05-for-else-while-else.md

Python for...else and while...else

Video Link: https://youtu.be/aoW_Y1MumN0

In this video, we learned to use the else clause in for and while loops with the help of examples.

Programs in the Video

• Python for...else
• Python while...else

---

Python for...else

Suppose we want to check if an element is present in a given list.

For that, let's iterate through each element and check if it is equal to our value:

languages = ['Python', 'JavaScript', 'C', 'C++', 'Java']

for language in languages:
    if language == "Java":
        print("Item found")

Output

Item found

Now, suppose we want to print Item not found if the item was not found in the list.

One way to do so is to use a flag and check it after the loop ends:

languages = ['Python', 'JavaScript', 'C', 'C++', 'Java']

flag = False

for language in languages:
    if language == "Java":
        flag = True

if flag:
    print("Item found")
else:
    print("Item not found")

Output

Item found

Let's test using another keyword "Rust":

languages = ['Python', 'JavaScript', 'C', 'C++', 'Java']

flag = False

for language in languages:
    if language == "Rust":
        flag = True

if flag:
    print("Item found")
else:
    print("Item not found")

Output

Item not found

We can implement this exact functionality using for...else.

languages = ['Python', 'JavaScript', 'C', 'C++', 'Java']

for language in languages:
    if language == "Java":
        print("Item found")
else:
    print("Item not found")

Output

Item found
Item not found

• First, we checked if every item was equal to Java,and since the last item is equal to Java, "Item found" was printed.
• The else clause of the for loop executes at the end, only if the loop completes normally. In this case the loop completed normally and Item not Found was printed at the end.

However, we can now use the break statement inside if to end the loop abruptly if the item was found:

languages = ['Python', 'JavaScript', 'C', 'C++', 'Java']

for language in languages:
    if language == "Java":
        print("Item found")
        break
else:
    print("Item not found")

Output

Item found

Changing the keyword to Rust:

languages = ['Python', 'JavaScript', 'C', 'C++', 'Java']

for language in languages:
    if language == "Rust":
        print("Item found")
        break
else:
    print("Item not found")

Output

Item not found

---

Python while...else

The working of an else clause of a while loop is very similar to that of a for loop.

Let's write a program to check if a number is prime.

num = int(input("Enter a number: "))

i = 2

while i < num:
    if num % i == 0:
        print(num, "is not a prime as it is", num // i, "times", i)
        break
    i += 1
else:
    print(num, "is a prime number")

Output 1

Enter a number: 34
34 is not a prime as it is 17 times 2

Output 2

Enter a number: 29
29 is a prime number