How can I correctly use if and elseif?

Hi there! I'm a newbie on the Matlab and I just don't know how to write this function.

I have two inputs : t and toe

I have to calculate

         tk = t-toe
         if tk > 302400  tf = tk - 604800
         if tk < -302400  tf tk + 604800

I created this function :

 function [tk] = time(a)
             a=t-toe;
          if tk>302400;       tf = tk-604800;
            elseif tk<-302400;       tf = tk+604800;
          end
          end

when I run this function I get this error.

% Undefined function 'time' for input arguments of type 'double'.

what does it mean "input arguments of type double"? what is the double there?

Thank you for your help!

P.S: How can I learn more quickly this program? It seems a bit complicated for me considering I've never learnt programming before. Could I find some exercises to help me learn it?

[Von Duesenberg]

The Matlab documentation would be a good place to start: https://fr.mathworks.com/help/matlab/ref/if.html

[KL]

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

and also https://www.mathworks.com/training-schedule/matlab-onramp.html

[Walter Roberson]

Notice by the way that used

              a=t-toe;

instead of tk = t-toe

Also, you have not defined toe.

Answer 1

[Stalin Samuel]

create a file with below code

   function [tf] = time(t,toe)
               tk=t-toe;
            if tk>302400;       tf = tk-604800;
            elseif tk<-302400;       tf = tk+604800;
            end
  end

while saving the code you must use "time" as file name.

[gblmtc]

Thanks Samuel! I applied your function, it works but it calculates just tk=t-toe. It doesn't either add or subtract 604800. Doesn't continue and do the if . Do you know why?

[Stephen23]

@gblmtc: What sizes are t and toe ?

Why did you accept this answer if it does not do what you want?

Answer 2

[Stephen23]

I guess that some of the inputs are non-scalar, in which case you should use indexing:

function tk = time(t,toe)
tk = t-toe;
idx = tk>302400;
tk(idx) = tk(idx)-604800;
idx = tk<-302400;
tk(idx) = tk(idx)+604800;
end

[gblmtc]

I apologize! I checked it again and it does work. At the beginning when I saw it didn't work I put unaccepted but didn't know how to change it back. I apologize again!

[gblmtc]

I'm new here and I don't really know how it works.

[Stephen23]

@gblmtc: indexing is a very basic concept that is used a lot in MATLAB. You can learn about indexing by doing the introductory tutorials, which are highly recommended for all beginners:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

and also by reading this article:

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

[gblmtc]

Thanks Stephen! I'll have a look on what you recommend. I need to learn Matlab but being at the very beginning it's quite challenging.

[Stephen23]

@gblmtc: I understand that there is a lot to learn, and it can seem quite daunting, but luckily MATLAB has easy-to-read documentation, which should always be the first place you look for information. You also might like to read this, it has some tips to keep in mind when learning MATLAB:

https://www.mathworks.com/matlabcentral/answers/228557-experts-of-matlab-how-did-you-learn-any-advice-for-beginner-intermediate-users

[Stephen23]

gblmtc's "Answer" moved here:

Yes, you are right! Sometimes it is really really daunting. Sometimes I just say it is not for me and I just can't do it. Thanks for your help!