# 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?
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;
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?

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