Question

I have a very basic question about loops. I’m trying to get into programming and am currently reading the book “Learn Python The Hard Way”, and got stuck on study drill number 5 on ex 33:
http://learnpythonthehardway.org/book/ex33.html

This is what my code looks like:

i = 0
numbers = []

def main(i, numbers):
    userloop = int(raw_input("Type in max value for the loop > "))
    while i < userloop:

        print "At the top i is %d" % i
        numbers.append(i)
        userincrease = int(raw_input("Increase > "))
        i += userincrease

        print "Numbers now: ", numbers
        print "At the bottom i is %d" % i


    print "The numbers: "
    print numbers
    for num in numbers:
    print num       


main(i, numbers)

The thing is that i want to use a for loop instead of the while loop. Is that even possible and then, how do I do it? I’ve tried simply exchanging

while i < userloop:

with

for i in range(userloop)

But then, the i value kinda resets every time it reaches the “top” of the loop.

Since I’m very much new to this, please don’t be too mean if i missed something obvious.

By the way, does anyone know whether this book is any good to begin with or am I wasting my time?

Answer 1

A for loop in python loops over a series of values provided by the expression after in. In Python we use the term iterable for anything that can produce such a series. That can be a range(), but it could be anything that can be iterated over (looped over). The loop variable i receives the next value on each iteration, whatever value there was before.

Thus, the following is legal:

for i in ('foo', 'bar', 'baz'):
    print i
    i = 'spam'

but the i = 'spam' essentially does nothing.

You’d have to approach it differently; skip those values instead:

print_at = 0
for i in range(userloop):
    if i >= print_at:
        print "At the top i is %d" % i
        numbers.append(i)
        userincrease = int(raw_input("Increase > "))
        print_at = i + userincrease

        print "Numbers now: ", numbers
        print "At the bottom i is %d" % i

Every time you ask the user for an increase, we update the value print_at to be the sum of the current value of i plus that increase, and then proceed to ignore most of the loop body until i catches up with print_at.