How jagged arrays can be used and when they would be preferred over normal arrays

This is a follow-up from this post. A reader asks:

Could you possibly do a post on how jagged arrays can be used and when they would be preferred over normal arrays? Thanks!

Sure, why not!? I encourage people to ask me questions as I sometimes run out of blog topics.

As I’m sure you already know, the array is one of the most common data structure in programming. There are usually two types of arrays, rectangular and jagged arrays. Rectangular arrays can usually be thought of as a an M by N matrix like this:

You can use rectangular arrays to store data, which can then be used in literally infinite amounts of ways. An example would be a table of student’s grades, with M being student IDs and N being their grades.

On the other hand, jagged arrays are arrays that are not rectangular in “shape”; that is, at least one row has a different amount of columns than the rest. Online and on textbooks you will see people call them “arrays of arrays”, but personally I find this confusing. Just look at this picture instead:

This is a jagged array example

Now, an example of when you might want to use a jagged array? I don’t have code for it, but let’s continue with our student’s grades example. Say you have 10 students in a class, and 4 test grades. Ideally, you would have a rectangular array of 10*4 = 40 fields. Now say some students miss class and some other students earn extra credit, then you could not a rectangular shaped array. You could make an even bigger array where all the grades would fit, but this has the potential to be inefficient. Why make a table larger than it has to be?

The above is kind of confusing, so I’ll illustrate it. Say you have 4 students and 4 tests, but one students drops class and another gets extra credit. Then, you have a table like this:

A B A A
C D
A A A A A
B A B A

To take in all the scores in a rectangular array, you would need a 4 by 5 array because otherwise we couldn’t fit student#3′s scores. This allocates memory for 20 fields, even though we only need 15! This may not seem like a big deal, but imagine this table is now millions of rows long and imagine you have to visit each field to calculate class averages. This would clearly result in performance issues because the computer spends time visiting empty fields. In any case, with jagged arrays we can create tables of exactly the size we want. As far as I know, that’s all the benefit there is to them.

Because they are so confusing, I avoid them like the plague. You should use cleaner things like linked lists.

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