I'm trying to sort a string array,
but I get the following error when sort() is called:

java.lang.NullPointerException
at processing.core.PApplet.sort_compare(PApplet.java:4934)

Can you see if something is wrong in my code ?

int imax=4;
String[] seq = new String[imax+1];
String[] seqSorted = new String[imax+1];

void setup() {
 size(100,100);
 noLoop();
}

void draw() {
 //
 println("A -------> string array is empty");
 for (int i=0; i<imax; i++) {
   println(seq[i]);
 }
 //
 float theta=1.45;
 int index=0;
 for (int c=0; c<imax; c++) {
     seq[index]=nf(c*theta%1,3,5)+" "+str(c);
     index++;
 }
 //
 println("B -------> string array is filled");
 for (int i=0; i<imax; i++) {
   println(seq[i]);
 }
 //
 seqSorted = sort(seq);// <========= runtime ERROR here
 for (int i=0; i<imax; i++) {
   println(seqSorted[i]);
 }
}

OK, Thanks (that was an unexpected error!)

Here is some explanation of what I'm trying to do:

In this program segment, I need to sort an array of float values, while keeping track of the index permutation.
So sorting {1.2, 0.7, 3.2} to {0.7, 1.2, 3.2},
has index going {0, 1, 2} to {1, 0, 2}.

To use the sort() function,
I concatenate Value and Index,
going {"1.2 0", "0.7 1", "3.2 2"} to {"0.7 1", "1.2 0", "3.2 2"}

Then I still need to extract the numeric values from the string.

Repeating the code:

int imax=4;
float[] seqFloat = new float[imax];
String[] seqString = new String[imax];
String[] seqStringSorted = new String[imax];

void setup() {
 size(100,100);
 noLoop();
}

void draw() {
 //
 //setup initial float array
 float theta=1.45;
 int index=0;
 for (int c=0; c<imax; c++) {
     seqFloat[index]=c*theta%1;
     index++;
 }
 //
 println("A -------> initial float array");
 for (int i=0; i<imax; i++)
   println(seqFloat[i]);
 //
 //setup string array
 for (int i=0; i<imax; i++) {
     seqString[i]=nf(seqFloat[i],3,5)+" "+str(i);
     index++;
 }
 //
 println("B -------> initial string array");
 for (int i=0; i<imax; i++)
   println(seqString[i]);
 //
 seqStringSorted = sort(seqString);
 println("C -------> sorted string array");
 for (int i=0; i<imax; i++)
   println(seqStringSorted[i]);
}