Intro

To decide and control the flow of a program, we have branching and looping techniques in Python. A good understanding of loops and if-else statements is necessary to write efficient programs in Python.

This Python loop exercise aims to help Python developers to learn and practice if-else conditions, for loop, range() function, and while loop.

Use the following tutorials to solve this exercise

• Control flow statements: Use the if-else statements in Python for conditional decision-making
• for loop: To iterate over a sequence of elements such as list, string.
• range() function: Using a for loop with range(), we can repeat an action a specific number of times
• while loop: To repeat a block of code repeatedly, as long as the condition is true.
• Break and Continue: To alter the loop’s execution in a certain manner.
• Nested loop: loop inside a loop is known as a nested loop

Exercise 1: Print First 10 natural numbers using while loop

Write python code to print first 10 natural numbers using while loop.

Example

1
2
3
4
5
6
7
8
9
10

Hint:

• Some hint for student

Solution:

# program 1: Print first 10 natural numbers
i = 1
while i <= 10:
    print(i)
    i += 1

Exercise 2: Print the following pattern

Write python code to print the following pattern

Example

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

Hint:

• Decide the row count, i.e., 5, because the pattern contains five rows
• Run outer for loop 5 times using for loop and range() function
• Run inner for loop i+1 times using for loop and range() function
  • In the first iteration of the outer loop, the inner loop will execute 1 time
  • In the second iteration of the outer loop, the inner loop will execute 2 time
  • In the third iteration of the outer loop, the inner loop will execute 3 times, and so on till row 5
• print the value of j in each iteration of inner loop (j is the the inner loop iterator variable)
• Display an empty line at the end of each iteration of the outer loop (empty line after each row)
• Print Patterns In Python
• Nested loops in Python

Solution:

print("Number Pattern ")

# Decide the row count. (above pattern contains 5 rows)
row = 5
# start: 1
# stop: row+1 (range never include stop number in result)
# step: 1
# run loop 5 times
for i in range(1, row + 1, 1):
    # Run inner loop i+1 times
    for j in range(1, i + 1):
        print(j, end=' ')
    # empty line after each row
    print("")

Exercise 3: Calculate the sum of all numbers from 1 to a given number

Write a program to accept a number from a user and calculate the sum of all numbers from 1 to a given number.

Example

If the user entered 10 the output should be 55 (=1+2+3+4+5+6+7+8+9+10)

Enter number 10
Sum is:  55

Hint:

Approach 1: Use for loop and range() function

• Create variable s = 0 to store the sum of all numbers
• Use Python 3’s built-in function input() to take input from a user
• Convert user input to the integer type using the int() constructor and save it to variable n
• Run loop n times using for loop and range() function
• In each iteration of a loop, add current number (i) to variable s
• Use the print() function to display the variable s on screen

Approach 2: Use the built-in function sum(). The sum() function calculates the addition of numbers in the list or range

Solution 1:

Using for loop and range() function

# s: store sum of all numbers
s = 0
n = int(input("Enter number "))
# run loop n times
# stop: n+1 (because range never include stop number in result)
for i in range(1, n + 1, 1):
    # add current number to sum variable
    s += i
print("\n")
print("Sum is: ", s)

Solution 2:

Using the built-in function sum()

n = int(input("Enter number "))
# pass range of numbers to sum() function
x = sum(range(1, n + 1))
print('Sum is:', x)

Exercise 4: Print multiplication table

Write a program to print multiplication table of a given number.

Example

For example, num = 2 so the output should be

2
4
6
8
10
12
14
16
18
20

Hint:

• Set n =2
• Use for loop to iterate the first 10 numbers
• In each iteration, multiply 2 by the current number.( p = n*i)
• Print p

Solution:

n = 2
# stop: 11 (because range never include stop number in result)
# run loop 10 times
for i in range(1, 11, 1):
    # 2 *i (current number)
    product = n * i
    print(product)

Exercise 5: Display numbers from a list using loop

Write a program to display only those numbers from a list that satisfy the following conditions

• The number must be divisible by five
• If the number is greater than 150, then skip it and move to the next number
• If the number is greater than 500, then stop the loop

Example

numbers = [12, 75, 150, 180, 145, 525, 50]

75
150
145

Hint:

• Use for loop to iterate each item of a list
• Use break statement to break the loop if the current number is greater than 500
• use continue statement move to next number if the current number is greater than 150
• Use number % 5 == 0 condition to check if number is divisible by 5
• break and continue in Python

Solution:

numbers = [12, 75, 150, 180, 145, 525, 50]
# iterate each item of a list
for item in numbers:
    if item > 500:
        break
    elif item > 150:
        continue
    # check if number is divisible by 5
    elif item % 5 == 0:
        print(item)

Exercise 6: Count the total number of digits in a number

Write a program to count the total number of digits in a number using a while loop.

Example

For example, the number is 75869, so the output should be 5.

Hint:

• Set counter = 0
• Run while loop till number != 0
• In each iteration of loop
  • Reduce the last digit from the number using floor division (number=number//10)
  • Increment counter by 1
• print counter

Solution:

num = int(input("Input a number: "))
count = 0
while num != 0:
    # floor division
    # to reduce the last digit from number
    num = num // 10

    # increment counter by 1
    count = count + 1
print("Total digits are:", count)

Exercise 7: Print the following pattern

Write a program to use for loop to print the following reverse number pattern.

Example

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

Hint:

• Set row = 5 because the above pattern contains five rows
• create an outer loop to iterate numbers from 1 to 5 using for loop and range() function
• Create an inner loop inside the outer loop in such a way that in each iteration of the outer loop, the inner loop iteration will be reduced by i. i is the current number of an outer loop
• In each iteration of the inner loop, print the iterator variable of the inner loop (j)
• Note:
• In the first iteration of the outer loop inner loop execute five times.
• In the second iteration of the outer loop inner loop execute four times.
• In the last iteration of the outer loop, the inner loop will execute only once

Solution:

n = 5
k = 5
for i in range(0,n+1):
    for j in range(k-i,0,-1):
        print(j,end=' ')
    print()

Exercise 8: Print list in reverse order

Write Python code to print list in reverse order using a loop.

Example

list1 = [10, 20, 30, 40, 50]

50
40
30
20
10

Hint:

Approach 1: Use the built-in function reversed() to reverse the list

Approach 2: Use for loop and the len() function

• Get the size of a list using the len(list1) function
• Use for loop and reverse range() to iterate index number in reverse order starting from length-1 to 0. In each iteration, i will be reduced by 1
• In each iteration, print list item using list1[i]. i is the current value if the index

Solution 1:

Using a reversed() function and for loop

list1 = [10, 20, 30, 40, 50]
# reverse list
new_list = reversed(list1)
# iterate reversed list
for item in new_list:
    print(item)

Solution 2:

Using for loop and the len() function

list1 = [10, 20, 30, 40, 50]
# get list size
# len(list1) -1: because index start with 0
# iterate list in reverse order
# star from last item to first
size = len(list1) - 1
for i in range(size, -1, -1):
    print(list1[i])

Exercise 9: Display numbers from -10 to -1 using for loop

Write Python number to display numbers from -10 to -1 using for loop

Example

-10
-9
-8
-7
-6
-5
-4
-3
-2
-1

Hint:

• Reverse range

Solution:

for num in range(-10, 0, 1):
    print(num)

Exercise 10: Use else block to display a message “Done” after successful execution of for loop

For example, the following loop will execute without any error.

Example

for i in range(5):
    print(i)

0
1
2
3
4
Done!

Hint:

• Same as the if statement, Python allows us to use an else block along with for loop. for loop can have the else block, which will be executed when the loop terminates normally. See else block in for loop.

Solution:

for i in range(5):
    print(i)
else:
    print("Done!")

Exercise 11: Display all prime numbers within a range

Note: A Prime Number is a number that cannot be made by multiplying other whole numbers. A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers

Examples:

• 6 is not a prime mumber because it can be made by 2×3 = 6
• 37 is a prime number because no other whole numbers multiply together to make it

Example

# range
start = 25
end = 50

Prime numbers between 25 and 50 are:
29
31
37
41
43
47

Hint:

• Some hint for student

Solution:

start = 25
end = 50
print("Prime numbers between", start, "and", end, "are:")

for num in range(start, end + 1):
    # all prime numbers are greater than 1
    # if number is less than or equal to 1, it is not prime
    if num > 1:
        for i in range(2, num):
            # check for factors
            if (num % i) == 0:
                # not a prime number so break inner loop and
                # look for next number
                break
        else:
            print(num)

Exercise 12: Display Fibonacci series up to 10 terms

The Fibonacci Sequence is a series of numbers. The next number is found by adding up the two numbers before it. The first two numbers are 0 and 1.

For example, 0, 1, 1, 2, 3, 5, 8, 13, 21. The next number in this series above is 13+21 = 34.

Example

Fibonacci sequence:
0  1  1  2  3  5  8  13  21  34

Hint:

• Set num1 = 0 and num2 =1 (first two numbers of the sequence)
• Run loop ten times
• In each iteration
  • print num1 as the current number of the sequence
  • Add last two numbers to get the next number res = num1+ num2
  • update values of num1 and num2. Set num1=num2 and num2=res

Solution:

# first two numbers
num1, num2 = 0, 1

print("Fibonacci sequence:")
# run loop 10 times
for i in range(10):
    # print next number of a series
    print(num1, end="  ")
    # add last two numbers to get next number
    res = num1 + num2

    # update values
    num1 = num2
    num2 = res

Exercise 13: Find the factorial of a given number

Write a program to use the loop to find the factorial of a given number.

The factorial (symbol: !) means to multiply all whole numbers from the chosen number down to 1.

For example: calculate the factorial of 5

Example

5! = 5 × 4 × 3 × 2 × 1 = 120

120

Hint:

• Set variable factorial =1 to store factorial of a given number
• Iterate numbers starting from 1 to the given number n using for loop and range() function. (here, the loop will run five times because n is 5)
• In each iteration, multiply factorial by the current number and assign it again to a factorial variable (factorial = factorial *i)
• After the loop completes, print factorial

Solution:

num = 5
factorial = 1
if num < 0:
    print("Factorial does not exist for negative numbers")
elif num == 0:
    print("The factorial of 0 is 1")
else:
    # run loop 5 times
    for i in range(1, num + 1):
        # multiply factorial by current number
        factorial = factorial * i
    print("The factorial of", num, "is", factorial)

Exercise 14: Reverse a given integer number

Example

Solution:

num = int(input())
reverse_number = 0
print("Given Number ", num)
while num > 0:
    reminder = num % 10
    reverse_number = (reverse_number * 10) + reminder
    num = num // 10
print("Revere Number ", reverse_number)

Exercise 15: Display elements from a given list present at odd index positions

Use a loop to display elements from a given list present at odd index positions.

Example

my_list = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]

20 40 60 80 100

Hint:

• Use list slicing. Using list slicing, we can access a range of elements from a list

Solution:

my_list = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
# stat from index 1 with step 2( means 1, 3, 5, an so on)
for i in my_list[1::2]:
    print(i, end=" ")

Exercise 16: Calculate the cube of all numbers from 1 to a given number

Write a program to rint the cube of all numbers from 1 to a given number.

Example

input_number = 6

Current Number is : 1  and the cube is 1
Current Number is : 2  and the cube is 8
Current Number is : 3  and the cube is 27
Current Number is : 4  and the cube is 64
Current Number is : 5  and the cube is 125
Current Number is : 6  and the cube is 216

Hint:

• Iterate numbers from 1 to n using for loop and range() function
• In each iteration of a loop, calculate the cube of a current number (i) by multiplying itself three times (c = i * i* i)

Solution:

input_number = int(input("Input a number: "))
for i in range(1, input_number + 1):
    print("Current Number is :", i, " and the cube is", (i * i * i))

Exercise 17: Find the sum of the series upto n terms

Write a program to calculate the sum of series up to n term. For example, if n =5 the series will become 2 + 22 + 222 + 2222 + 22222 = 24690

Example

# number of terms
n = 5

24690

Solution:

n = 5
# first number of sequence
start = 2
sum_seq = 0

# run loop n times
for i in range(0, n):
    print(start, end="+")
    sum_seq += start
    # calculate the next term
    start = start * 10 + 2
print("\nSum of above series is:", sum_seq)

Exercise 18: Print the following pattern

Write a program to print the following start pattern using the for loop

Example

* 
* * 
* * * 
* * * * 
* * * * * 
* * * * 
* * * 
* * 
*

Hint:

• Split pattern into 2 part: upper and lower.
• Use two for loops. First for loop to print the upper pattern and second for loop to print lower pattern.

Solution:

rows = 5
for i in range(0, rows):
    for j in range(0, i + 1):
        print("*", end=' ')
    print("\r")

for i in range(rows, 0, -1):
    for j in range(0, i - 1):
        print("*", end=' ')
    print("\r")
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