I need to have an error message for when the user inputs a letter into the a,b,c, or d inputs but im not sure how to do it, ive tried using isnan but could not figure out how to implement it into my code for it to work.

I need to have an error message for when the user inputs a letter into the a,b,c, or d inputs but im not sure how to do it, ive tried using isnan but could not figure out how to implement it into my code for it to work.

clc clear all close all; %variables% start=1; while start==1 %rerun loop close all %closing plot after loop

    %VARIABLE MENU
    v=inputdlg({'enter value for a','enter value for b','enter value for c','enter value for d'}, 'Constant',[1 50; 1 50; 1 50; 1 50]); %input values of constants
    a= str2num (v{1});b= str2num (v{2});c= str2num (v{3});d= str2num (v{4});
    %VARIABLE MENU

    %DOMAIN AND STEP MENU
    r=inputdlg({'enter minimum x-value','enter maximum x-value','enter step size'}, 'Domain',[1 50; 1 50; 1 50;]);%input values for graph dimensions
    e= str2num (r{1});f= str2num (r{2});g= str2num (r{3});
    x= e:g:f; %domain
    %DOMAIN AND STEP MENU

    %FUNCTIONS
    y=a*x+b; %y(x) = ax + b
    y2=(a*x.^2)+(b*x)+c; %y=ax^2 + bx + c
    y3=(a*x.^3)+(b*x.^2)+(c*x)+d; %y=ax^3+bx^2+cx+d
    %FUNCTIONS

    %USER INPUT TOLERANCE OF ZERO
    tolerance=inputdlg({'Enter lower tolerance, Ex: -0.001','Enter upper tolerance, Ex: 0.001'}, 'Tolerance of zero',[1 50; 1 50]);%lower and upper tolerance of zero
    lower= str2num (tolerance{1});upper= str2num (tolerance{2});
    %USER INPUT TOLERANCE OF ZERO

    %PLOT  FUNCTIONS   
    plot (x,y,'r')%plot function #1 in red
    hold on
    plot (x,y2,'b')%plot function #2 in blue
    hold on
    plot (x,y3,'g')%plot function #3 in green
    hold on
    %PLOT  FUNCTIONS END

   %FINDING ZEROS START

   %ROOT OF 1st FUNCTION
   for x1=e:1e-5:f 
     zero1=([a b]);%function vector
     tol=a*x1+b; %evaluating function for every value in domain
       if tol>=lower && tol<=upper %grabbing zeros only from user set tolerance
          msg{1}=sprintf ('Approximate root for y=%1.3fx+%1.3f is: x= %1.6f \n',a,b,x1);
          tol=zero1;
          plot (x1,zero1,'rx')%plot root #3 in green x
       else
         %if no roots exist in domain fucntions ends
       end

   end
   hold on

   %ROOT OF 2nd FUNCTION
   for x2=e:1e-5:f 
     zero2=([a b c]); %function vector
     tol2=(a*x2.^2)+(b*x2)+c; %evaluating function for every value in domain
       if tol2>=lower && tol2<=upper %grabbing zeros only from user set tolerance
          msg{2}=sprintf('Approxiate root for y=%1.3fx^2+%1.3fx+%1.3f is: x= %1.6f \n',a,b,c,x2);
          tol2=zero2;
          plot (x2,zero2,'bx')%plot root #2 in blue x

       else
         %if no roots exist in domain fucntions ends
       end
   end
   hold on

   %ROOT OF 3rd FUNCTION1
   for x3=e:1e-5:f 
    zero3=([a b c d]);%function vector
    tol3=(a*x3.^3)+(b*x3.^2)+(c*x3)+d;%evaluating function for every value in domain
        if tol3>=lower && tol3<=upper %grabbing zeros only from user set tolerance
            tol3=zero3;
            plot (x3,zero3,'gx')%plot root #3 in green x
            msg{3}=sprintf('Approximate root for y=%1.3fx^3+%1.3fx^2+%1.3fx+%1.3f is: x= %f \n',a,b,c,d,x3);
        else 
           %if no roots exist in domain fucntions ends
        end 
   end
    hold on
    %FINDING ZEROS END

    %ROOTS MESSAGE
     msgbox(msg,'roots','help');
     %ROOTS END

    %GRAPH LEGEND START
    xlabel('x');
    ylabel('y');
    title('Functions and Roots');
    leg{1}= sprintf('%1.3fx + %1.3f',a,b);
    leg{2}= sprintf('%1.3fx^2 + %1.3fx + %1.3f',a,b,c);
    leg{3}= sprintf('%1.3fx^3 + %1.3fx^2 + %1.3fx + %1.3f',a,b,c,d);
    legend(leg)
    %GRAPH LEGEND END

    %RE-RUN MENU
    start=menu('Return to start?','Yes','End');
if start==2 
   close all
end
    %RE-RUN MENU
end

[Rena Berman]

(Answers Dev) Restored edit

Answer 1

[Eric]

str2num will return empty if it cannot convert, so you would need to use isempty(). Alternatively, there is a second output from str2num, which is a flag that you could check to see if the conversion was successful.

Edit:

For example,

 [a, success_a] = str2num('c');

will return an empty a and success_a will be false/0, so you can check using either of the following conditions (only one is necessary):

 if isempty(a) || ~success_a
     % Do error message like:
     error('ERROR: This is a character, not a number!');
 end

If you want to keep asking until it's right...

 a = str2num(mynum);
 while isempty(a) % or if you prefer the success flag, ~success_a
     %%%Complain/Error and get new 'a' here %%%
     a = str2num(mynum);
 end