ the letter is not a/b/c’)

Output 1:

Enter a character :e

e is a vowel

Output 2:

Enter a character :x

x the letter is not a/b/c

3. Iteration or Looping constructs

Iteration or loop are used in situation when the user need to execute a block of code several of times or till the condition is satisfied. A loop statement allows to execute a statement or group of statements multiple times.

Python provides two types of looping constructs:

(i) while loop

(ii) for loop

(i) while loop

The syntax of while loop in Python has the following syntax:

Syntax:

while <condition>:

statements block 1

[else:

statements block2]

In the while loop, the condition is any valid Boolean expression returning True or False. The else part of while is optional part of while. The statements block1 is kept executed till the condition is True. If the else part is written, it is executed when the condition is tested False. Recall while loop belongs to entry check loop type, that is it is not executed even once if the condition is tested False in the beginning.

Example 6.6: program to illustrate the use of while loop - to print all numbers from 10 to 15

i=10  # intializing part of the control variable

while (i<=15):       # test condition

print (i,end='\t')    # statements - block 1

i=i+1 # Updation of the control variable

Output:

10 11 12      13      14      15

Note

That the control variable is i, which is initialized to 10, the condition is tested i<=15, if true value of i gets printed, then the control variable i gets updated as i=i+1 (this can also be written as i +=1 using shorthand assignment operator). When i becomes 16, the condition is tested False and this will terminate the loop.

Note

print can have end, sep as parameters. end parameter can be used when we need to give any escape sequences like ‘\t’ for tab, ‘\n’ for new line and so on. sep as parameter can be used to specify any special characters like, (comma) ; (semicolon) as separator between values (Recall the concept which you have learnt in previous chapter about the formatting options in print()).

Following is an example for using else part within while loop.

Example 6.7: program to illustrate the use of while loop - with else part

i=10  # intializing part of the control variable

while (i<=15):       # test condition

print (i,end='\t')    # statements - block 1

i=i+1 # Updation of the control variable

else:  

print ("\nValue of i when the loop exit ",i)

Output: 1

10 11 12 13 14 15

Value of i when the loop exit 16

(ii) for loop

for loop is the most comfortable loop. It is also an entry check loop. The condition is checked in the beginning and the body of the loop(statements-block 1) is executed if it is only True otherwise the loop is not executed.

Syntax:

for counter_variable in sequence:

statements-block 1

[else:         # optional block

statements-block 2]

The counter_variable mentioned in the syntax is similar to the control variable that we used in the for loop of C++ and the sequence refers to the initial, final and increment value. Usually in Python, for loop uses the range() function in the sequence to specify the initial, final and increment values. range() generates a list of values starting from start till stop-1.

The syntax of range() is as follows:

range (start,stop,[step])

Where,       

start  –        refers to the initial value

stop   –        refers to the final value

step   –        refers to increment value, this is optional part.

Example 6.8:Examples for range()

range (1,30,1) will start the range of values from 1 and end at 29

range (2,30,2) will start the range of values from 2 and end at 28

range (30,3,-3) - will start the range of values from 30 and end at 6

range (20)   will consider this value 20 as the end value(or upper limit) and starts the range count from 0 to 19 (remember always range() will work till stop -1 value only)

Example 6.9: #program to illustrate the use of for loop - to print single digit even number

for i in range (2,10,2):

print (i, end=' ')

Output:

2 4 6 8

Following is an illustration using else part in for loop

Example 6.10: #program to illustrate the use of for loop - to print single digit even number with else part

for i in range(2,10,2):

print (i,end=' ')

else:

print ("\nEnd of the loop")

Output:

2 4 6 8

End of the loop

Note

In Python, indentation is important in loop and other control statements. Indentation only creates blocks and sub-blocks like how we create blocks within a set of { } in languages like C, C++ etc.

Here is another program which illustrates the use of range() to find the sum of numbers 1 to 100

Example 6.11: # program to calculate the sum of numbers 1 to 100

n = 100

sum = 0

for counter in range(1,n+1):

sum = sum + counter

print("Sum of 1 until %d: %d" % (n,sum))

Output:

Sum of 1 until 100: 5050

In the above code, n is initialized to 100, sum is initialized to 0, the for loop starts executing from 1, for every iteration the value of sum is added with the value of counter variable and stored in sum. Note that the for loop will iterate from 1 till the upper limit -1 (ie. Value of n is set as 100, so this loop will iterate for values from 1 to 99 only, that is the reason why we have set the upper limit as n+1)

Note

range () can also take values from string, list, dictionary etc. which will be dealt in the later chapters.

Following is an example to illustrate the use of string in range()

Example 6.12: program to illustrate the use of string in range() of for loop

for word in 'Computer':

print (word,end=' ')

else:

print ("\nEnd of the loop")

Output

C o m p u t e r

End of the loop

(iii) Nested loop structure

A loop placed within another loop is called as nested loop structure. One can place a while within another while; for within another for; for within while and while within for to construct such nested loops.

Following is an example to illustrate the use of for loop to print the following pattern 1

1 2

1 2 3

1 2 3 4

1 2 3 4 5

Example 6.13: program to illustrate the use nested loop -for within while loop

i=1

while (i<=6):

for j in range (1,i):

print (j,end='\t')

print (end='\n')

i +=1

4. Jump Statements in Python

The jump statement in Python, is used to unconditionally transfer the control from one part of the program to another. There are three keywords to achieve jump statements in Python : break, continue, pass. The following flowchart illustrates the use of break and continue.

(i) break statement

The break statement terminates the loop containing it. Control of the program flows to the statement immediately after the body of the loop.

A while or for loop will iterate till the condition is tested false, but one can even transfer the control out of the loop (terminate) with help of break statement. When the break statement is executed, the control flow of the program comes out of the loop and starts executing the segment of code after the loop structure.

If break statement is inside a nested loop (loop inside another loop), break will terminate the innermost loop.

The working of break statement in for loop and while loop is shown below.

Program to illustrate the use of break statement inside for loop

for word in “Jump Statement”:

if word = = “e”:

break

print (word, end= ' ')

Output:

Jump Stat

The above program will repeat the iteration with the given “Jump Statement” as string. Each letter of the given string sequence is tested till the letter ‘e’ is encountered, when it is encountered the control is transferred outside the loop block or it terminates. As shown in the output, it is displayed till the letter ‘e’ is checked after which the loop gets terminated.

One has to note an important point here is that ‘if a loop is left by break, the else part is not executed’. To explain this lets us enhance the previous program with an ‘else’ part and see what output will be:

Program to illustrate the use of break statement inside for loop

for word in “Jump Statement”:

if word = = “e”:

break

print (word, end=”)

else:

print (“End of the loop”)

print (“\n End of the program”)

Output:

Jump Stat

End of the program

Note that the break statement has even skipped the ‘else’ part of the loop and has transferred the control to the next line following the loop block.

(ii) continue statement

Continue statement unlike the break statement is used to skip the remaining part of a loop and start with next iteration.

Syntax

continue

The working of continue statement in for and while loop is shown below.

Program to illustrate the use of continue statement inside for loop

for word in “Jump Statement”:

if word = = “e”:

continue

print (word, end=”)

print (“\n End of the program”)

Output:

Jump Statement

End of the program

The above program is same as the program we had written for ‘break’ statement except that we have replaced it with ‘continue’. As you can see in the output except the letter ‘e’ all the other letters get printed.

(iii) pass statement

pass statement in Python programming is a null statement. pass statement when executed by the interpreter it is completely ignored. Nothing happens when pass is executed, it results in no operation.

pass statement can be used in ‘if’ clause as well as within loop construct, when you do not want any statements or commands within that block to be executed.

Syntax:

Pass

Program to illustrate the use of pass statement

a=int (input(“Enter any number :”))

if (a==0):

pass

else:

print (“non zero value is accepted”)

Output:

Enter any number :3

non zero value is accepted

When the above code is executed if the input value is 0 (zero) then no action will be performed, for all the other input values the output will be as follows:

Note

pass statement is generally used as a placeholder. When we have a loop or function that is to be implemented in the future and not now, we cannot develop such functions or loops with empty body segment because the interpreter would raise an error. So, to avoid this we can use pass statement to construct a body that does nothing.

Example 6.18: Program to illustrate the use of pass statement in for loop

for val in “Computer”:

pass

print (“End of the loop, loop structure will be built in future”)

Output:

End of the loop, loop structure will be built in future.