Как найти обратное число python

In this tutorial, you’ll learn how to use Python to reverse a number. While this is similar to learning how to reverse a string in Python, which you can learn about here, reversing a number allows us to use math to reverse our number. You’ll learn how to use a Python while loop, string indexing, and how to create an easy-to-read function to reverse a number in Python.

The Quick Answer: Use a Python While Loop

Reverse a Python Number Using a While Loop

Python makes it easy to reverse a number by using a while loop. We can use a Python while loop with the help of both floor division and the modulus % operator.

Let’s take a look at an example to see how this works and then dive into why this works:

# How to Reverse a Number with a While Loop
number = 67890
reversed_number = 0

while number != 0:
    digit = number % 10
    reversed_number = reversed_number * 10 + digit
    number //= 10
    
print(reversed_number)

# Returns: 9876

Let’s break down what we do here:

1. We instantiate two variables, number and reversed_number. The first stores our original number and the second is given the value of 0
2. While our number variable is equal to anything by 0, we repeat our actions below
3. We instantiate digit, and assign it the modulus (remainder) of our number divided by 10
4. We multiply our reversed number by 10 and add our digit
5. Finally, we return the floored result of our number divided by 10 (this essentially removes the number on the right)
6. This process is repeated until our original number is equal to zero

It’s important to note, this approach only works for integers and will not work for floats.

In the next section, you’ll learn how to use Python string indexing to reverse a number.

Reverse a Python Number Using String Indexing

Another way that we can reverse a number is to use string indexing. One of the benefits of this approach is that this approach will work with both integers and floats.

In order to make this work, we first turn the number into a string, reverse it, and turn it back into a number. Since we’ll need to convert it back to its original type, we need to first check the numbers type.

Let’s take a look at how we can accomplish this in Python:

# How to Reverse a Number with String Indexing
number = 67890.123

reversed_number_string = str(number)[::-1]
if type(number) == float:
    reversed_number = float(reversed_number_string)
elif type(number) == int:
    reversed_number = int(reversed_number_string)
    
print(reversed_number)

# Returns 321.09876

Python indexing allows us to iterate over an iterable object, such as a string. The third parameter is optional, but represents the step counter, meaning how to traverse an item. By default, the value is 1, which means it goes from the first to the last. By using the value of -1, we tell Python to generate a new string in its reverse.

We use type checking in order to determine what type of number to return back to.

In the next section, you’ll learn how to create a custom function that makes the code easier to follow and understand.

Reverse a Python Number Using a Custom Function

In this section, you’ll learn how to turn what you learned in the section above into an easy-to-read function. While the code may seem intuitive as we write it, our future readers may not agree. Because of this, we can turn our code into a function that makes it clear what our code is hoping to accomplish.

Let’s take a look at how we can accomplish this in Python:

def reverse_number(number):
    """Reverses a number (either float or int) and returns the appropriate type.

    Args:
        number (int|float): the number to reverse

    Returns:
        int|float: the reversed number
    """
    if type(number) == float:
        return float(str(number)[::-1])
    elif type(number) == int:
        return int(str(number)[::-1])
    else:
        print('Not an integer or float')

print(reverse_number(12345.43))
print(reverse_number(456))

# Returns:
# 34.54321
# 654

Our function accepts a single parameter, a number. The function first checks what the type is. If the type is either float or int, it reverses the number and returns it back to the same type. If the type is anything else, the function prints out that the type is neither a float nor an int.

Conclusion

In this post, you learned how to reverse a number using both math and string indexing. You also learned how to convert the string indexing method to a function that will make it clear to readers of your code what your code is doing.

If you want to learn more about string indexing in Python, check out the official documentation for strings here.

Additional Resources

To learn more about related topics, check out the resources below:

• Python: Reverse a String (6 Easy Ways)
• How to Reverse a Python List (6 Ways)
• Python range(): A Complete Guide (w/ Examples)

Python is an interpreted, high-level, general-purpose programming language with different applications. To learn the fundamental concepts of Python, there are some standard programs which would give you a brief understanding of all the concepts practically. Reverse a number in Python is one of these programs which gives the learner a deep understanding of loops and arithmetic operators. This blog will help you understand and implement the ways to reverse a number. It will cover the following topics –

• How to reverse a number in Python?
• Python program to reverse a number
  • Using loops
  • Using recursion

How to reverse a number in Python?

It’s simple! You can write a Python program which takes input number and reverse the same. The value of an integer is stored in a variable which is checked using a condition and then each digit of the number is stored in another variable, which will print the reversed number. Numbers can be reversed in Python using different methods, let us take a look at the Python program to implement the same.

Python program to reverse a number

There are two ways to reverse a number in Python programming language –

• Using a Loop
• Using Recursion

Reverse a Number using Loop

 # Get the number from user manually 
num = int(input("Enter your favourite number: "))

# Initiate value to null
test_num = 0

# Check using while loop

while(num>0):
  #Logic
  remainder = num % 10
  test_num = (test_num * 10) + remainder
  num = num//10

# Display the result
print("The reverse number is : {}".format(test_num))

Output:

Program Explanation

User value: Number = 123456 and Reverse = 0

First Iteration
Reminder = Number %10
Reminder = 123456%10 = 6
Reverse = Reverse *10 + Reminder
Reverse = 0 * 10 + 6 = 0 + 6 = 6
Number = Number //10
Number = 123456 //10 = 12345

Second Iteration
From the first Iteration the values of both Number and Reverse have been changed as: Number = 12345 and Reverse = 6
Reminder = Number % 10
Reminder = 12345 % 10 = 5
Reverse = Reverse *10+ Reminder = 6 * 10 + 5
Reverse = 60 + 5 = 65
Number = Number //10 = 12345 //10
Number = 1234

Third Iteration
From the Second Iteration, the values of both Number and Reverse have been changed as: Number = 1234 and Reverse = 65
Reminder = Number %10
Reminder = 1234%10 = 4
Reverse = Reverse *10+ Reminder = 65 * 10 + 4
Reverse = 650 + 4 = 654
Number = Number //10 = 1234//10
Number = 123

Fourth Iteration
From the Second Iteration the values of both Number and Reverse have been changed as: Number = 123 and Reverse = 654
Reminder = Number %10
Reminder = 123 %10 = 3
Reverse = Reverse *10+ Reminder = 654 * 10 + 3
Reverse = 6540 + 3 = 6543
Number = Number //10 = 123//10
Number = 12

Fifth iteration
From the Second Iteration the values of both Number and Reverse have been changed as: Number = 12 and Reverse = 6543
Reminder = Number %10
Reminder = 12 %10 = 2
Reverse = Reverse *10+ Reminder = 6543 * 10 + 2
Reverse = 65430 + 2 = 65432
Number = Number //10 = 12//10
Number = 1

Sixth iteration
From the Second Iteration, the values of both Number and Reverse have been changed as, Number = 1 and Reverse = 65432
Reminder = Number %10
Reminder = 1 %10 = 1
Reverse = Reverse *10+ Reminder = 65432 * 10 + 1
Reverse = 654320 + 1 = 654321
Number ended:

Reverse a Number using Recursion

 
# Python Program to Reverse a Number using Recursion

Num = int(input("Please Enter any Number: "))

Result = 0
def Result_Int(Num):
    global Result
    if(Num > 0):
        Reminder = Num %10
        Result = (Result *10) + Reminder
        Result_Int(Num //10)
    return Result

Result = Result_Int(Num)
print("n Reverse of entered number is = %d" %Result)

Output:

With this, we come to an end of this blog on “Reverse a Number in Python”. I hope it added value to your knowledge of Python programming.

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I’m creating a python script which prints out the whole song of ’99 bottles of beer’, but reversed. The only thing I cannot reverse is the numbers, being integers, not strings.

This is my full script,

def reverse(str):
   return str[::-1]

def plural(word, b):
    if b != 1:
        return word + 's'
    else:
        return word

def line(b, ending):
    print b or reverse('No more'), plural(reverse('bottle'), b), reverse(ending)

for i in range(99, 0, -1):
    line(i, "of beer on the wall")
    line(i, "of beer"
    print reverse("Take one down, pass it around")
    line(i-1, "of beer on the wall n")

I understand my reverse function takes a string as an argument, however I do not know how to take in an integer, or , how to reverse the integer later on in the script.