How do you make a program read a function for a script and creating a function?

I am very new to matlab. So sorry if I am a noob at programming simple things like this. What I am trying to do is, insert a vector and if the vector is a negative number turn it into a 0 and keep the rest of the positive number the same. so for this code it should pop out [12 0 4 0] but all I get is 12.

[Kenny Dang]

I have to use for and if statements

[Jan]

@Kenny: Please post the code as text in following questions. Screenshots do not allow to copy&paste. Thanks.

Answer 1

[Jan]

Omit the first loop, because it does not perform anything.

   for n = 1:1:x(i)

should be fixed to:

for i = 1:length(x)

to create a loop over all elements of the vector x.

The rest of your loop is fine, when you omit the return. Set a semicolon behind "y(i) = x(i)" to suppress the output.

[Kenny Dang]

This is my code. But when I run it, it gives two answers, and they are both correct. Why am I getting both answers? I only want one.

global x
for i =1:length(x)
  if x(i) >0
      y(i) =x(i)
else y(i)=0;
  end
end

[Image Analyst]

What are the two "answers"? After the loop you have the x, still as it was originally. And you have y, which just has one set of values. What is the other "answer"?

[Jan]

@Kenny: Perhaps you mean theoutput to the command window caused by the forgotton semicolon after "y(i)=x(i)"? Avoid global variables. They cause more problems than they solve. A proper indentation helps to see the structure of the code:

for i = 1:length(x)
  if x(i) > 0
    y(i) = x(i);
  else
    y(i) = 0;
  end
end

[Guillaume]

As Jan says, avoid global. There's absolutely no reason to have global x. While we're at it, you should also preallocate y before the loop (otherwise it gets resized at each loop iteration, slowing the code down), so:

y = zeros(size(x));  %make y the same size and shape as x

And I would also replace length by numel, so that the loop works whether x is a vector or a matrix:

for i = 1:numel(x)   %numel is a lot safer than length

Answer 2

[Image Analyst]

Get rid of lines 2-13 and have this:

x(x<0) = 0;

[Kenny Dang]

Forgot to say I have to use if and for statements

[Image Analyst]

And you forgot to tag it as homework. I've done that for you. Get rid of lines 3 and 4 and make line 5 like Jan has. You might also initialize y before the loop

y = x;

and change lines 6-11 to this

if y(i) < 0
    y(i) = 0
end

You certainly don't want line 8 in there - why return from your function while you're looping????

Answer 3

[Guillaume]

Your code is very puzzling. You have for loop using i that does nothing:

 for  i = 1:j
 end             %there's nothing in the body of the loop!

At the end of this loop, i is equal to j (what meaningless variable names!). Then you have an n loop that goes from 1 to the value of x(i), which has established is x(j), and thus -5. Hence that loop will not even execute. Even if it did start, there's nothing in the loop that depends on n.

I really don't understand what you were trying to do with these loop. In any case, no loop is needed:

x = [12 -3 4 -5];
y = x;
y(y<0) = 0