I'm trying to execute a block of code if one of several conditions is met. Again, for clarity, if any of the conditions are true, I'd like for the code to execute (chain of logical ORs).
When I type this code into Matlab:
if strmatch(T,'Sd') || strmatch(T,'SuperDev') || ...
strmatch(T,'lrnTrialVD') || strmatch(T,'lrnTrialVIS') || ...
strmatch(T,'lrnTrialTARGET') || strmatch(T,'lrnTrialAUD')
I get this error:
??? Operands to the || and && operators must be convertible to logical scalar values.
Can someone please tell me where I've gone wrong?
/blz
EDIT: I used the wrong function. strcmp is what I meant to use!
I believe this is because the return value of strmatch() is an array, not a scalar, and || might not be defined on array arguments. I don't have MATLAB in front of me (only Octave), but does [1, 2, 3] || [4, 5, 6, 7] work for you?
Additonally, it would be better to match a regular expression like (Sd|SuperDev|lrnTrial(VD|VIS|TARGET|AUD)) which is a bit more compact, more readable, and only needs to inspect the string 'T' once (not six times.)
This would look like:
octave-3.2.4.exe:10> T1 = "Sd"
T1 = Sd
octave-3.2.4.exe:11> T2 = "Lollipop"
T2 = Lollipop
octave-3.2.4.exe:12> regexp(T1,"(Sd|SuperDev|lrnTrial(VD|VIS|TARGET|AUD))" )
ans = 1
octave-3.2.4.exe:13> regexp(T2,"(Sd|SuperDev|lrnTrial(VD|VIS|TARGET|AUD))" )
ans = [](1x0)
strmatch doesn't necessarily return something which can be converted to a logical scalar value. It might, for example, return a vector of match locations which does not convert without further effort on your part. It's all there in the error message and the product documentation, it really is !
Related
I am importing a file into a function using [filepath, name, ext]=fileparts(thisFile);. I would then like to use a series of if statements that will operate on the file depending on its type; however, come files will have the same initial set up and so I would like to be able to include them all in an if statement. My current implementation: elseif (ext == '.s3p'). If I put in something like: elseif (ext == '.s2p' || '.s3p') the compiler whines,
Operands to the || and && operators must be convertible to logical scalar values.
You see my logic. Is there anything that I can do to make this work?
Thank you in advance!
You probably want strcmp, not ==. With a==b you get element-wise comparison. So you get a logical vector as result; and if a and b have different number of characters you get an error.
The fact that the output of == is a logical vector of element-wise comparisons is what causes the m-lint message. || and && require scalar arguments.
Lastly, in (ext == '.s2p' || '.s3p') you probably meant (ext == '.s2p' || ext == '.s3p').
Combining all of the above:
elseif strcmp(ext,'.s2p') || strcmp(ext,'.s3p')
For clarity and readability, you may prefer to use the ismember function with a cell array of the extensions to compare with:
elseif ismember(ext, {'.s2p' ,'.s3p'})
or, as pointed out by Cris Luengo,
elseif any(strcmp(ext, {'.s2p', '.s3p'}))
Better yet, instead of a series of elseif statements you may prefer to use switch, which implicitly applies strcmp / ismember for each case:
switch ext
case {'sp2', 'sp3'}
% Do stuff
case {'aaa', 'bbb'}
% Do stuff
otherwise
error('Unrecognized extension')
end
Consider the following code
t = ones(3,5)
Ind2save = find(t(1,:) == 0,1,'first')
So for example I am trying to find if even the first zero of the first row, so if the first element is a non zero then
if(Ind2save ~= 1 )
disp('no')
end
now for the above condition it doesn't display 'no' because the condition is not fulfilled but because all the rows are filled and Ind2save is an empty matrix so we another condition to check if it is fully filled then
if(Ind2save > 1 || isempty(Ind2save))
disp('no')
end
I get the following error
Operands to the || and && operators must be convertible to logical scalar values.
I searched for the reasons due to which this error is caused and in majority of the cases people were comparing two vectors so a better idea was to replace || with | but in my case the conditions are never vectors but Ind2save > 1 returns an empty matrix , does anyone know why is the reason for that? How can I accommodate both the conditions?
The issue is because in your case Ind2save is empty ([]) therefore the first part of your condition can't be used with || since [] > 1 doesn't yield a logical scalar (it results in []).
In order to fix this, you can to flip the order of your conditions such that you check if the array is empty first.
if isempty(Ind2save) || Ind2save > 1
The reason that this works is that if Ind2Save is empty, then the first condition evaluates to true therefore short-circuiting the rest of the checks.
You may have other issues if for some reason Ind2save is a vector. In that case you could need to so something to convert it to a logical scalar:
if isempty(Ind2save) || ismember(1, Ind2save)
I have this section of code. I want to find the closest node among the two sets of base stations (ABS and RBS). The way i am doing it is like this but the find function gives me an error. Both the distance matrices is of the same size.
closest_dist=min(distance_ua(iduser,:),distance_ur(iduser,:));
closest_node=(find(distance_ua(iduser,:)==closest_dist)) || (find(distance_ur(iduser,:)==closest_dist));
find returns an array of index values and || only works for values which can be converted to logical scalars. An array of integers cannot be convert to a logical scalar.
[1 2 3] || [1 2 3]
Operands to the || and && operators must be convertible to logical scalar
values.
If you want to use the logical OR, you're better off using find after the operation which would be performed directly on the logical arrays. Additionally, you'll want to use | instead of || to compare two logical arrays.
closest_node = find(distance_uid(iduser, :) == closest_dist | ...
distance_ur(iduser, :) == closest_dist);
MATLAB operators usually translate into a function form as in the following examples:
~A => not(A)
A + B => plus(A,B)
A(...) => subsref(...)
A(...) = ... => subsasgn(...)
etc.
Now please consider the operators && and ||.
The various documentation (1-doc for or, 2-doc for and, 3-the MATLAB Programming Fundamentals ebook), does not shed any light on this, and nor do help and, help or, help relop.
This also didn't help: profile('on','-detail','builtin').
What I can say is that | seems to be redirected to or() judging by the following example:
>> 1 || [0,0]
ans =
1
>> 1 | [0,0]
ans =
1 1
>> or(1,[0,0])
ans =
1 1
>> 1 && [0,0]
Operands to the || and && operators must be convertible to logical scalar values.
So my question is: assuming it's possible - how can one explicitly call the underlying function of && and ||?
(note: this question deals with the aspect of "how", not "why")
There can't be a function implementing the underlying functionality. Assume there is a function scor which implements this operator, then calling scor(true,B) would evaluate B before calling scor, but the operator does not evaluate B.
Obviously scor could be defined scor=#(x,y)(x||y), but it will evaluate B in the upper case.
/Regarding the comment using function handles, this might be a workaround:
%not printing a:
true||fprintf('a')
%printing a:
scor=#(x,y)(x||y)
scor(true,fprintf('a'))
%not printing a:
scor(true,#()(fprintf('a')))
What is the difference between the | and || logical operators in MATLAB?
I'm sure you've read the documentation for the short-circuiting operators, and for the element-wise operators.
One important difference is that element-wise operators can operate on arrays whereas the short-circuiting operators apply only to scalar logical operands.
But probably the key difference is the issue of short-circuiting. For the short-circuiting operators, the expression is evaluated from left to right and as soon as the final result can be determined for sure, then remaining terms are not evaluated.
For example, consider
x = a && b
If a evaluates to false, then we know that a && b evaluates to false irrespective of what b evaluates to. So there is no need to evaluate b.
Now consider this expression:
NeedToMakeExpensiveFunctionCall && ExpensiveFunctionCall
where we imagine that ExpensiveFunctionCall takes a long time to evaluate. If we can perform some other, cheap, test that allows us to skip the call to ExpensiveFunctionCall, then we can avoid calling ExpensiveFunctionCall.
So, suppose that NeedToMakeExpensiveFunctionCall evaluates to false. In that case, because we have used short-circuiting operators, ExpensiveFunctionCall will not be called.
In contrast, if we used the element-wise operator and wrote the function like this:
NeedToMakeExpensiveFunctionCall & ExpensiveFunctionCall
then the call to ExpensiveFunctionCall would never be skipped.
In fact the MATLAB documentation, which I do hope you have read, includes an excellent example that illustrates the point very well:
x = (b ~= 0) && (a/b > 18.5)
In this case we cannot perform a/b if b is zero. Hence the test for b ~= 0. The use of the short-circuiting operator means that we avoid calculating a/b when b is zero and so avoid the run-time error that would arise. Clearly the element-wise logical operator would not be able to avoid the run-time error.
For a longer discussion of short-circuit evaluation, refer to the Wikipedia article on the subject.
Logical Operators
MATLAB offers three types of logical operators and functions:
| is Element-wise — operate on corresponding elements of logical arrays.
Example:
vector inputs A and B
A = [0 1 1 0 1];
B = [1 1 0 0 1];
A | B = 11101
|| is Short-circuit — operate on scalar, logical expressions
Example:
|| : Returns logical 1 (true) if either input, or both, evaluate to true, and logical 0 (false) if they do not.
Operand: logical expressions containing scalar values.
A || B (B is only evaluated if A is false)
A = 1;
B = 0;
C =(A || (B = 1));
B is 0 after this expression and C is 1.
Other is, Bit-wise — operate on corresponding bits of integer values or arrays.
reference link
|| is used for scalar inputs
| takes array input in if/while statements
From the source:-
Always use the && and || operators when short-circuiting is required.
Using the elementwise operators (& and |) for short-circuiting can
yield unexpected results.
Short-circuit || means, that parameters will be evaluated only if necessarily in expression.
In our example expr1 || expr2 if expr1 evaluates to TRUE, than there is no need to evaluate second operand - the result will be always TRUE. If you have a long chain of Short-circuit operators A || B || C || D and your first evaluates to true, then others won't be evaluated.
If you substitute Element-wise logical | to A | B | C | D then all elements will be evaluated regardless of previous operands.
| represents OR as a logical operator. || is also a logical operator called a short-circuit OR
The most important advantage of short-circuit operators is that you can use them to evaluate an expression only when certain conditions are satisfied. For example, you want to execute a function only if the function file resides on the current MATLAB path. Short-circuiting keeps the following code from generating an error when the file, myfun.m, cannot be found:
comp = (exist('myfun.m') == 2) && (myfun(x) >= y)
Similarly, this statement avoids attempting to divide by zero:
x = (b ~= 0) && (a/b > 18.5)
You can also use the && and || operators in if and while statements to take advantage of their short-circuiting behavior:
if (nargin >= 3) && (ischar(varargin{3}))