Random(0,2) gives bad results

Question

All I need to do is generate a random integer between 0 and 3 for example. I am using this function:

void Crossover(int parents[2][3])
{
  int ranDpart;
  int ranDparent;
  for(int i=0; i<10; i++)
  {
    for(int j=0; i<3; j++)
    {
      ranDpart = random(0,3);
      ranDparent = random(0,2);
      newGen[i][j] = parents[ranDparent][ranDpart];
    }
  }
}

where as you see parents is input with size of 2x3 and newGen is 10x3 2D array. As seen here in the code, this function is supposed to randomly take one of parent and a part of this parent to make a new "child". Am I doing something wrong? I think that random gives me bad results. I tried some online compiler and random gives me numbers like "1804289383" even if I tried to make it long. Can you guys help me with random int?

Answer 1

stdlib.h defines a few psuedo-random number functions. I am not familiar with a random function taking two arguments, presumably the low and high end of the desired range, that you've specified in your code.

For what it's worth, in plain old C use the modulo operator to achieve the range you want using stdlib's srand and rand functions:

/* seed psuedo-random number generator during setup with something */
srand(0xdeadbeef);

and in your function simply use modulus to get your range:

int ranDpart = rand() % 3; /* range: 0,1,2 */
int ranDparent = rand() % 2; /* range: 0,1 */

If your ranges should be inclusive e.g. [0,3] and [0,2] then

int ranDpart = rand() % 4; /* 0,1,2,3 */
int ranDparent = rand() % 3; /* 0,1,2 */

Answer 2

Unfortunatly, "real random" value don't really exist in computer. If you just call a random function many time, you'll get (in many cases) same result. For that, before calling a random generator, you have to set a "seed". The example here: https://www.arduino.cc/reference/en/language/functions/random-numbers/random/ give a solution, using:

 randomSeed(analogRead(0));

If the PIN 0 (used in this example) is not connected, you'll get some "noise" on it and this will create the seed for your random function.

Answer 3

The random(a,b) function does work. If it doesn't there is something else going on. You mention the program is long. A couple of things to remember when programming Arduinos is that

1. In C there is no boundary checking, so when an array is indexed out of range unexpected behavior or crashes occur.
2. Arduinos have very little RAM. The UNO only has 2K. Part of that is used for the stack. When memory runs low the stack can overwrite program variables and again create unexpected results or crashes e.g. endless reboots.

There are some functions that return the amount of free RAM available at run time, use those with Serial.print() for debugging. There are some freeRam() functions here, the last one works well for me (with UNO)