Float array is working w/ max() & min() for me:

float[] a = {-10.4f, 6, 5.9999f, 65e3f, 2e-4};

println( max(a[0], a[1]) );
println( min(a[3], a[2]) );

I think I should explain this a bit better. All the numbers I need to use are from a separate file. These numbers are grouped into rows, with 3 numbers per row, separated by commas. I want 1 float[] to refer to the 1st number on each row, another for the 2nd, and another for the 3rd (AX, AY, and AZ respectively).

1. String[] lines;
2. float[] AX, AY, AZ, GX, GY, GZ;
3. float MX, MY, MZ = 0.0;
4. float mX, mY, mZ = 0.0;
5. int yo;
6. 
   
7. void setup() {
8.   size(500, 500);
9. 
   
10.   smooth();
11.   noLoop();
12.   lines = loadStrings("file.csv");
13. 
    
14.   AX = new float[1000];
15.   AY = new float[1000];
16.   AZ = new float[1000];
17. }
18. void draw() {
19.   translate(0, yo);
20. 
    
21.   stroke(255);
22.   strokeWeight(2);
23.   line(0, 0, width, 0);
24. 
    
25.   for (int k = 1; k < lines.length; k++) {
26.     String[] pieces = split(lines[k], ',')
27.     for (int i = 0; i < pieces.length; i++) {
28.       AX[i] = float(pieces[1]);
29.       AY[i] = float(pieces[2]);
30.       AZ[i] = float(pieces[3]);
31. 
    
32.       //This is where I figure I define the maximum and minimum values of each column. I tried:
33. 
    
34.       MX = max(AX[i]);
35.       MY = max(AY[i]);
36.       MZ = max(AZ[i]);
37. 
    
38.       mX = min(AX[i]);
39.       mY = min(AY[i]);
40.       mZ = min(AZ[i]);
41.     }
42.   }
43.   point(100, MX);
44.   point(100, mX);
45.   point(250, MY);
46.   point(250, mY);
47.   point(400, MZ);
48.   point(400, mZ);
49. }

Oh! I didn't know that max() & min() were able to compare whole arrays! 

I don't have access to your CSV file. So I'm debugging your code blindly!  

Line #26 -> I believe pieces[] contains all elements present in the current line, right?

Lines #27 - #30 -> you have already decided you wanna pick 2nd + 3rd + 4th elements from pieces[] .
So, why is this for loop block for, since you're grabbing a big bite outta pieces[] at once?
Generally, for loops are used to interact w/ one element at a time, repeating this procedure until all elements are dealt with.

My advice is to just get rid of this for (int i = 0; i < pieces.length; i++) . That lone for (int k = 1; k < lines.length; k++) is enough!  (Warning: UNTESTED!)

for (int k = 1; k = lines.length;) {
    String[] pieces = split(lines[k++], ',');

    float AX = float( pieces[1] );
    float AY = float( pieces[2] );
    float AZ = float( pieces[3] );

    float MX = max(AX);
    float MY = max(AY);
    float MZ = max(AZ);

    float mX = min(AX);
    float mY = min(AY);
    float mZ = min(AZ);

    point(100, MX);
    point(100, mX);
    point(250, MY);
    point(250, mY);
    point(400, MZ);
    point(400, mZ);
}