If/else Statement Being Ignored

Hello, I'm relatively new to MATLAB and I think I have an if/else statement that appears to be ignored by my code. I am using the statement to define variables for later in my code. When I say "ignored" it appears as though my code only recognizes the second set of variables listed. Meaning, if I set A2 = 10, the code reads A2 = 10 for all t (time), even when t<50. If i set it such that A2 = 10 is in the "if" portion (t<50), it will use A2 = 0 for the entire run. I am going to only include the relevant portions as best I can because the rest of the entire document is simply just a long list of variables and their values that is then called by another script. Any ideas or help would be greatly appreciated.

global k Xind B C D E F G I K W Z Y A1 A2 X0

tTreat = 50;

if (t < tTreat)
    B = 0;
    C = 0;
    D = 0;
    E = 0;
    F = 0;
    G = 0;
    I = 0;
    K = 0;
    W = 0;
    Y = 0;
    Z = 0;
    A1 = 0;
    A2 = 0;

else
    B = 0;
    C = 0;
    D = 0;
    E = 0;
    F = 0;
    G = 0;
    I = 0;
    K = 0;
    W = 0;
    Y = 0;
    Z = 0;
    A1 = 0;
    A2 = 10;
end

X0 = zeros(404,1);

k = zeros(148,1);

Xind = zeros (138,1);

tspan = 0:0.01:100;
X0(135) = 1.41e3; %(etc.)

[Adam]

Learn to use the debugger. It will help you immensely to quickly understand what is going on. If 't' is scalar or vector will be immediately obvious if you just put a breakpoint in.

[Image Analyst]

Adam's answer is very good. Sooner or later (hopefully sooner) you'll need to learn to use the debugger (like the rest of us) to step through your code to examine variables and see what lines of code get executed. See this tutorial http://blogs.mathworks.com/videos/2012/07/03/debugging-in-matlab/

[Guillaume]

While totally unrelated to the problem (which has been answered), the use of so many global variable is a likely source for future bugs. The further lack of imagination in the naming of these globals will also in great likelyhood lead to bugs, and certainly won't help with maintaining the code.

Answer 1

[Image Analyst]

Setting B=10 has no effect on the test in the "if" statement. The if statement only looks at whether t is less than tTreat, and doesn't care about whether B is 10 or any other number. Why do you think B should have any effect on whether the "if" or the "else" block is executed?

[Matt]

I didn't mean to imply the two were related. There was also a typo, there should not have been a B in the post. I know that the it/else statement is not related to the actual values when it is executing, but for some reason the code is choosing to ignore the if/else statement as a whole it appears.

Answer 2

[Walter Roberson]

Your "t" is a vector or array, and not all of the elements are < 50 . or t is empty.

If you have vector t then t<tTreat calculates a vector of logical results. Then the if statement only considers the condition to be true if all of the results are non-zero, which fails if the result is empty and fails if some of the results were true but others were not.

If you are working with vectors or arrays then you should be using loops to handle the situation one at a time, or you should be switching to using logical indexing to construct vectors of results. For example,

    zt = zeros( size(t) );
    B = zt;
    C = zt;
    D = zt;
    E = zt;
    F = zt;
    G = zt;
    I = zt;
    K = zt;
    W = zt;
    Y = zt;
    Z = zt;
    A1 = zt;
    A2 = zt;

    mask = t >= tTreat;
    A2(mask) = 10;

Here all of the variables would have values the same size as t so that you can select the value on a per-member-of-t basis. Then you use logical indexing to set the other value only in the portions of the variable that need the other value.

[Matt]

My "t" should not be a vector or array I believe, it should be a scalar (it represents time). Using this seems to only generate an error message: "In an assignment A(:) = B, the number of elements in A and B must be the same."

[Guillaume]

Well, clearly if you get this error one or both of t or tTreat is not scalar.

'My "t" should not be a vector or array I believe'

There should not be any should or I believe. Use the debugger as you've been told (just double click in the grey margin to put a breakpoint on the if line) and see what it actually is.