5.5: Loop Else

Learning Objectives

By the end of this section you should be able to

• Use loop else statement to identify when the loop execution is interrupted using a break statement.
• Implement a loop else statement with a for or a while loop.

Loop else

A loop else statement runs after the loop's execution is completed without being interrupted by a break statement. A loop else is used to identify if the loop is terminated normally or the execution is interrupted by a break statement.

Ex: A for loop that iterates over a list of numbers to find if the value 10 is in the list. In each iteration, if 10 is observed, the statement "Found 10!" is printed and the execution can terminate using a break statement. If 10 is not in the list, the loop terminates when all integers in the list are evaluated, and hence the else statement will run and print "10 is not in the list." Alternatively, a Boolean variable can be used to track whether number 10 is found after loop's execution terminates.

Example 5.4: Finding the number 10 in a list

In the code examples below, the code on the left prints "Found 10!" if the variable i's value is 10. If the value 10 is not in the list, the code prints "10 is not in the list.". The code on the right uses the seen Boolean variable to track if the value 10 is in the list. After loop's execution, if seen's value is still false, the code prints "10 is not in the list.".

Table 5.3

numbers = [2, 5, 7, 11, 12] for i in numbers: if i == 10: print("Found 10!") break else: print("10 is not in the list.")

numbers = [2, 5, 7, 11, 12] seen = False for i in numbers: if i == 10: print("Found 10!") seen = True if seen == False: print("10 is not in the list.")

Checkpoint: Loop else template

Concepts in Practice \(\PageIndex{1}\): Loop else practices

What is the output?

n = 16
exp = 0
i = n
while i > 1:
  if n%2 == 0:
    i = i/2
    exp += 1
  else:
    break
else:
  print(n,"is 2 to the", exp)

1. 16 is 2 to the 3
2. 16 is 2 to the 4
3. no output

Answer

b. The variable i is initialized to 16 , and the while loop executes with i = 16, 8 , 4 , and 2 . In each loop's iteration, the variable exp's value is incremented by 1. Hence, at the end of the loop, exp = 4. Since the loop terminates without interruption, the else statement after the loop is executed and prints 16 is 2 to the 4.

Concepts in Practice \(\PageIndex{2}\): Loop else practices

What is the output?

n = 7
exp = 0
i = n
while i > 1:
  if n%2 == 0:
    i = i//2
    exp += 1
  else:
    break
else:
  print(n,"is 2 to the", exp)

1. no output
2. 7 is 2 to the 3
3. 7 is 2 to the 2

Answer

a. The initial i's value is 7 . Hence, in the first loop's iteration, the else statement within the loop executes, and the break statement terminates the loop. Since the loop's execution is interrupted, the else after the loop will not execute and there will be no output.

Concepts in Practice \(\PageIndex{3}\): Loop else practices

What is the output?

numbers = [1, 2, 2, 6]
for i in numbers:
  if i >= 5:
    print("Not all numbers are less than 5.")
    break
  else:
    print(i)
    continue
else:
  print("all numbers are less than 5.")

1. 1
   2
   2
   6
   Not all numbers are less than 5.
   

2. 1
   2
   2
   Not all numbers are less than 5.
   

3. 1
   2
   2
   6
   all numbers are less than 5.

Answer

b. The loop iterates over all integer values in the list. If a value is less than 5 , the value will be printed, and if the value is greater than 5 , the code prints "Not all numbers are less than 5." and the code's execution terminates.

Try It: Sum of values less than 10

Write a program that, given a list, calculates the sum of all integer values less than 10. If a value greater than or equal to 10 is in the list, no output should be printed. Test the code for different values in the list.

Interactive Code

list1 = [1, 5, 7, 5, 9, 2, 4]
list2 = [1, 3, 10, 5]

total = 0

for i in list1: # Test the code with list2 as well
    # TO DO: add the value of integer i to total if i < 10

Answer

list1 = [1, 5, 7, 5, 9, 2, 4]
list2 = [1, 3, 10, 5]

total = 0

for i in list1:
if i >= 10:
break
total += i
else:
print("sum =", total)