Python program to find out the third largest number in a list

Python 3 program to find out the third largest number in a list :

In this python tutorial, we will learn how to find out the third largest number in a list. For example the third largest number among 1,5,4,2,7,9 is 5. Our program will iterate through the list only for one time i.e. the time complexity of this program is O(n).

Python program :

#1
num = [2,3,7,4,5,6,10,11,120]

#2
largest_num = num[0]
second_largest_num = num[0]
third_largest_num = num[0]

#3
for i in num :
    #4
    if i > largest_num :
        third_largest_num = second_largest_num
        second_largest_num = largest_num
        largest_num = i
    #5
    elif i > second_largest_num :
        third_largest_num = second_largest_num
        second_largest_num = i
    #6
    elif i > third_largest_num :
        third_largest_num = i

#7
print("Third largest number of the list is {}".format(third_largest_num))

Explanation :

The commented numbers in the above program denote the step numbers below :

  1. First, create one number list holding few random numbers. We will find the third largest among these numebers.
  2. Create three variables to hold the largest number,second largest number and third largest number of the list. Assign them all the value of the first number of the given list.
  3. Run one for loop to read each number of the list one by one.
  4. First check if the current number is greater than the largest number. If yes, assign the value of second largest number to the third largest number, value of largest number to the second largest number and the current value of the number to the largest number. Basically, we are updating the value for these three variables.
  5. If the first condition failed, check if it is larger than second largest number or not. If yes, assign the value of second largest number to the third largest number and assign the current reading value to the second largest number.
  6. Similarly, check if the current number is only greater than third largest number.If yes, assign its value to the third largest number variable.
  7. Finally print out the result , i.e. value of the third largest number variable.

Output :

Third largest number of the list is 10

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