IF statement with multiple logical OR

I'm running a function that solves a given PDE using Euler's method

I attempt to plot the numerical solutions to the PDE at specific times (0.1, 0.5, 1.0 and 2.0) by testing equalities for t in an IF statement

However, only plots for t == 0.1 and t == 0.5 are plotted for some reason

I've tried debugging with breakpoints and the IF statement tests true when t == 0.1 and 0.5, but false when t == 1 and 2, even though the variable t in the workspace == 1 and 2

The relevant code is:

t = 0;
dt = 0.1;

for a = 1 : timesteps

      for b = 1 : nodes
          u = %solve PDE;
      end

      t = t + dt;
      round(t,1);

      if t == 0.1 || t ==0.5 || t == 1.0 || t == 2.0
          figure
          plot(t,u)
      end
  end

[John D'Errico]

NEVER test for exact equality of a floating point number. That test will fail most of the time. Note that 0.1 is not representable exactly in a binary arithmetic.

[George Holt]

Many thanks! I tested for inequalities instead and that works

Answer 1

[Jan]

This line does not perform ynathing, because theoutput is not stored:

round(t,1);

You cannot expect, that 0.1+0.1+0.1 is exactly 0.3, because the most decimal floating point values do not have an exact representation in the binary system, such that rounding errors occur.

Better use an arbitrary, but matching threshold:

if any(abs(t - [0.1, 0.5, 1.0. 2.0]) < 1e8)
   figure
   plot(t,u)
end

Add a meaningful comment to explain the size of the threshold.

Since R2015a:

if ismembertol(t, [0.1, 0.5, 1.0. 2.0])

[George Holt]

I used

if (0 < t) && (t < 0.2) || (0.4 < t) && (t < 0.6) %etc

but your solution is much nicer!

Thanks