Page 309 - Computer Science V2.0 Class 11

11.4.2 A Number with Digits in Reverse Order
Next, let's develop a function called reverseDigits(num) that takes a non-negative integer as an argument and
returns a number whose digits are in the opposite order of the argument's digits. For example, reverseDigits(1234)
will return 4321. Before we jump into the coding task, let us understand the process of obtaining a number (say,
reverseNum) whose digits are in the opposite order of the argument's digits. Note that 1234 = ((1x10 + 2)x10 + 3)
x10+4. So, to build reverseNum, we must compute ((4x10 + 3)x10 + 2)x10+1. For this purpose, we begin with a
clean slate, i.e., we initialise reverseNum as 0. Next, we extract the unit's digit (4) and add it to reverseNum.
So, reverseNum becomes 4. Now that we do not require the unit's digit any more, we discard it by setting num =
num // 10. Now num is equal to 123. Again, we multiply reverseNum by 10 and add to it the unit's digit (3) from
current value of num (123). Thus, reverseNum becomes 43. Again, discarding the units's digit of num, we get num
equal to 12. We multiply reverseNum by 10 and add to it the unit's digit (2) from current value of num (12). Thus,
reverseNum becomes 432. Now, discarding the units's digit (2) of num (12), we get the num equal to 1. Again, we
multiply reverseNum by 10 and add to it the unit's digit (1) from current value of num (1). Thus, reverseNum
becomes 4321. Now, discarding the units's digit of num, we get num equal to 0, which indicates that there are no more
digits to be processed and we have been able to build reverseNum as desired.

Program 11.6 Write a function reverseDigits(num) to display reverse of a number.

01 # Program Objective: To display reverse of a number
02 def reverseDigits(num):
03 """
04 Objective: To find reverse of the number
05 Inputs:
06 num : number whose reverse has to be returned
07 Return value:
08 reverseNum : reverse of num
09 """
10 num1 = num
11 reverseNum = 0
12 while num > 0:
13 remainder = num % 10
14 reverseNum = reverseNum * 10 + remainder
15 num = num // 10
16 return reverseNum
17 number = int(input("Enter a number : "))
18 reverseNum = reverseDigits(number)
19 print("The reverse of ", number, " is :", reverseNum)
Sample Output:
>>> Enter a number : 1234
The reverse of 1234 is : 4321 Enter in loop
As shown in the syntax for the while statement, it may include
an optional else clause. It operates like the else clause in a for Condition False
statement, i.e., the else clause gets executed on a smooth exit
from the while loop. However, if control moves out of the while True
loop on execution of the break statement, the else clause is
ignored.
else block

Encounter Body
break
no

yes
Exit from loop

Looping in Python 295