if statement gives wrong results or does not work?

Hello,

I've got the following if statement within a for loop. In the beggining, i thought it's quite straight forward. I've got a scalar and an array, I have to compare each value of the array with that scalar and after that, to make certain calculations.

a_sat = -1.2;

a = [-5 -2 -7 0 -1 3 5];

for i = 1:length(a)

if a(1:length(a) ) >=a_sat

    b = 0.45;

else if a(1:length(a)) < a_sat

       b(i) = 3.*a;

    end

end

end

This gives an undefined value of b. If I try to remove the counter in the if's, then I get a wrong result for the value of b. can anyone please help me with that? Thank you!

Answer 1

[Iain]

"any" and "all" are your friends,

You have actually written

 if ALL of a >= a_sat
  set "b"
 else, if ALL of a < sat
  set "b"
 else % (i.e. some of a > sat and some < sat)
  do nothing
 end

I don't know what you're actually wanting.

I suspect what you want is something like this

 if a(i) > a_sat % if just the "ith" part of a is > a_sat
  b = 0.45; % set b to "max"
 else %if not...
  b = 3*a(i);  % b = 3* the "ith" part of a. 
 end

[Christina]

Hi Iain

Yes, that's similar to what I need.

How will I use 'any' or 'all' for that purpose though?

Thank you

[Iain]

It really depends on what you need.

 if all( a >= a_sat ) %if all of a is greater/equal to a_sat
  b = maxi;
 else %under all other circumstances
  b = something_else;
 end

 if any( a >= a_sat ) %if any of a is greater/equal to a_sat
  b = maxi;
 else % if none of a is greater/equal to a_sat
  b = something_else;
 end

What do you actually need?

[Chin Wan Elijah]

Hi Iain, I need something similar, but I have already put a 'min' in the 'if' condition, i.e. if min(result)<=-0.005 display('hello') end I checked the value for result, some of it actually fulfill the condition but there is no 'hello' displayed, what can this be caused by?

[Iain]

If result is a matrix, when you take the min, you get a row vector. For that to go into the "then" part of the if statement, the ENTIRE row vector needs to be less than or equal to -0.005.