Printing from a list inside of a while loop

Python
1

Here is the program that I am working on. The first part (with the “for loop”) works as expected. It prints out the following list:

Month 1 is January
Month 2 is February

  • through*
    Month 12 is December

I am trying to get the same output using a “while loop” but I am getting an error that x is undefined but the same coding works for the “for loop”.

make list for months of the year. Run a loop to generate month number and print Month number is correlated with month name

months_of_the_year = [“January”, “February”, “March”, “April”, “May”, “June”, “July”, “August”, “September”, “October”, “November”, “December”]
for x in range(0, 12):
print("Month " + (str(x+1)) + " is " + months_of_the_year)
print()
print()
i = 1
while i <= 12:
print("Month " + str(i) + " is " + (months_of_the_year)
i = i + 1

Thanks for looking at this. As I am tying to learn difference between using “for loop” and “while loop”.

2

I was able to get the while loop working. Not sure exactly why it works but here is what I ended up doing:

i = 0
while i in range(0, 12):
print("Month " + str(i+1) + " is " + months_of_the_year[i])
i = i+1

I think I need to better understand how the 2 different loops function and how they execute there process.

Thanks to those who considered this issue.

3

This is a classic off-by-one issue. Your array is indexed as 0 - 11. In your for loop, you referenced x+1 to generate 1-12. You used the RANGE function. That stops at 11. So x+1 becomes 1-12. While x itself is 0-11. So the index works fine.

When you did the while loop, you started with 1. That puts you one-off. If you want your i variable to start at 1, then you have to minus 1 for your months_of_the_year index because those are 0-11.

Many things in the world of programming start with 0 instead of 1. That’s because these are OFFSETS not COUNTS. You count from 1 because 1 means you have 1 item. But offsets (in lists) means distance from beginning. So you start those with 0. That’s why arrays are always indexed from 0. It’s always confusing. :slight_smile:

When you see errors like this, it’s invariably an off-by-one issue.

Hopefully that helps to explain what happened.

Otherwise: The difference between a while loop and a for loop … for loops are great for working through a list. While loops are great for working until you hit a condition. Both can be effective at any task. But that’s the main difference. WHILE is like looking for a result. FOR is like working through a list.

Jerry

4

Thanks for responding. I will try (-1). Thanks again.