what does the expression " if ( ) && ( ) " do

 if (i>=njmp1) && (i<=njmp2) 
                        fcs(i)=a*i+b;
                    elseif (i>njmp2)
                        fcs(i)=fc2;
                        
                    end

Answer 1

[James Tursa]

&& is the "logical and" operator. You can read about it here:

https://www.mathworks.com/help/matlab/ref/logicaloperatorsshortcircuit.html

[ad lyn]

but when we use it in the previous statement is it gonna keep the "AND" fonctionnality?

Answer 2

[Walter Roberson]

Except for line numbering and issues about when interruptions are permitted, the code you posted is equivalent to

 if (i>=njmp1)
     if (i<=njmp2) 
        fcs(i)=a*i+b;
     elseif (i>njmp2)
        fcs(i)=fc2;
     else
         %do nothing
     end
 elseif (i>njmp2)
    fcs(i)=fc2;
 else
     %do nothing
 end

complete with the implication that i<=njmp2 will not be executed at all if i>=njmp1 is false.

Imagine that you have a situation where you have a variable in which a non-nan second element is to be used, provided that a second element exists:

 ub = inf;
 if length(bounds) > 1
    if ~isnan(bounds(2))
      ub = bounds(2);
    end
 end

this cannot be coded as

 ub = inf;
 if length(bounds) > 1 & ~isnan(bounds(2))
    ub = bounds(2);
 end

because the & operator always evaluates all if its parts, and so would always try to access bounds(2) even if length(bounds)>1 were false. The & operator is not smart about stopping if the condition cannot be satisfied. But

 ub = inf;
 if length(bounds) > 1 && ~isbounds(2))
   ub = bounds(2);
 end

is fine. The && operator is smart about not evaluating the second operand if the first is false -- not just "smart" but guarantees that the second part will not be evaluated. (In such a situation to just say it was "smart" implies that it is a matter of optimization, that it thinks it is "more efficient" not to evaluate the second operand -- but that implies that in some circumstances, perhaps involving parallel processing, if it happened to be more efficient to evaluate the second clause, that it could potentially be evaluated. But the && operand promises that the second operand will not be evaluated if the first one is false, so it is safe to use && with chains of tests that rule out the possibility of exceptional behaviour.)

[ad lyn]

thank you!!!