Output Argument Error - How to Define The Output Argument for a Function

I'm trying to create and use the following function:

function [dctmatrix]=dctmatrix(i,j,N)

if i==1
    dctmatrix=sqrt(1/N);
elseif i>=2
    dctmatrix=sqrt(2/N)*cos((pi*(2*j-1)*(i-1))/(2*N));
end
end

But whenever I try to use it as follows, I get this error:

i=1:512;
j=1:512;
Y=dctmatrix(i,j,512);
Yinv=transpose(dctmatrix(i,j,512));
X_gray1comp=Y*X_gray1*Yinv;

Output argument "dctmatrix" (and maybe others) not assigned during call to "dctmatrix".

Error in Project2code (line 100)
Y=dctmatrix(i,j,512);

I thought that since I said "dctmatrix=..." in the code for the function itself, that that counted as an output argument. I don't understand how what I've done is wrong. :/

Answer 1

[James Tursa]

Don't have the return variable name the same as the function name. Use a different name for the return variable (e.g., "result"). Also, you need to use vectorized calculations if you want your function to operate on non-scalar inputs for i and j. So you will need to adjust your i==1 and i>=2 tests to account for vector inputs, and you will need to adjust your cos(etc) calculation for vector inputs.

[Mel Smith]

I made the following adjustments and got the same error:

function dct=dctmatrix(i,j,N)

if i==[1]
    dct=sqrt(1/N);
elseif i>=[2]
    dct=sqrt(2/N)*cos((pi*(2.*j-1).*(i-1))/(2*N));
end
end

i=[1:512];
j=[1:512];
Y=dctmatrix(i,j,512);
Yinv=transpose(dctmatrix(i,j,512));
X_gray1comp=Y*X_gray1*Yinv;

[Steven Lord]

The body of the if section will be executed only if ALL of the elements of i == 1 are true. You're calling this function with i = 1:512. Are ALL the elements of 1:512 equal to 1?

Similarly, the body of the elseif section will be executed only if ALL the elements of i >= 2 are true. Are ALL the elements of 1:512 greater than or equal to 2?

You're probably going to want to use logical indexing in this situation.

[Walter Roberson]

Suppose an input was less than 1, or was in the range (1, 2) exclusive such as sqrt(2), then you would not generate an output.