Why Matlab only run the "else" function equation even "a" fulfils the condition in "if" function? The graph is plotted based on the equation in "else" function only no matter whether "a" is less than 198 or not.

 EL=15;
 for X=3:3:EL;
    dx=600/X;
    a=0:dx:600;
    if a<198
        y=113+113*(cos(pi*a/200));
    else
        y=(exp(-a/100)).*(((113)*(cos(pi*a/200)))+(113)); 

 end

 figure(4)
 plot(a,y,'-ob')
 title('ABC','fontsize',15)
 xlabel('x-coordinate');
 ylabel('y-coordinate');
 end

Answer 1

[Rik]

If you are on R2018a you will see an m-lint warning that a is a non-scalar operator (and maybe earlier show it as well). The if statement uses an implicit call to all if you put a logical array as a parameter. Pre-allocate and use indexing. Also, you are overwriting all your data each and every loop.

EL=15;
figure(4),clf(4)
for X=3:3:EL
    dx=600/X;
    a=0:dx:600;
    y=zeros(size(a));
    a_=a(  a<198 );
    y   (  a<198 )=113+113*(cos(pi*a_/200));
    a_=a(~(a<198));
    y   (~(a<198))=(exp(-a_/100)).*(((113)*(cos(pi*a_/200)))+(113));
    plot(a,y,'-ob'),hold on
end
title('ABC','fontsize',15)
xlabel('x-coordinate');
ylabel('y-coordinate');

[Rik]

Did this suggestion solve your problem? If so, please consider marking it as accepted answer. If this didn't solve your question, please comment with what problems you are still having.

Answer 2

[the cyclist]

The reason is that when you check a vector of condition, such as

a=[0 200 400 600];
if a<198

then all of the elements of a must be less than 198, in order to enter the if portion of the control structure.