Loop Trough Time t = t+dt

I have a simple code like this

t = 1380;

dt = 0.1

for i = 1:1000

t = t+dt;

disp(t)

end

I believe the answer should be obvious that the final answer should be

t = 1480. I dont know why Matlab shows the answer a little bit different which is 1479.99999999991.

Anyone know why?

Answer 1

[Star Strider]

You have encountered floating-point approximation error.

See the documentation section on Floating-Point Numbers, and the colon operator.

[Obaja Triputera Wijaya]

Is there anything that we can do? because inside the loop, I would like to add If statement.

if t == 1400

a = 2

end

but the problem is the t nver reach that value because of the floating error

[Star Strider]

Yes.

Change it to:

if abs(t - 1400) < 0.05
    a = 2
end

Since the code counts up by ‘dt’, this will introduce a tolerance in the calculation, so the floating-point approximation error are taken into account.

To see this graphically:

t = linspace(1399, 1401);

figure
plot(t, (abs(t - 1400) < 0.05))
grid

That will show the effect of using the inequality to test for a range of values for ‘t’ near 1400.

Experiment to get the result you want.

[darova]

Don't use equal sign for float numbers

if abs(t-1400) < 1e-6    % tolerance
    a = 2;
end