Time of execution cannot be correctly measured.

Projects Programming
1

Hello Dear All

I'm writing a code to do matrices calculation where i'm having trouble measuring the time of execution.
If i only do the matrices calculation, the time couldn't be recorded correctly.
However, after the time printing, if i choose to print at least one cell of the array, the execution time of the calculation increases to a value.
For 1k or 100k calculation cycles, the time is always 4us which cannot be right.
After i enable the print code in red below, the time shows linearity vs the number of execution cycles.

I have also printed out the whole result array and they show the correct values.
Please help.

Thanks in advance.

Please see below Code:

byte i;    //array calculation index
byte j;
unsigned long a;

int mX[13][13];  
int mY[13][13];
long mZ[13][13];
unsigned long time_start;  //calculation time record
unsigned long time_end;
int sum=0;


void setup() {
Serial.begin(9600);
}

void loop() {
//Generate two 13x13 random matrices------------------------------------------------
for(i=0;i<13;i++)
{
  for(j=0;j<13;j++)
  { 
    mX[i][j]  = random(-128,127);
    mY[i][j]  = random(-128,127);
  }
}


  time_start=millis();  //startintg time--------------------------------------------

  for(a = 0; a < 1000; a++)
  { 
        for(i = 0; i < 13; i++)
        {
              for(j = 0; j < 13; j++)
              {
                     mZ[i][j] =2*mX[i][j]+mX[i][0]*mY[0][j]+ mX[i][1]* mY[1][j]+ mX[i][2]* mY[2][j]+ mX[i][3]* mY[3][j]+ mX[i][4]* mY[4][j]+ mX[i][5]* mY[5][j]+ mX[i][6]* mY[6][j]+ mX[i][7]* mY[7][j]+ mX[i][8]* mY[8][j]+ mX[i][9]* mY[9][j]+ mX[i][10]* mY[10][j]+ mX[i][11]* mY[11][j]+ mX[i][12]* mY[12][j]; //Z=2X+XY
              }

        }
   }
  time_end=millis();    //end time
  Serial.println((time_end - time_start));  //execution time

  //Serial.print(mZ[0][0]); [color=red]//!!!!!!!!!!!!! If i enable this line of code, the measured time changes.[/color]
   

  while(Serial.available()==0){}
  }
2

...and that's why we ask you to please remember to use code tags when posting code

3

To make it easy for people to help you please modify your post and use the code button </> so your code looks like this and is easy to copy to a text editor. See How to use the Forum

Your code is too long for me to study quickly without copying to a text editor.

...R

4

Consider contacting your classmate to discuss the assignment.

5

the time is always 4us which cannot be right.

You found that out by subtracting two millis values?

6

The compiler is optimizing the entire calculation out of existence because it has detected that the results of the calculation aren't being used - except when you print a result.
Try declaring the mZ array to be volatile which should make the compiler leave it alone.

volatile long mZ[13][13];

Pete

7

el_supremo:
The compiler is optimizing the entire calculation out of existence because it has detected that the results of the calculation aren't being used - except when you print a result.
Try declaring the mZ array to be volatile which should make the compiler leave it alone.

volatile long mZ[13][13];

Pete

Thanks a lot Pete.
This is indeed the problem where if i never use the matrix value again after calculation, the compiler just skipped the calculation.

Kind regards