Loops ​

To calculate 1 + 2 + 3, we can directly write the expression:

>>> 1 + 2 + 3
6

However, calculating 1 + 2 + 3 + ... + 10000 would be nearly impossible using just expressions. To perform thousands of repetitive calculations, we need loop statements.

Python supports two types of loops: for...in loops and while loops.

for...in Loops ​

The for...in loop iterates over each element in a list or tuple. For example:

names = ['Michael', 'Bob', 'Tracy']
for name in names:
    print(name)

Executing this code will print each element in the list names:

Michael
Bob
Tracy

In the for x in ... loop, each element is assigned to the variable x, and the indented block is executed.

To calculate the sum of integers from 1 to 10, we can use a variable sum for accumulation:

sum = 0
for x in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]:
    sum = sum + x
print(sum)

If we want to calculate the sum from 1 to 100, manually writing the list would be difficult. Fortunately, Python provides the range() function, which generates a sequence of integers. For example, range(5) generates a sequence of integers from 0 to less than 5:

>>> list(range(5))
[0, 1, 2, 3, 4]

Using range(101), we can generate the sequence 0 to 100:

sum = 0
for x in range(101):
    sum = sum + x
print(sum)

Running the above code will give the result 5050, just as Gauss famously calculated.

while Loops ​

A while loop continues executing as long as the condition is true. When the condition becomes false, the loop exits. For example, to calculate the sum of all odd numbers below 100:

sum = 0
n = 99
while n > 0:
    sum = sum + n
    n = n - 2
print(sum)

In this example, the variable n is decreased by 2 in each iteration until it reaches -1, at which point the while condition is no longer satisfied, and the loop exits.

Exercise ​

Print "Hello, xxx!" for each name in the list:

L = ['Bart', 'Lisa', 'Adam']
for name in L:
    print(f'Hello, {name}!')

break ​

The break statement allows us to exit a loop prematurely. For example, the following code prints numbers from 1 to 100:

n = 1
while n <= 100:
    print(n)
    n = n + 1
print('END')

To stop the loop early, we can use the break statement:

n = 1
while n <= 100:
    if n > 10:  # When n = 11, break the loop
        break
    print(n)
    n = n + 1
print('END')

The output will be numbers from 1 to 10, followed by "END." The break statement exits the loop when the condition is met.

continue ​

The continue statement skips the current iteration and proceeds to the next iteration:

n = 0
while n < 10:
    n = n + 1
    if n % 2 == 0:  # Skip even numbers
        continue
    print(n)

The output will be 1, 3, 5, 7, 9, skipping even numbers.

Summary ​

Loops are an effective way to make the computer perform repetitive tasks.

• The break statement exits the loop, while the continue statement skips to the next iteration. Both are often used with if statements.
• Avoid overusing break and continue as they can make the code more complex. In most cases, loops can be written without them.
• Be cautious of infinite loops (deadlocks). Use Ctrl+C to exit or forcibly terminate the Python process.

Exercise: Write an infinite loop program.