1 Python’s “for” is not Count Based

The Python for loop is a range based or for each loop. If you come from C++ or Java… that is definitely… confusing.

2 A Quick Problem

Suppose that we want to output the even numbers between two (2) and one hundred (100), including both 2 and 100, that are not divisible by four (4).

We would start with…

def main():
    for num in range(2, 102, 2):
        print(f"{num:>3}")


if __name__ == "__main__":
    main()

The loop will grab every num starting at 2 and ending at 100. At each step num will be incremented by 2. The next step is to add a conditional block to skip numbers that are divisible by 4

def main():
    for num in range(2, 102, 2):
        if num % 4 != 0:
            print(f"{num:>3}")


if __name__ == "__main__":
    main()

Although my preference would be to flip the condition and continue (or skip) to the next loop iteration.

def main():
    for num in range(2, 102, 2):
        if num % 4 == 0:
            continue

        print(f"{num:>3}")


if __name__ == "__main__":
    main()

This decreases nesting (and is a useful trick for making code more readable). Imagine a problem where instead of one (1) such check we have three (3) or four (4) conditions to check.

3 Reversing the Problem

Suppose that we want to output numbers starting at 100 and stopping at 2. A quick call to reversed will do the trick.

def main():
    for num in reversed(range(2, 102, 2)):
        if num % 4 == 0:
            continue

        print(f"{num:>3}")


if __name__ == "__main__":
    main()

4 Is That It?

That is it… for now. We will see for again in future lectures on input files and list.