Problem with my if else command

Hi all,

I have my code written as below, however it does not give me the plot that I want. Instead of using two equations according to the conditions. It gives me a graph using the else equation regardless of the value of x_15. Can anyone help me with this?

x_15=0:40:600;

if x_15<200

y_15=75*(b+500)/(a+50)*(cos(pi*x_15/200))+75*(b+500)/(a+50);

else

y_15=(exp(-x_15/150)).*(((7*(b+110)/(a+10))*(cos(pi*x_15/200)))+(7*(b+110)/(a+10)));

end

scatter(x_15,y_15);

title('S-Shaped Pipeline FEA Model');

xlabel('X position');

ylabel('Y Position');

Thank you.

Regards \

Wei Kang

Answer 1

[Stephen23]

An IF statement is not appropriate (without a loop). The simple MATLAB way is to use indexing:

>> a = 1;
>> b = 1;
>> x_15 = 0:40:600;
>> idx = x_15<200; % INDEX!
>> y_15 = (exp(-x_15/150)).*(((7*(b+110)/(a+10))*(cos(pi*x_15/200)))+(7*(b+110)/(a+10)));
>> y_15(idx) = 75*(b+500)/(a+50)*(cos(pi*x_15(idx)/200))+75*(b+500)/(a+50);
%      ^^^^^                                 ^^^^^ INDEXING!

And checking:

>> plot(x_15,y_15)

[Luna]

+1 :)