Java中2D数组的实际大小（2D数组的填充区域）

Question

I'm having problem with my homework assignment, and hoping to find some help.

I have to read a text file into a 2D int array. The width of the 2D array is fixed at 5 and will never change. However, the height of the array may change depend on the length of the text file.

I have to begin with a 40x5 array; any file over 40 lines will not acceptable, but less than 40 will be OK. So, I may get only X rows of my array be filled (X <= 40).

I have to calculate the average of each column, so I need to how many elements in a column; Is there any way to get that? Or can I change the size of my original array (with all the data in it) to Xx5 after I know what X is?

The sample input file:

034 080 055 078 045
060 100 056 078 078
070 010 066 078 056
034 009 077 078 020
045 040 088 078 055

output should be:

48.6 47.8 68.4 78.0 50.8

Answer 1

I wouldn't recommend using a plain array for a dynamically expanding matrix like this. I recommend reading up on the List & ArrayList documentation for a good implementation.

From the Java doc: [ArrayList is a] resizable-array implementation of the List interface.

In effect, this is Java's solution to the constantly-defined array limitation of the C days.

Answer 2

What you need to do is calculate a sum of each separate column (easy; you don't need to keep the whole state, just the running total for each column) plus a count of the number of rows. Given that, computing the average of each column should then be easy. You can do all this in a single pass reading just one row at a time, and your solution should be able to handle files of any length.