Conditional statements

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:

while condition do 
{instructions}

The code flows as follows: first, the condition is evaluated, and then, if it is satisfied, all the instructions within the curly brackets are executed one by one. This will be repeated over and over again until the condition does not hold anymore.

The main loop in our simulation can be programmed using a while statement like this

delta = tolerance;   // safe initial bet; could also be a large number
while (count < niter && delta >= tolerance) do {
  // specify boundary conditions for T
  count += 1;        // increase the iteration counter by one
  Tnew = T;          // will be replaced: calculate Tnew from T
  // update delta, the greatest difference between Tnew and T
  T = Tnew;          // update T once all elements of Tnew are calculated
  // print the temperature at [iout,jout] if the iteration is multiple of nout
}

Let’s focus, first, on printing the temperature every 20 interations. To achieve this, we only need to check whether count is a multiple of 20, and in that case, to print the temperature at the desired position. This is the type of control that an if statement give us. The general syntax is:

if condition then 
  {instructions A} 
else 
  {instructions B}

In our case we print the temperature at the desired position if the iteration is multiple of niter:

if count%nout == 0 then writeln('Temperature at iteration ', count, ': ', T[iout,jout]);

To print 0th iteration, we can insert just before the time loop

writeln('Temperature at iteration ', count, ': ', T[iout,jout]);

Note that when only one instruction will be executed, there is no need to use the curly brackets. % returns the remainder after the division (i.e. it returns zero when count is multiple of 20).

Let’s compile and execute our code to see what we get until now, 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=1000   # in MB
#SBATCH --output=solution.out
./baseSolver

and then submitting it:

$ chpl baseSolver.chpl -o baseSolver
$ sbatch serial.sh
$ tail -f solution.out

Temperature at iteration 0: 25.0
Temperature at iteration 20: 25.0
Temperature at iteration 40: 25.0
...
Temperature at iteration 500: 25.0

Of course the temperature is always 25.0, as we haven’t done any computation yet.