Control flow

Chapel, as most high-level programming languages, has different staments to control the flow of the program or code. The conditional statements are the if statement and the while statement.

The general syntax of a while statement is one of the two:

while condition do 
  instruction;
  
while condition {
  instruction1;
  ...
  instructionN;
}

With multiple instructions inside the curly brackets, all of them are executed one by one if the condition evaluates to True. This block will be repeated over and over again until the condition does not hold anymore.

In our Julia set code we don’t use the while construct, however, we need to check if our iteration goes beyond the \(|z|=4\) circle – this is the type of control that an if statement gives us. The general syntax is:

if condition then
  instruction1;
else
  instruction2;

and you can group multiple instructions into a block by using the curly brackets {}.

Let’s package our calculation into a function: for each complex number we want to iterate until either we reach the maximum number of iterations, or we cross the \(|z|=4\) circle:

proc pixel(z0) {
  const c: complex = 0.355 + 0.355i;   // Julia set constant
  var z = z0*1.2;   // zoom out
  for i in 1..255 {
    z = z*z + c;
    if abs(z) >= 4 then
      return i;
  }
  return 255;
}

Let’s compile and run our code using the job script serial.sh:

#!/bin/bash
#SBATCH --time=00:05:00      # walltime in d-hh:mm or hh:mm:ss format
#SBATCH --mem-per-cpu=3600   # in MB
#SBATCH --output=solution.out
./juliaSetSerial

$ chpl juliaSetSerial.chpl
$ sbatch serial.sh
$ tail -f solution.out