IF argument question I am seriously new to coding.

Ok, Im trying to output c1 for x from 0 to 10. Im looking for it to return c1=0 0 0 0 0 0 0 0 1 2 but it keeps giving me c1=-8 -7 -6 -5 -4 -3 -2 -1 0 1 2

if the function c is less than zero I am trying to tell it to make c1=0, and if c is greater than zero to make c1 whatever c would be.

function beam_displac(x)
L=0:x
c=(L-8)
if c <= 0
    c1= 0
else c1=c
end

What am I doing wrong? How might I do this? Thanks

Answer 1

[Image Analyst]

You mean like this:

c1 = max(0,x-8)

???

[Image Analyst]

By the way what you were going wrong was using c in the if statement. c<=0 produces a whole vector of 10 values, not a single true false value. It looks at the first value, which was negative so it went straight to the else and just had c1=c. If you wanted to do a comparison element by element you should have done this

% Initialize
c = x - 8;
% Find negative indexes as a logical vector.
negValues = c < 0;
% Initialize
c1 = c;
% Now assign only those to zero
c1(negValues) = 0;

Or, more compactly

c1 = x-8;
c1(x<0) = 0;

[Jason]

c1 = max(0,x-8)

Thank you! Thats exactly what I needed to make my code work. Can you explain exactly what this means? I read it as your telling matlab that c1 can only have values between 0 & x-8. is that accurate?

I thought max function just picked out the maximum value in an array. How is this different?

[Image Analyst]

max does pick out the max. So if you're comparing 0 and -7 or -6 or any negative number, the max will be 0. If you pass in two array, or an array and a number, it compares element by element. So it compares

     -8    -7    -6    -5    -4    -3    -2    -1     0     1     2

to

      0     0     0    0     0     0     0     0     0     0     0

and the max element-by-element will be

0 0 0 0 0 0 0 0 1 2

because 0 is bigger than -8.....-1 so those elements will be set to 0.

Answer 2

[Walter Roberson]

Your "c" is a vector. When you have "if" with a vector argument, it is considered to be true only if all the vector values are considered true. Any non-zero value is considered true. If your x value that you pass in is less than 8, then none of your c values will be 0 and so the "if" will be considered true. But if your "x" value that you pass in is 8 or higher, then one of your c values is going to end up as 0 and the "if" will be considered false.

You should use a "for" loop, or you should use "logical indexing"

Answer 3

[Roger Stafford]

The trouble with your code is that 'c' as you have defined it, is a vector with many elements. When you write "if c<=0", the quantity c<=0 is a corresponding logical vector, and the 'if' condition is considered satisfied only if all the elements in the logical vector are true. That is clearly not what you intended. You apparently had in mind that the if-else-end form would be operated element-by-element making as many decisions as there are elements in c. That is not how if-else-end works. It either goes one way or the other way, one time only. Therefore you have to adopt some method which forces matlab to make individual decisions for each element. The relevant 'if' documentation reads: "An evaluated expression is true when the result is nonempty and contains all nonzero elements (logical or real numeric). Otherwise, the expression is false."