How to use vectors in conditional functions?

I have a function which accepts four numbers:

function [ force, stiff ] = myfunction( d_a, d_b, v_a, v_b)

After running my function I want to display a surf plot by putting 4 vectors through the function and then plotting.

In the function myfunction, when I introduce conditions (e.g. if, &&, ==) the function does not work. How can I get around this problem?

[Gerd]

Hi John,

can you please post some code because otherwise it is hard to tell where the problem is.

[Harry MacDowel]

Could you post the codes to further clarify your questions? I do not understand how you write your if condition codes so I cannot tell you what is your problem and how to get around it.

[John]

Thank you for your suggestions thus far. I have found a solution and will clarify the problem now for future people who come across this problem.

I have 4 vectors which get sent into a function. In that function the vectors run through if statements and return a single vector. This is where the error occurs:

-----??? Operands to the || and && operators must

-----be convertible to logical scalar values.

The solution I have found is using Paulo Silva's answer of idx and indexing the values where the conditions are true. This works well.

[Walter Roberson]

When you have vectors, you need to use | and & instead of || and && .

However, those will generate vectors of logical values. If you want to make decisions based upon the individual logical values, you need to loop or you need to use logical indexing. With a vector of logical values, you can use all() or any() if what you care about is the group behavior of the vector of values.

Answer 1

[Paulo Silva]

For John's example code:

idx=a>1 & a<3 %mark the index values where your condition is true
idx1=a>2      %mark the index values where your second condition is true
c=a(idx)+a(idx1)*2

[Sean de Wolski]

+1; and note Paulo's use of a single '&' for vector logical conditions.

[John]

This does not work as I get an error. Also note (from my example), 'a' may or may not be a vector. Any further ideas?

[Oleg Komarov]

@Paulo: idx = a > 1 && a < 3

[Walter Roberson]

&& won't work if "a" is a vector. & is more robust here.

John, what error do you get with Paulo's version?

[Oleg Komarov]

OMG I got confused.

[John]

Paulo Silva,

So far it this method is working, I will test it further now to see if it works under different conditions.

Thanks.

Answer 2

[Daniel Shub]

Without any more information regarding what doesn't work, I am just guessing ...

The == operator with a scalar returns a scalar, with an array it returns an array. You may want to replace

x == y

with

any(x == y)

or

all(x == y)

you may also want to replace && with just & and with |.

Answer 3

[John]

Thank you for your replies. I have created a sample code to illustrate what I am doing.

a = 3;  % Say, a is a vector
if a>1 && a<3
    b=1;
elseif a>2
    b=2;
end
c=a*b;  % Say, c is the resulting vector
        % with the relevant 'b' for each number in vector

I realise I can do the same thing with a for loop, however, I think there is a much easier method.

[Walter Roberson]

John, if "a" is in the range of being greater than 2 and less than 3, then that satisfies a>1 & a<3, so b=1 would be appropriate for that case according to your code. But then you have the "elseif" for a>2 : are you sure that for the range (2,3) you want b=1 and not the b=2 of the elseif ? Otherwise you might as well (for the vector case) use

if a>1 && a<3

b = 1;

elseif a>=3

b = 2;

end