What caused this error message?

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% Define the input signal x[n]
n = 0:49;
x = (-1).^n .* (n >= 0);
% Define the length of the input signal
N = length(x);
% Initialize the output signal y[n]
y = zeros(1, N);
% Define the initial conditions
y_minus_1 = 4;
y_minus_2 = -2;
% Calculate the output y[n] using the difference equation
for n = 1:50
    if n == 1
        y(n) = x(n) + x(n - 1) - (1/4) * y_minus_1 + (1/8) * y_minus_2;
    elseif n == 2
        y(n) = x(n) + x(n - 1) - (1/4) * y(n - 1) + (1/8) * y_minus_1;
    else
        y(n) = x(n) + x(n - 1) - (1/4) * y(n - 1) + (1/8) * y(n - 2);
    end
end

Array indices must be positive integers or logical values.

% Plot the output signal y[n]
stem(0:N-1, y);
grid on;
xlabel('n');
ylabel('y[n]');
title('Output of the System');

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Array index should be positive integer or logical value.

Error: HW2_2_59_c (18th line)

y(n) = x(n) + x(n - 1) - (1/4) * y_minus_1 + (1/8) * y_minus_2;

Answer 1

[Raj]

Hi @대선

In MATLAB, array indices start from 1, so attempting to access x(0) or y(0) would result in this error.

 y(n) = x(n) + x(n - 1) - (1/4) * y_minus_1 + (1/8) * y_minus_2;

Here you are trying to access x(n-1) which is essentially x(0) when n=1.

This will throw the error stating array index should be positive or logical value. Make appropiate changes in your code to reolve this.

I hope this helps!

[John D'Errico]

Exactly so. Since this happens only in this branch of the if clause:

if n == 1
    y(n) = x(n) + x(n - 1) - (1/4) * y_minus_1 + (1/8) * y_minus_2;

it means @대선 neeeds to choose an appropriate boundary condition for the case where n==1, since x(0) is not accesible in MATLAB. However, @대선 has this information in hand, in the form of:

x = (-1).^n .* (n >= 0);

There we see that when n==0, we would have

x(0) = (-1)^0 = 1

This provides the boundary condition to apply, replacing x(n-1) with 1 in that initial branch.

if n == 1
    y(n) = x(n) + 1 - (1/4) * y_minus_1 + (1/8) * y_minus_2;

[대선]

Thank you!

But now I have another eror message..

What does it mean that index exceeds 1..??

Index exceeds the number of array elements. Index should not exceed 1.

Error: HW2_2_59_c (in line 14)

 y(n) = x(n) + x(n - 1) - (1/4) * y(n - 1) + (1/8) * y_minus_1;

[Raj]

Hi @대선, when are you facing this error?

As rightly mentioned by @John D'Errico, you can pre-compute x(0)=1 and calcualte y(1).

y(1)=x(1)+x(0)-(1/4)* y_minus_1 + (1/8) * y_minus_2;

You can later have the limit of for loop from n=2:50

for n = 2:50

    if n == 2
        y(n) = x(n) + x(n - 1) - (1/4) * y(n - 1) + (1/8) * y_minus_1;
    else
        y(n) = x(n) + x(n - 1) - (1/4) * y(n - 1) + (1/8) * y(n - 2);
    end
end

Check if this is what you intend on doing.