### 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:*

- First, create one number list holding few random numbers. We will find the third largest among these numbers.
- 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. - Run one
*for loop*to read each number of the list one by one. - First, check if the current number is
*greater*than the*largest*number. If yes, assign the value of the*second-largest*number to the*third-largest*number, the value of the*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*. - If the first condition failed, check if it is
*larger*than the*second-largest*number or not. If yes, assign the value of the*second-largest*number to the*third-largest*number and assign the current reading value to the*second largest number*. - Similarly, check if the current number is only greater than
*third-largest*number. If yes, assign its value to the*third-largest*number variable. - Finally, print out the result, i.e. the value of the
*third largest number*variable.

### Output :

`Third largest number of the list is 10`

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