Weird behavior of modulo function

Why is this code plotting a straight line?

syms x(t);
x(t) = mod(t, 3);
tSamp = -10:0.01:10;
figure(1);
plot(tSamp, x(tSamp));

mod(a, b) should return a value in the range [0, b), and here it is like mod(t, 3)==t.

What is happening???

Answer 1

[Walter Roberson]

Symbolic mod() does not do what you think it does. And that's a nuisance that has caught me multiple times as well!!

"If a is a polynomial expression, then mod(a,b) finds the modulus for each coefficient."

t is a polynomial in t, equivalent to 1*t + 0 . That is coefficients [1 0]. mod([1 0], 3) is [1 0], so mod(t,3) is 1*t + 0, which is just t.

You have to use something like t - 3*floor(t/3)

[Steven Lord]

Or just don't use symbolic variables and functions at all.

>> tSamp = -10:0.01:10;
>> plot(tSamp, mod(tSamp, 3))

or

>> tSamp = -10:0.01:10;
>> f = @(t) mod(t, 3);
>> plot(tSamp, f(tSamp))

[Milosh]

Thank you very much!

[Milosh]

@Steven No, I need the symbolic variables for some integration later...