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Many years ago when I did a tour of duty as an enlisted man in the U.S. Air Force, the drill sergeant had a habit of lining us up and telling us to "count off."
What that meant was that the first person in the line called out the number one , the person behind him called out the number two , the person behind him called out the number three , etc.
(Since learning about computer programming, I now wonder if the first person should have called out zero. I notice that thefirst case of Ebola in the United States is being identified by the CDC as case 0.)
I'm sure the drill sergeant didn't realize that what he was doing was assigning an ordinal index value to each person in the line (and neither did I realize it at the time) .
Even though he didn't know the technical details of ordinal indices, he didn't have any difficulty yelling, "Number six, wash dishes, number fourteen,peel potatoes, number twenty-two, carry out the garbage, etc."
That is what using an index is all about -- using an ordinal index to select an item. As you will see in a future module titled Itse1359-1080-Lists Part 1 , this is also referred to as a subscription in Python. In the context of this module, indexing is the process of assigning an ordinal index value to each data item contained in some sort of acontainer.
In other words, we assign an ordinal number to each item, which describes the numerical position of the item in the container.
A dozen eggs:
For example, if you were very careful, you could use a felt tip pen to assign an ordinal index to each of the twelve eggs contained in acarton containing a dozen eggs. (Should you start with zero or one?)
Then you could extract the egg whose index value is 9 from the container and eat it for breakfast.
Having assigned the index, we can use that index to access the data item corresponding to that index, as in "Number six, wash dishes."
In this module, we will be using the index values that are automatically assigned to the characters in a string for the purpose of accessing thosecharacters, both individually, and in groups.
Here is what Magnus Lie Hetland has to say on the topic of slicing (and indexing as well.) Although this quotation was taken from a discussion of lists, it applies equallywell to strings. (Note that some material was deleted from the quotation for brevity.)
According to Magnus Lie Hetland:
"One of the nice things about lists is that you can access their elements separately or in groups, through indexing and slicing.
Indexing is done (as in many other languages) by appending the index in brackets to the list. (Note that the first element has index 0) ...
(This is the answer to the question about the first egg -- Baldwin)
Slicing is almost like indexing, except that you indicate both the start and stop index of the result, with a colon (":") separating them: ...
Notice that the end is non-inclusive. If one of the indices is dropped, it is assumed that you want everything in that direction. i.e. list[:3]means "every element from the beginning of list up to element 3,non-inclusive." ...
list[3:] would, on the other hand, mean "everyelement from list, starting at element 3 (inclusive) up to, and including, the last one."
For really interesting results, you can use negative numbers too: list[-3]is the third element from the end of the list..."
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