Control Statements (Loops & Nested Loops)

do-while loop

Reason — Python does not support do-while loop as a built-in looping construct. Only for loop and while loop are used to perform iterative tasks in Python.

for

Reason — The for loop is the best example of a fixed iterative loop because the number of iterations is known and fixed in advance using the range() function or any other iterable.

range(start, end, step)

Reason — The range() function can be used with one, two or three parameters. The complete form is range(start, end, step) where start is the initial value, end is the stop value and step is the update value.

while

Reason — The while loop checks the condition before executing the loop body. If the condition is false at the very beginning, the control will not enter the loop block and hence the loop will not execute even once.

A for loop is used for fixed iterations, while a while loop is used for conditional statements.

Reason — The for loop is a fixed iterative loop where the number of iterations are known in advance. The while loop is an unfixed iterative loop which executes based on a conditional expression and continues until the condition becomes false.

stop value is mandatory

Reason — The for loop is a fixed iterative loop where the number of iterations is determined using the range() function. In range(), the initial value and step value are optional because their default values are 0 and 1 respectively, but the stop value is mandatory as it specifies the ending limit of the loop.

for

Reason — The for loop works with a predefined sequence or iterable, so the number of iterations is fixed in advance unlike the while loop which depends on a condition.

generate a sequence of numbers

Reason — The range() function is a built-in function that generates a sequence of integer numbers based on the initial value, stop value and update parameters. This sequence is then used by the for loop for iteration.

eleven times

Reason — Whole numbers start from 0. Therefore, the whole numbers up to 10 are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, which makes a total of 11 numbers. Hence, the loop runs eleven times.

while

Reason — An infinite loop can be created with the help of the while loop by giving an ever-true condition. Since the condition will never become false, the loop will keep on executing infinitely.

10 times

Reason — The function range(10) generates a sequence of numbers from 0 to 9, which contains 10 values. Hence, the loop will execute 10 times.

nested loop

Reason — A loop written inside another loop is called a nested loop. In this type of loop, the inner loop executes completely for every iteration of the outer loop.

m = 6
n = 11

Let’s break down the code line by line:

1. m, n = 2, 15: This statement uses multiple assignment, where m is assigned 2 and n is assigned 15 in a single statement.

2. for i in range(1, 5):: The for loop runs for the values 1, 2, 3, 4. Therefore, the loop executes 4 times.

3. Iterations

• First Iteration: m = 3, n = 14
• Second Iteration: m = 4, n = 13
• Third Iteration: m = 5, n = 12
• Fourth Iteration: m = 6, n = 11

4. print("m =", m): Displays m = 6

5. print("n =", n): Displays n = 11

6. Therefore, the final output is:

m = 6
n = 11

120

Let’s break down the code line by line:

1. k = 1: The variable k is assigned the value 1.

2. i = 2: The variable i is assigned the value 2.

3. while(i < 6):: The while loop iterates as long as i < 6.

4. The iterations are as follows:

• i = 2: k = 1 * 2 = 2, then i becomes 3
• i = 3: k = 2 * 3 = 6, then i becomes 4
• i = 4: k = 6 * 4 = 24, then i becomes 5
• i = 5: k = 24 * 5 = 120, then i becomes 6

5. When i = 6, the condition becomes false and the loop terminates. Hence, the final value of k = 120 is printed.

99 98 97 96 

Let’s break down the code line by line:

1. p = 100: The variable p is assigned the value 100.

2. for i in range(1, 5):: The for loop runs for the values 1, 2, 3, 4. Therefore, the loop executes 4 times.

3. Iterations of the loop:

• First Iteration: p = 100 - 1 = 99
• Second Iteration: p = 99 - 1 = 98
• Third Iteration: p = 98 - 1 = 97
• Fourth Iteration: p = 97 - 1 = 96

4. print(p, end=' '): Displays the value of p after each iteration in the same line separated by spaces.

5. Therefore, the final output is: 99 98 97 96

-90 -81 -72 -63 -54 -45 -36 -27 -18 -9 0 

Let’s break down the code line by line:

1. p = -99: The variable p is assigned the value -99.

2. while(p < 0):: The while loop executes as long as the value of p is less than 0.

3. In each iteration, the statement p = p + 9 increases the value of p by 9 and print(p, end=' ') displays the updated value in the same line separated by spaces.

4. The values of p become -90, -81, -72, -63, -54, -45, -36, -27, -18, -9 and 0.

5. When p becomes 0, the condition p < 0 becomes False and the loop terminates.

6. Therefore, the output is -90 -81 -72 -63 -54 -45 -36 -27 -18 -9 0.

0 
1 0 
2 1 0 
3 2 1 0 

The outer loop runs from 0 to 3. For each value of i, the inner loop starts from i and decreases up to 0 using the step value -1. Therefore, when i = 0, the output is 0; when i = 1, the output is 1 0; when i = 2, the output is 2 1 0; and when i = 3, the output is 3 2 1 0. The print() statement moves the output to the next line after each iteration of the outer loop.

1 2 
2 4 
3 6 

The outer loop runs with values 1, 2, and 3. For each value of x, the inner loop runs with values 1 and 2. During each iteration, the product of x*y is printed. Thus, when x = 1, the outputs are 1 and 2; when x = 2, the outputs are 2 and 4; and when x = 3, the outputs are 3 and 6. The print() statement moves the cursor to the next line after each row is completed.

Both A and R are true and R is not the correct explanation of A.

Reason — A loop is a repetitive structure used to execute statements repeatedly until the required number of iterations are over. The for loop is one type of loop that is used when the number of iterations is fixed. Therefore, both statements are true, but the second statement does not explain the first statement.

A is true but R is false.

Reason — The range() function is a built-in function used with the for loop to generate a sequence of numbers. In the range() function, the start value and step value are optional because their default values are 0 and 1 respectively. However, the stop value is mandatory. Therefore, the first statement is true but the second statement is false.

(a) Line 1: num

(b) Line 2: and

(c) Line 3: c+1

The complete program is given below:

# a program to display numbers divisible by 3 and 5 in the list
c = 0
My_List = [10, 15, 24, 30, 20, 45, 25, 60]
print("Given List:", My_List)
print("The numbers divisible by 3 as well as 5 in the list are:")
for num in My_List:                    # line 1
    if(num%3 == 0 and num%5 == 0):     # line 2
        print(num)
        c = c+1                        # line 3
print("Number of elements divisible by 3 as well as 5: ", c)

for i in range(1, 11):
    print(i*i, end=" ")

1 4 9 16 25 36 49 64 81 100

n = 1
for i in range(1, 11):
    print(n, end=" ")
    n = n + i

1 2 4 7 11 16 22 29 37 46 

n = 2
for i in range(1, 11):
    print(n, end=" ")
    n = n + (2*i + 1)

2 5 10 17 26 37 50 65 82 101 

s = 0
for i in range(1, 21):
    s = s + i*i
print("Sum of the series =", s)

Sum of the series = 2870

s = 0
for i in range(1, 20):
    s = s + (i * (i + 1))
print("Sum of the series =", s)

Sum of the series = 2660

n = int(input("Enter the value of n: "))
s = 0

for i in range(1, n+1):
    t = 0
    for j in range(1, i+1):
        t = t + j
    s = s + t

print("Sum of the series =", s)

Enter the value of n: 6
Sum of the series = 56

n = int(input("Enter the value of n: "))
s = 0
p = 1

for i in range(1, n+1):
    p = p * i
    s = s + p

print("Sum of the series =", s)

Enter the value of n: 5
Sum of the series = 153

n = int(input("Enter the value of n: "))
s = 0
f = 1

for i in range(1, n+1):
    f = f * i
    s = s + f

print("Sum of the series =", s)

Enter the value of n: 6
Sum of the series = 873

s = 0
print("Enter any ten natural numbers:")
for i in range(10):
    n = int(input())
    s = s + n
print("Sum of ten natural numbers =", s)

Enter any ten natural numbers:
5
4
6
7
3
2
9
8
10
23
Sum of ten natural numbers = 77

ps = 0
ns = 0

for i in range(20):
    n = int(input("Enter a number: "))

    if(n > 0):
        ps = ps + n
    elif(n < 0):
        ns = ns + n

print("Sum of positive numbers =", ps)
print("Sum of negative numbers =", ns)

Enter a number: 10
Enter a number: -5
Enter a number: 6
Enter a number: 9
Enter a number: -23
Enter a number: -4
Enter a number: -6
Enter a number: -11
Enter a number: 22
Enter a number: 33
Enter a number: 44
Enter a number: 10
Enter a number: 12
Enter a number: 6
Enter a number: 7
Enter a number: 8
Enter a number: -9
Enter a number: -2
Enter a number: -3
Enter a number: 19
Sum of positive numbers = 186
Sum of negative numbers = -63

n = int(input("Enter a number: "))
s = 0

while(n != 0):
    d = n % 10
    s = s + d
    n = n // 10

print("Sum of digits =", s)

Enter a number: 642
Sum of digits = 12

c = 0
n = int(input("Enter a number: "))
for a in range(1, n+1):
    if n % a == 0:
        c = c + 1

if c > 2:
    print(n, "is a composite number")
else:
    print(n, "is not a composite number")

Enter a number: 9
9 is a composite number

Enter a number: 13
13 is not a composite number

n = int(input("Enter a number: "))
greatest = n
smallest = n

for i in range(1, 10):
    n = int(input("Enter a number: "))

    if(n > greatest):
        greatest = n

    if(n < smallest):
        smallest = n

print("Greatest number =", greatest)
print("Smallest number =", smallest)

Enter a number: 12
Enter a number: 56
Enter a number: 45
Enter a number: 78
Enter a number: 19
Enter a number: 20
Enter a number: 30
Enter a number: 41
Enter a number: 87
Enter a number: 90
Greatest number = 90
Smallest number = 12

n = int(input("Enter a number: "))
p = n
s = 0

while n > 0:
    d = n % 10
    s = s + d
    n = n // 10

if p % s == 0:
    print(p, "is a Niven Number")
else:
    print(p, "is not a Niven Number")

Enter a number: 126
126 is a Niven Number

Enter a number: 167
167 is not a Niven Number

n = int(input("Enter a number: "))
r = 0

while n > 0:
    d = n % 10
    r = r * 10 + d
    n = n // 10

print("Reversed number:", r)

Enter a number: 467
Reversed number: 764

for i in range(5, 11):
    print("Table of", i)

    for j in range(1, 11):
        print(i, "*", j, "=", i*j)

    print()

Table of 5
5 * 1 = 5
5 * 2 = 10
5 * 3 = 15
5 * 4 = 20
5 * 5 = 25
5 * 6 = 30
5 * 7 = 35
5 * 8 = 40
5 * 9 = 45
5 * 10 = 50

Table of 6
6 * 1 = 6
6 * 2 = 12
6 * 3 = 18
6 * 4 = 24
6 * 5 = 30
6 * 6 = 36
6 * 7 = 42
6 * 8 = 48
6 * 9 = 54
6 * 10 = 60

Table of 7
7 * 1 = 7
7 * 2 = 14
7 * 3 = 21
7 * 4 = 28
7 * 5 = 35
7 * 6 = 42
7 * 7 = 49
7 * 8 = 56
7 * 9 = 63
7 * 10 = 70

Table of 8
8 * 1 = 8
8 * 2 = 16
8 * 3 = 24
8 * 4 = 32
8 * 5 = 40
8 * 6 = 48
8 * 7 = 56
8 * 8 = 64
8 * 9 = 72
8 * 10 = 80

Table of 9
9 * 1 = 9
9 * 2 = 18
9 * 3 = 27
9 * 4 = 36
9 * 5 = 45
9 * 6 = 54
9 * 7 = 63
9 * 8 = 72
9 * 9 = 81
9 * 10 = 90

Table of 10
10 * 1 = 10
10 * 2 = 20
10 * 3 = 30
10 * 4 = 40
10 * 5 = 50
10 * 6 = 60
10 * 7 = 70
10 * 8 = 80
10 * 9 = 90
10 * 10 = 100

for i in range(1, 21):
    n = int(input("Enter a number: "))
    
    if n > 1:
        c = 0
        
        for j in range(1, n + 1):
            if n % j == 0:
                c = c + 1
        
        if c == 2:
            print(n, "is a prime number")

Enter a number: 4
Enter a number: 5
5 is a prime number
Enter a number: 6
Enter a number: 7
7 is a prime number
Enter a number: 8
Enter a number: 9
Enter a number: 2
2 is a prime number
Enter a number: 3
3 is a prime number
Enter a number: 12
Enter a number: 14
Enter a number: 15
Enter a number: 16
Enter a number: 78
Enter a number: 91
Enter a number: 20
Enter a number: 17
17 is a prime number
Enter a number: 39
Enter a number: 41
41 is a prime number
Enter a number: 19
19 is a prime number
Enter a number: 29
29 is a prime number

n = int(input("Enter a number: "))
p = n

while n > 9:
    s = 0
    while n > 0:
        d = n % 10
        s = s + d*d
        n = n // 10
    n = s

if n == 1:
    print(p, "is a Happy Number")
else:
    print(p, "is not a Happy Number")

Enter a number: 31
31 is a Happy Number

Enter a number: 12
12 is not a Happy Number

for i in range(1, 6):
    
    for j in range(5, i - 1, -1):
        print(j, end=" ")
    
    print()

5 4 3 2 1 
5 4 3 2 
5 4 3 
5 4 
5 

for i in range(5, 0, -1):
    
    for j in range(1, 2 * i, 2):
        print(j, end=" ")
    
    print()

1 3 5 7 9 
1 3 5 7 
1 3 5 
1 3 
1 

for i in range(1, 6):
    
    for j in range(5, 5 - i, -1):
        print(j, end=" ")
    
    print()

5 
5 4 
5 4 3 
5 4 3 2 
5 4 3 2 1 

for i in range(1, 6):
    
    n = 2 * i - 1
    
    for j in range(1, i + 1):
        print(n, end=" ")
        n = n + 2
    
    print()

1 
3 5 
5 7 9 
7 9 11 13 
9 11 13 15 17 

for i in range(1, 6):
    
    for j in range(i, 6):
        print(j, end=" ")
    
    print()

1 2 3 4 5 
2 3 4 5 
3 4 5 
4 5 
5 

for i in range(1, 6):
    
    for j in range(9, 9 - (2 * i), -2):
        print(j, end=" ")
    
    print()

9 
9 7 
9 7 5 
9 7 5 3 
9 7 5 3 1 

A loop is a repetitive control structure in which a statement or a set of statements are executed repeatedly until the given condition is false or the desired number of iterations are over.

The for loop is a fixed iterative looping construct in Python. In this looping system, the number of iterations is fixed. The loop starts with a given value of the control variable and executes the block of statements by updating the control variable unless the last limit of the loop is reached. This loop is preferred when the user is aware of the number of times the process is to be repeated.

Syntax:

for <variable> in range(<parameters>):
    <statement or a set of statements>

Example:

for i in range(1, 6):
    print(i, end=' ')

Output: 1 2 3 4 5

This code displays the numbers from 1 to 5 using a for loop.

The range() function is a built-in function in Python which is used with the for loop to generate a sequence of integer numbers. It plays a significant role in defining the iteration limit of the for loop.

The range() function takes three parameters — initial value, stop value and update. The for loop iterates over the sequence generated by range() and executes the loop body for each value in the sequence. This makes the for loop simple, concise and easy to control.

The parameters needed to create a for loop are passed through the range() function. These are:

1. Initial value — It is an optional parameter that defines the starting value of the loop. The default value is 0.
2. Stop value — It is a mandatory parameter that indicates the end condition of the loop. The loop will execute up to stop value - 1.
3. Update — It is an optional parameter used to increase or decrease the value of the control variable. The default update value is 1.

(a) General format of for loop:

for <variable> in range(<parameters>):
    <statement or a set of statements>

(b) General format of while loop:

while <conditional expression>:
    statement 1
    statement 2
    ...............
    <update expression>

An infinite loop or an endless loop is a looping construct in which the number of iterations will never come to an end. The set of statements within the loop block keeps executing infinitely because the given condition never becomes false.

Example:

a = 5
while(a > 0):
    print(a*a)
    a = a + 1

In the above code, the value of 'a' will always be positive. Hence, the condition (a > 0) will never become false, resulting in never-ending iterations of the loop. Press Ctrl+C keys together to break the running of such an endless loop.

Similarity: Both for loop and while loop are used to repeat the execution of a block of statements until a particular condition is satisfied.

Difference: The for loop is a fixed iterative loop in which the number of iterations is known in advance, whereas the while loop is an unfixed iterative loop in which the user may not be aware of the exact number of iterations that will take place during execution.

The differences between step loop and continuous loop are:

A nested loop is defined as a loop programmed within another loop. It is used in such a way that the inner loop repeats for a given number of times for each iteration of the outer loop.

A nested loop is used when a loop has to be programmed within another loop to perform a specific task. In a nested loop, the inner loop repeats for a number of times for each iteration of the outer loop. Nested loops are widely used in pattern printing.