How do I create a loop for an equation using if statements?

I want to define an equation/function, to later plot, that changes every cycle. If we consider each cycle to last for example 2 seconds, I want the equation to be defined as:

• x = y for the first cycle (from t=0s to t=2s);
• x = -y for every even number cycle (from t=2s to t=4s, from t=6s to t=8s, etc)
• x= c, where c is a constant for every odd number cycle (from t=4s to t=6s, from t=8s to t=10s, etc)

Initially, I thought of using if statements like here, but this would require making an if statement for each cycle, and therefore if I required a large number of cycles to be plotted the if statement would be very long.

How can I write this down in a way that I don't require a statement for each cycle to be plotted/equated?

[Cris LaPierre]

Take a look at Ch 13 of MATLAB Onramp. It introduces both if statements and for loops.

Answer 1

[Jan]

Use "logical indexing":

y = rand(1, 100);
t = 0:99;  % Prefer the cyclenumber, not the seconds
c = 17;

index    = (t == 0);
x(index) = y(index);
index    = (t > 0) & (rem(t, 2) == 0);
x(index) = -y(index);
index    = (t > 0) & (rem(t, 2) == 1);
x(index) = c;