Second Largest Number in a List in Python
Once a list is sorted, its largest numbers sit at the end — and negative indexes count from the end.
Quick answer
Sort the list and take nums[-2]: after nums.sort() the largest number is at index -1 and the second largest at -2. If the list can repeat a number, remove duplicates first with sorted(set(nums))[-2]. Without sorting, keep two variables (first and second) and update them in one loop.
The question
Read a line of different integers separated by spaces and print the second largest number.
The code
program.py
nums = list(map(int, input().split()))
nums.sort()
print("Second largest =", nums[-2])Input
52 95 31 35 23 19 86
Output
Second largest = 86
How it works
- 1Read the numbers into a list.
- 2nums.sort() arranges them from smallest to largest.
- 3nums[-1] is the largest, so nums[-2] is the second largest.
What happens to the sample list
| Step | nums |
|---|---|
| after input | [52, 95, 31, 35, 23, 19, 86] |
| nums.sort() | [19, 23, 31, 35, 52, 86, 95] |
| nums[-1] | 95 (largest) |
| nums[-2] | 86 (second largest) |
Common mistakes
- Using nums[1], which is the second smallest, not the second largest.
- Writing nums = nums.sort() — sort() returns None, so the list is lost.
Second largest number without sorting (one loop)
first holds the largest value seen so far and second the one below it. When a bigger number arrives, the old first moves down to second. One pass, and the list is not changed.
program.py
nums = list(map(int, input().split()))
first = second = None
for x in nums:
if first is None or x > first:
second = first
first = x
elif x != first and (second is None or x > second):
second = x
print("Second largest =", second)Input
12 45 7 33
Output
Second largest = 33
When the list has duplicate numbers
With 10 40 40 25, nums[-2] after sorting is 40 again. set() removes the repeats first, so the second largest distinct value is found.
program.py
nums = list(map(int, input().split()))
print("Second largest =", sorted(set(nums))[-2])Input
10 40 40 25
Output
Second largest = 25
Using max() and remove()
Take the largest out of the list, and the largest of what is left is the answer. Easy to explain in a viva.
program.py
nums = list(map(int, input().split()))
nums.remove(max(nums))
print("Second largest =", max(nums))Input
8 3 15 11
Output
Second largest = 11
Questions students ask
- How do you find the second largest number in a list in Python?
- Sort the list with nums.sort() and print nums[-2]. Negative indexes count from the end, so -1 is the largest and -2 the second largest. sorted(set(nums))[-2] does the same when numbers can repeat.
- What does nums[-2] mean in Python?
- The second item from the end of the list. nums[-1] is the last item and nums[-2] the one before it. On a sorted list those are the largest and the second largest values.
- Can I find the second largest number without sort()?
- Yes. Walk through the list once with two variables, first and second. When a number is bigger than first, the old first becomes second; when it is between them, it becomes the new second.
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Preparing for NIELIT O Level?
This program is the kind asked in the M3 (Python) theory paper and the PR-3 practical exam.
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