Nested loop optimized with Java Iterators

Few days ago, i saw a piece of a geoprocessing algorithm code. This algorithm get a vector of points, each point with two spatial coordinates: (x,y), and calculate the euclidean distance between of each point with the whole points of the vector. The result is a beatifull matrix of distance between each points.

The problem was the follow: this algorithm use "Iterators" on the vector, as follows:

Iterator i1=layer.iterator();
while(i1.hasNext()){
i1.next();
Iterator i2=layer.iterator();
while(i2.hasNext()){
i2.next();
// calculate Euclidean distance
}
}

but it has a problem in optimization terms, because the distance between point #1 and point #2 is the same that between point #2 and point #1, i.e.,an optimized algorithm only need compute the superior diagonal of the matrix, beacuse the matrix of distances is simetric.

In above sample, with optimized algorithm, we need 3 iterations on the vector and symmetrize the matrix; in the other hand without optimization we need 9 iterations: n^2.

If we assume that the objects in the vector are unique, the best solution in order of do not lose the semantic of Iterators is the use the follows code:

Iterator i1 = l.iterator();
while (i1.hasNext()) {
Integer ii1 = (Integer) i1.next();
Iterator i2 = l.subList(l.indexOf(ii1),l.size()).iterator();
while (i2.hasNext()){
i2.next();
}
}

If we use two index in order to follow the loops, we can use the follows code:

int i=0;
Iterator i1 = l.iterator();
while (i1.hasNext()) {
c=0:
Integer ii1 = (Integer) i1.next();
Iterator i2 = l.subList(i,l.size()).iterator();
while (i2.hasNext()){
i2.next();
long distance = ....
matrix[i][j] = distance;
matrix[j][i] = distance;
c++;
}
i++;
}

In this way we can reduce the computing time in n/2.

Smart pointers. shared_ptr and normal pointer

Shared pointer can make our code more secure. shared_pointer is a kind of pointer that allow to others pointer share his container. And when no pointer points to a container, the garbage collector erase the container thus ensuring the reliable of the program in order to no consume the heap of the process.

If we use the next code, the above behaviour.

The follow code show the common error, creating new pointers and not erase the container.


If we compile and execute this two codes, we obtain the next statistical result:

Parallel algorithms. Java threads. Using multiple cores.

Today, computar machines has a multiples cores into his processors. The main problem is that the software engineers do not design the software accord this situation because they ignore this new technology or simply because they doesn´t want design parallel algorithms. This problem imply that the multiple core machines are underused.

I propose a simple problem: Adding an integer array of ten million positions with a due core machine.

1. Only one process
2. Four threads. A piece of vector by thread
3. Two threads. A piece of vector by thread

The results:

Java Readers and Writer. Mutual exclusion.

Think about this situation, you have a buffer and into the bufer has one data. We have several elements in this situation: One Writer, that provide tha data into the buffer, and many Readers, thtat need the data into the buffer; and one restriction, only one persona can read or write into the buffer at the same time.

In order to resolve this problem we can use a Monitor. A monitor is a Class that provide a mutual exclusion in this methods. To write this in Java, we can introduce the keyword synchronized in the method. For ejample, in Buffer.java:




We can see complete execution over Main class



Output:

run:
Reader 0 is waiting to get data (total buffer=0)
Reader 9 is waiting to get data (total buffer=0)
Reader 8 is waiting to get data (total buffer=0)
Reader 7 is waiting to get data (total buffer=0)
Reader 3 is waiting to get data (total buffer=0)
Reader 6 is waiting to get data (total buffer=0)
Reader 2 is waiting to get data (total buffer=0)
Reader 1 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 0 unlock
Reader 0 has read a data (buff=0)
........reader 9 unlock
Reader 9 is waiting to get data (total buffer=0)
Reader 4 is waiting to get data (total buffer=0)
Reader 5 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 8 unlock
Reader 8 has read a data (buff=0)
........reader 7 unlock
Reader 7 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 3 unlock
Reader 3 has read a data (buff=0)
........reader 6 unlock
Reader 6 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 2 unlock
Reader 2 has read a data (buff=0)
........reader 1 unlock
Reader 1 is waiting to get data (total buffer=0)
Reader 28 is waiting to get data (total buffer=0)
Reader 29 is waiting to get data (total buffer=0)
Reader 20 is waiting to get data (total buffer=0)
Reader 11 is waiting to get data (total buffer=0)
Reader 13 is waiting to get data (total buffer=0)
Reader 16 is waiting to get data (total buffer=0)
Reader 27 is waiting to get data (total buffer=0)
Reader 10 is waiting to get data (total buffer=0)
Reader 18 is waiting to get data (total buffer=0)
Reader 24 is waiting to get data (total buffer=0)
Reader 23 is waiting to get data (total buffer=0)
Reader 17 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 9 unlock
Reader 9 has read a data (buff=0)
........reader 4 unlock
Reader 4 is waiting to get data (total buffer=0)
Reader 21 is waiting to get data (total buffer=0)
Reader 22 is waiting to get data (total buffer=0)
Reader 19 is waiting to get data (total buffer=0)
Reader 12 is waiting to get data (total buffer=0)
Reader 25 is waiting to get data (total buffer=0)
Reader 26 is waiting to get data (total buffer=0)
Reader 14 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 5 unlock
Reader 5 has read a data (buff=0)
........reader 7 unlock
Reader 7 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 6 unlock
Reader 6 has read a data (buff=0)
........reader 1 unlock
Reader 1 is waiting to get data (total buffer=0)
Reader 15 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 28 unlock
Reader 28 has read a data (buff=0)
........reader 29 unlock
Reader 29 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 20 unlock
Reader 20 has read a data (buff=0)
........reader 11 unlock
Reader 11 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 13 unlock
Reader 13 has read a data (buff=0)
........reader 16 unlock
Reader 16 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 27 unlock
Reader 27 has read a data (buff=0)
........reader 10 unlock
Reader 10 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 18 unlock
Reader 18 has read a data (buff=0)
........reader 24 unlock
Reader 24 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 23 unlock
Reader 23 has read a data (buff=0)
........reader 17 unlock
Reader 17 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 4 unlock
Reader 4 has read a data (buff=0)
........reader 21 unlock
Reader 21 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 22 unlock
Reader 22 has read a data (buff=0)
........reader 19 unlock
Reader 19 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 12 unlock
Reader 12 has read a data (buff=0)
........reader 25 unlock
Reader 25 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 26 unlock
Reader 26 has read a data (buff=0)
........reader 14 unlock
Reader 14 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 7 unlock
Reader 7 has read a data (buff=0)
........reader 1 unlock
Reader 1 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 15 unlock
Reader 15 has read a data (buff=0)
........reader 29 unlock
Reader 29 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 11 unlock
Reader 11 has read a data (buff=0)
........reader 16 unlock
Reader 16 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 10 unlock
Reader 10 has read a data (buff=0)
........reader 24 unlock
Reader 24 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 17 unlock
Reader 17 has read a data (buff=0)
........reader 21 unlock
Reader 21 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 19 unlock
Reader 19 has read a data (buff=0)
........reader 25 unlock
Reader 25 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 14 unlock
Reader 14 has read a data (buff=0)
........reader 1 unlock
Reader 1 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 29 unlock
Reader 29 has read a data (buff=0)
........reader 16 unlock
Reader 16 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 24 unlock
Reader 24 has read a data (buff=0)
........reader 21 unlock
Reader 21 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 25 unlock
Reader 25 has read a data (buff=0)
........reader 1 unlock
Reader 1 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 16 unlock
Reader 16 has read a data (buff=0)
........reader 21 unlock
Reader 21 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 1 unlock
Reader 1 has read a data (buff=0)
........reader 21 unlock
Reader 21 is waiting to get data (total buffer=0)
The writer is writing data (buffer:1)
........reader 21 unlock
Reader 21 has read a data (buff=0)
The writer is writing data (buffer:1)
The writer is writing data (buffer:2)
The writer is writing data (buffer:3)
The writer is writing data (buffer:4)
The writer is waiting to write data. The buffer is full