Errors Within an If Statement

Question

Han 2020-5-1

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/522296-errors-within-an-if-statement

回答： Hussein 2023-8-20

I keep getting these errors and I'm not sure why. Could somebody check it over and see where I'm exactly going wrong?

function questionsasked()
question1=questdlg('Are you ready to start?',...%question box
    'Questions',...
    'Yes','No','No');
question2=questdlg('Do you know the keys on the piano?',...
    'Questions',...
    'Yes','Somewhat','No','No');
question3=questdlg('How did you learn?',...
    'Questions',...
    'Self-Taught','Lessons','Other','Other')
question4=questdlg('How long did you take lessons for?',...
    'Questions',...
    '<1 year','1-10 years','>10 years','>10 years')
question5=questdlg('Are you still taking lessons?',...
    'Questions',...
    'Yes','No','No')
%user must click yes in order to continue
while ~strcmp(question1,'No')%<SM:WHILE>
    if strcmp(question1,'Yes')
        if strcmp(question2,'Yes')
            if strcmp(question3,'Self-Taught')
            else strcmp(question3, 'Lessons')
                if strcmp(question4,'<1 Year')
                    if strcmp(question5,'Yes')
                    else strcmp(question5,'No')
                    end
                else strcmp(question4,'1-10 year')
                    if strcmp(question5,'Yes')
                    else strcmp(question5,'No')
                    end
                else strcmp(question4, '>10 years') %error
                    if strcmp(question5,'Yes')
                    else strcmp(question5,'No')
                    end 
            else strcmp(question3, 'Other')%error                         
        else strcmp(question2,'No')%error
                    
                end
            end 
        end       
    end                     
question1=questdlg('Are you ready to start?',...%question box
    'Questions',...
    'Yes','No','No');
end 

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Han 2020-5-1

It says parse error at else. But not all of the else's, only that ones that have the error comment. I'm unsure as to what this means.

Charleston Chan 2021-7-12

There are 2 elses:

else strcmp(question3, 'Lessons')

else strcmp(question3, 'Other')%error

You need to change them to an elseif conditional

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Answer 1

Toder 2020-5-1

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/522296-errors-within-an-if-statement#answer_429693

编辑：Toder 2020-5-1

在 MATLAB Online 中打开

For if elsif else conditionals, Matlab executes the statements in the first block who's logical expression true, then it jumps to end. Only one block of statements is executed. The else is synonymous with "otherwise". It has the statments which are executed if all the previous if and elseif conditionals fail, and only one else can be paired with one if.

Your error is because of the second else. You can have as many elseif conditionals as you like, they just need to appear between the if and the else. I think elseif is what you intend to use in your code. Hopefully this example is helpful.

if x < 5
    disp('less than 5')
elseif x > 5
    disp('greater than 5')
else
    disp('equal to 5')
end

Suppose we set x=6 and then execute this code. Matlab first evaluates the expression x<5, which is not true, so it does not execute disp('less than 5'). Next it evaluates the expression x>5, which is true, so it executes disp('greater than 5') and jumps to end.

Now suppose we instead set x=5. Matlab first evaluates x<5, which is false, so it moves to the elseif conditional. x>5 is false, so it moves to the next block. The last block does not have any expression to evaluate (else statement are NOT allowed to have a logical expression), so disp('equal to 5') is executed, then Matlab jumps to end.

See https://www.mathworks.com/help/matlab/ref/if.html for more examples.

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Han 2020-5-1

在 MATLAB Online 中打开

Yes of course! And would you mind looking at my code. I think i figured most of it, put some of the graphs won't pop up.

function questionsasked()
question1=questdlg('Are you ready to start?',...%question box
    'Questions',...
    'Yes','No','No');
if strcmp(question1,'Yes')
question2=questdlg('Do you know the keys on the piano?',...
    'Questions',...
    'Yes','Somewhat','No','No');
    if strcmp(question2,'Yes')
    question3=questdlg('How did you learn?',...
    'Questions',...
    'Self-Taught','Lessons','Other','Other');
        if strcmp(question3,'Lessons')
        question4=questdlg('How long did you take lessons for?',...
        'Questions',...
        '<1 year','1-10 years','>10 years','>10 years');
            if strcmp(question4,'<1 year')
            question5=questdlg('Are you still taking lessons?',...
            'Questions',...
            'Yes','No','No');
                if strcmp(question5,'Yes')
                    X = categorical({'You','Me'});
                    X = reordercats(X,{'You','Me'});
                    Y = [3 5];
                    bar(X,Y)
                else 
                    X = categorical({'You','Me'});
                    X = reordercats(X,{'You','Me'});
                    Y = [2 5];
                    bar(X,Y)
                end 
           elseif  strcmp(question4,'1-10 years')
            question5=questdlg('Are you still taking lessons?',...
            'Questions',...
            'Yes','No','No');
                 if strcmp(question5,'Yes')
                    X = categorical({'You','Me'});
                    X = reordercats(X,{'You','Me'});
                    Y = [4 5];
                    bar(X,Y)
                else 
                    X = categorical({'You','Me'});
                    X = reordercats(X,{'You','Me'});
                    Y = [3 5];
                    bar(X,Y)
                end    
            elseif strcmp(question4,'>10 years')
            question5=questdlg('Are you still taking lessons?',...
            'Questions',...
            'Yes','No','No');
                if strcmp(question5,'Yes')
                    X = categorical({'You','Me'});
                    X = reordercats(X,{'You','Me'});
                    Y = [5 5];
                    bar(X,Y)
                else 
                    X = categorical({'You','Me'});
                    X = reordercats(X,{'You','Me'});
                    Y = [4 5];
                    bar(X,Y)
                end 
               
    elseif strcmp(question2,'No')
        X = categorical({'You','Me'});
        X = reordercats(X,{'You','Me'});
        Y = [0 5];
        bar(X,Y)
    elseif strcmp(question2,'Somewhat')
        X = categorical({'You','Me'});
        X = reordercats(X,{'You','Me'});
        Y = [1 5];
        bar(X,Y)
            end
        end
    end
end
end 

Han 2020-5-1

Nevermind I actually got it to work, thank you so much for your help though! My ends were in the wrong place.

Toder 2020-5-1

编辑：Toder 2020-5-1

Glad I could help, and glad you got it working. For the future, if you find yourself using a lot of elseifs, you may want to consider using a switch. You can read about them in the documentation; they are just a lot easier to read than a whole bunch of elseifs.

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

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