Write a program to find the largest and the second largest elements in a given list of elements.

Programming Problems · Q3

Q. Write a program to find the largest and the second largest elements in a given list of elements.

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Concept understanding — List Operations

A shopping list on paper is more than the items written on it — it is the things you can do with it: read the third entry, cross one out, add another at the bottom, check whether "milk" is already there. A Python list is the program's version of that paper: an ordered collection that can hold many values under one name, written in square brackets. List operations are the standard actions the language gives you for working with the whole collection or any part of it.

marks = [78, 92, 65, 88, 71]

The core operations every student must command:

Indexing — fetch one element by its position: marks[0] is 78. Counting starts at zero, so the last of five items is marks[4]. Negative indexes count from the end: marks[-1] is the last element, no matter how long the list grows.
Slicing — fetch a sub-list: marks[1:4] gives the elements at positions 1, 2 and 3. The start is included, the stop is excluded. Leaving an end open means "to the edge": marks[:3] is the first three, marks[2:] is everything from position 2 onward.
Concatenation — joining two lists with the plus sign produces a new, longer list.
Repetition — a list times an integer produces repeated copies, just as with strings: [0] * 5 builds a list of five zeros, a handy way to prepare storage.
Membership — the in and not in tests answer yes or no: 92 in marks is True. No loop needed.
Traversal — a for loop visits every element in order, which is how totals, searches and printouts are written.
Length — len(marks) reports how many elements the list currently holds.

Unlike a string, a list is mutable — it can be changed in place. Assigning marks[2] = 70 genuinely replaces the third element; nothing similar is allowed on a string.

Watch out

Asking for an index that does not exist — marks[10] on a five-item list — crashes with IndexError. A slice, oddly, forgives the same mistake: marks[10:20] just returns an empty list. Examiners love this contrast.

These operations matter because the list is the workhorse container of the whole course: marks of a class, items in a bill, rows read from a file all arrive as lists, and every later technique — searching, counting frequencies, computing averages — is built by combining exactly these few operations.

Sort a copy descending; largest = first element, second largest = first different value (handles duplicate maxima).

s = sorted(lst, reverse=True)
largest = s[0]
second = next((v for v in s[1:] if v != largest), None)
print("Largest:", largest, " Second largest:", second)

lst = [12, 45, 7, 45, 30, 2]  →  Largest: 45  Second largest: 30

✓Final answer

sorted(lst, reverse=True) then skip duplicates of the top value — e.g. 45 and 30 for the sample list.

Sort a copy of the list in descending order; the largest is at index 0 and the second largest is the first later element that differs from it (so duplicates of the maximum

don't get counted twice). A one-pass loop version is also shown.

The idea

"Second largest" usually means the second largest distinct value — in [9, 9, 5] it is 5,

not 9. Sorting makes this easy: after sorting in descending order, walk forward until the value changes.

Program (sort-based — simplest)

# Largest and second largest element of a list
n = int(input("How many elements? "))
lst = []
for i in range(n):
    lst.append(int(input("Enter element " + str(i + 1) + ": ")))

s = sorted(lst, reverse=True)      # descending copy; original list untouched
largest = s[0]

second = None
for value in s[1:]:
    if value != largest:           # first value different from the maximum
        second = value
        break

print("Largest        :", largest)
if second is None:
    print("Second largest : does not exist (all elements are equal)")
else:
    print("Second largest :", second)

Sample run

How many elements? 6
Enter element 1: 12
Enter element 2: 45
Enter element 3: 7
Enter element 4: 45
Enter element 5: 30
Enter element 6: 2
Largest        : 45
Second largest : 30

One-pass alternative (no sorting)

largest = second = None
for x in lst:
    if largest is None or x > largest:
        second, largest = largest, x
    elif x != largest and (second is None or x > second):
        second = x

Key lines explained:

sorted(lst, reverse=True) returns a new descending list (unlike lst.sort(), it doesn't change the original).
The if value != largest guard is what handles duplicates — with [45, 45, 30, ...] it skips the second 45 and correctly reports 30.
The one-pass version updates two trackers as it scans: a new maximum demotes the old one to second.

✓Final answer

Sort descending, take s[0] as the largest and the first value!= s[0] as the second largest — for [12, 45, 7, 45, 30, 2] : largest 45, second largest 30 (duplicates of the maximum are skipped).

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Q.Write a program to find the largest and the second largest elements in a given list of elements.

The idea

Program (sort-based — simplest)

Sample run

One-pass alternative (no sorting)

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