If Statement not working

Hello,

I am new to matlab and have a very, very basic issue. This is a simplified function, but it illustrates the problem I have in my other function.

Matlab seems to bypass my if conditions. Here is the function:

function y = functry(x)

Speed = 100;

if (x> 100)
    Speed = 50;
end
y = Speed * x;

end 

However, the Speed is taken to be 100 even when I input a high range, for example:

y_try = functry(1:150)
plot(1:150, y_try)

Here is still get a linear plot and no change for the Speed when x>100.

Any help would be much appreciated! Thank you!

Answer 1

[Ameer Hamza]

If x is a vector then the line

if (x> 100)

does not work as expected. The correct syntax is

function y = functry(x)
    Speed1 = 100;
    Speed2 = 50;
    mask = x > 100;
    y = (~mask)*Speed1.*x + mask*Speed2.*x;
end

[isabella Anglin]

Thank you! It works!

However, I don't understand, what does the ~ mean? and why do we add the parts together?

[Ameer Hamza]

~ is equivalent to not (~) logical operator. It reverses the logical value, i.e., true to false and false to true.

variable mask is a logical array. ~mask and mask has opposite logical state, so for each row only a single value is added.

[isabella Anglin]

Okay great thank you!

However, eventually in my function i'll want to have multiple "Speeds". Do i just make additional masks then?

[Ameer Hamza]

Yes. Let me show you a more general approach

function y = functry(x)
    Speed1 = 100;
    Speed2 = 50;
    mask1 = x < 100;
    mask2 = x >= 100;
    y = (mask1)*Speed1.*x + mask2*Speed2.*x;
end

Just keep making a mask like this, and it will work. Note that If you want to have a condition that x is between 150 to 250, then the correct syntax to write that is

mask = (150 < x) & (x < 250) % mask = 150 < x < 250 is a common mistake.