if function not working

Hi, I'm trying to simulate an earthquake signal by adding 12 sine waves together (A,B,C...L). Each sine waves needs to start and stop at different times, and I assumed I can do this by creating an if function for each sine wave individually - however I cant get it to work properly. When I run the code and plot (t,earthquake) the graph shows nothing which suggests all the sine waves are zero the whole time :(

So I'm guessing there is a problem with the if functions?

Here is my code, would appreciate help if someone can tell me what's wrong with it. Thanks!!

close all;clc;

%%EARTHQUAKE = SUM OF 12 SINE WAVES A,B,C,D,E,F,G,H,I,J,K,L:
SCALER  = 1.7;
%%INPUTS
%S = acceleration(g) T = period(s)  start --------> end
Sa = 0.12;           Ta = 0.1;      A_start = 0;    A_end = 22;
Sb = 0.15;           Tb = 0.13;     B_start = 1;    B_end = 18;
Sc = 0.16;           Tc = 0.14;     C_start = 2;    C_end = 16;
Sd = 0.165;          Td = 0.15;     D_start = 3;    D_end = 15;
Se = 0.17;           Te = 0.17;     E_start = 4;    E_end = 14;
Sf = 0.166;          Tf = 0.2;      F_start = 3;    F_end = 19;
Sg = 0.14;           Tg = 0.4;      G_start = 2;    G_end = 28;
Sh = 0.12;           Th = 0.4;      H_start = 1;    H_end = 31;
Si = 0.1;            Ti = 0.5;      I_start = 0;    I_end = 40;
Sj = 0.04;           Tj = 1;        J_start = 0;    J_end = 20;
Sk = 0.025;          Tk = 1.5;      K_start = 0;    K_end = 45;
Sl = 0.02;           Tl = 2;        L_start = 0;    L_end = 45;

%%IMPLEMENTING SINE WAVE DURATIONS
if t>A_start
    A = Sa*SCALER*sin(t* 2*pi/Ta);
elseif t>A_end
    A = Sa*SCALER*sin(t* 2*pi/Ta);
else
    A = 0;
end

if t>B_start
    B = Sb*SCALER*sin(t* 2*pi/Tb);
elseif t>Bend
    B = Sb*SCALER*sin(t* 2*pi/Tb);
else
    B = 0;
end

if t>C_start
    C = Sc*SCALER*sin(t* 2*pi/Tc);
elseif t>C_end
    C = Sc*SCALER*sin(t* 2*pi/Tc);
else
    C = 0;
end

if t>D_start
    D = Sd*SCALER*sin(t* 2*pi/Td);
elseif t>D_end
    D = Sd*SCALER*sin(t* 2*pi/Td);
else
    D = 0;
end

if t>E_start
    E = Se*SCALER*sin(t* 2*pi/Te);
elseif t>E_end
    E = Se*SCALER*sin(t* 2*pi/Te);
else
    E = 0;
end

if t>F_start
    F = Sf*SCALER*sin(t* 2*pi/Tf);
elseif t>F_end
    F = Sf*SCALER*sin(t* 2*pi/Tf);
else
    F = 0;
end

if t>G_start
    G = Sg*SCALER*sin(t* 2*pi/Tg);
elseif t>G_end
    G = Sg*SCALER*sin(t* 2*pi/Tg);
else
    G = 0;
end

if t>H_start
    H = Sh*SCALER*sin(t* 2*pi/Th);
elseif t>H_end
    H = Sh*SCALER*sin(t* 2*pi/Th);
else
    H = 0;
end

if t>I_start
    I = Si*SCALER*sin(t* 2*pi/Ti);
elseif t>I_end
    I = Si*SCALER*sin(t* 2*pi/Ti);
else
    I = 0;
end

if t>Jstart
    J = Sj*SCALER*sin(t* 2*pi/Tj);
elseif t>J_end
    J = Sj*SCALER*sin(t* 2*pi/Tj);
else
    J = 0;
end

if t>K_start
    K = Sk*SCALER*sin(t* 2*pi/Tk);
elseif t>K_end
    K = Sk*SCALER*sin(t* 2*pi/Tk);
else
    K = 0;
end

if t>L_start
    L = Sl*SCALER*sin(t* 2*pi/Tl);
elseif t>L_end
    L = Sl*SCALER*sin(t* 2*pi/Tl);
else
    L = 0;
end

%%SUM & PLOT
earthquake = A+B+C+D+E+F+G+H+I+J+K+L;

plot(t,earthquake)
grid on

[Stephen23]

There is no problem with if. You just did not read the documentation, in particular this line: "An expression is true when its result is nonempty and contains only nonzero elements (logical or real numeric). Otherwise, the expression is false"

Think about what that means if the expression is non-scalar: what does that mean if there are two elements (answer: they both have to be true).

In any case doing this using if statements is going to be pointlessly complicated: you should simply use indexing and vectorized code.

Answer 1

[KL]

Hi,

Assuming t is a vector, which it should be, you should use indexing to compare it in the if conditions, for example,

if t(i,1)>=A_start && t(i,1)<=A_end
  % code
else
  % code
end

what exactly is the difference between if and elseif in your code?

[Cameron Aldred]

Thanks for the reply! there is no difference between if and elseif, I was just trying to split up the condition "A_start < t < A_end".

Slightly confused by what your answer means, do I need to define i as a variable?

[KL]

yes. something like this perhaps.

t = (0:0.01:2)';
A = zeros(length(t),1);
for i=1:length(t)
    if t(i,1)>=0 && t(i,1)<=1
      A(i,1) = 2*sin(2*pi*t(i,1));
    else
      A(i,1) = 0;
    end
end
plot(A)

[Stephen23]

Or learn how to use MATLAB efficiently by using logical idnexing:

>> t = (0:0.01:2)';
>> A = zeros(size(t));
>> idx = t>=0 & t<=1;
>> A(idx) = 2*sin(2*pi*t(idx));
>> A(~idx) = 0;

Why waste your life writing ugly loops as if MATLAB was some ugly low-level language like C++ ?

[KL]

oh yeah, that's indeed the better way. Thanks Stephen.