# How can I correctly use if and elseif?

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gblmtc on 22 Mar 2018
Commented: Stephen Cobeldick on 22 Mar 2018
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 on 22 Mar 2018
The Matlab documentation would be a good place to start: https://fr.mathworks.com/help/matlab/ref/if.html
KL on 22 Mar 2018
Walter Roberson on 22 Mar 2018
Notice by the way that used
a=t-toe;
instead of tk = t-toe
Also, you have not defined toe.

Stalin Samuel on 22 Mar 2018
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 on 22 Mar 2018
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?
Stephen Cobeldick on 22 Mar 2018
@gblmtc: What sizes are t and toe ?
Why did you accept this answer if it does not do what you want?

### More Answers (1)

Stephen Cobeldick on 22 Mar 2018
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

Show 3 older comments
gblmtc on 22 Mar 2018
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.
Stephen Cobeldick on 22 Mar 2018
@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:
Stephen Cobeldick on 22 Mar 2018
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!