Python Programming

pass

pass is a simple statement that simply does not do anything.

It is useful as a placeholder, especially when you need to have an indented block in a compound statement like if and for but are not ready to write your code.

words = ["be", "very", "quiet"]
for word in words:
    pass

Altering the flow of loops

In (strict) structured programming, there is a rule that each block must have only one entry point and one exit point, [**put road illustration here**]

This prevents programmers from arbitrarily ‘jumping’ to different lines in code. This was usually done with a goto statement in the good old days (probably before many of you were born!) This made it hard to read, understand and debug codes.

However, sometimes it is easier to allow the programmer to alter the flow of a loop, for example exiting the for-loop earlier if some conditions are met. Two simple statements break and continue allow this.

break

The break statement will force the program to exit a loop, terminating the loop containing it.

Here is a possible use case (what does it do?)

while True:
    line = input("> ")
    if line == "done":
        break
    print(line)

print("Done!")

And another example (guess what it does):

target = 3
items = [1, 5, 3, 2, 1, 7, 6]
for (i, item) in enumerate(items):
    if item == target:
        print(f"The number {item} found at index {i}!")
        break

Exercise: Of course, we can write the programs above without any break statements. How? Try it yourself first before peeking at my answer at the end of this page!

There are, however, times when having the break statement can make your code more readable. For example, you can use break statements can be useful to get out of very very deep nested loops.

continue

The continue statement also alters the flow of control, but instead of exiting the loop, it skips any instructions after it (inside the loop) and jump straight back to the condition check (the beginning of the loop).

Here is an example: (as usual, try to understand what the code does)

numbers = [1, 4, 3, 4, 8, 9, 7, 6, 3, 2]
for number in numbers:
    if number % 3 == 0:
        continue 
    print(number)

And the answer to the earlier task…

Here is how I implemented the code from earlier without the break statement (strictly structured program)

target = 3
items = [1, 5, 3, 2, 1, 7, 3, 6]
found = False
found_index = -1
i = 0
while i < len(items) and not found: 
    if items[i] == target:
        found = True
        found_index = i
    i = i + 1

if found:
    print(f"The number {target} found at index {found_index}!")