Help about function with if statement

I have problem with creating function. I don´t know how to use command function properly. Can you guys tell me where I was wrong it will be very helpful

function [y]=task1(x)

if x>=8

       y = 14*(5*x)^(1/2)+9;
end

if (-1<=x<8)

         y=9*x +9;
end

if (x<-1) y=9;

end

[Jan]

Why do you assume, that something is wrong? Which problem do you observe? Do you get an error message?

Answer 1

[laaj]

function output=task1(x)

if x>=8

         y = 14*(5*x)^(1/2)+9;
  end

if (-1<=x<8)

           y=9*x +9;
  end

if (x<-1) 
         y=9;

end
output=[y];

I don't know much but I just hope it will solve the problem.

Answer 2

[Jan]

"if (-1<=x<8)" does most likely not do, what you expect. Matlab processes this expression from left to right: "-1<=x" replies TRUE or FALSE depending on the value of x. In the next step you have "TRUE<8" or "FALSE<8". While TRUE is converted to 1, FALSE is a 0, and both values are smaller than 8. You need two comparisons: "-1 <= x" and "x < 8":

function y = task1(x)
if x >= 8
  y = 14 * (5 * x)^(1 / 2) + 9;
elseif -1 <= x && x < 8
  y = 9 * x + 9;
elseif x < -1
  y = 9;
end

Now you can simplify this: If you have excluded x>=8 already, you do not have to check for x<8 anymore:

function y = task1(x)
if x >= 8
  y = 14 * sqrt(5 * x) + 9;   % SQRT is cheaper than ^0.5
elseif -1 <= x
  y = 9 * x + 9;
else
  y = 9;
end