Searching a List

Searching a List

Linear search looks at the elements one by one, from the first, until it finds the value it wants or reaches the end. It works on any list, sorted or not.

roll_numbers = [12, 7, 25, 3, 18]
key = 25
position = -1
for i in range(len(roll_numbers)):
    if roll_numbers[i] == key:
        position = i
        break
if position == -1:
    print("Not found")
else:
    print("Found at position", position + 1)   # Found at position 3

The program prints position + 1 because people count positions from 1 while Python counts indexes from 0. The break stops the search as soon as the value is found, so the program does not keep looking for nothing.

Python's own key in roll_numbers and roll_numbers.index(key) do the same search for you, but writing it yourself shows exactly how it works.

nums = list(map(int, input().split()))
key = int(input())
position = -1
# search nums for key
if position == -1:
    print("Not found")
else:
    print("Found at position", position + 1)