How do you get a variable to recognized in function

Everytime I run this function it say D is not recognized

GetUserInput();
filename = ['ENGR131_Lab4_CatMap_', D];  
load(filename, '-mat');  
PlotMap(C,D)
% B
function [C,D]=GetUserInput()
W = ['A', 'B']; 
X = ['b', 'r', 'm', 'c', 'y', 'g'];
C = '';  
D = '';
for i=1:2
    switch i
        case 1
            %unable to get it too work without error W=options
            prompt = 'Enter head marker body color (b, r, m, c, y, g): '
            
        case 2
            %unable to get it too work without error X=options
            prompt1 = 'File (A,B): '
            for I=1:2 
                if I == 1
                    while true  
                        C = input(prompt, 's');  
                        if any(C == X)
                            
                            break;  
                            
                        end
                    end
                else I == 2
                    
                    while true  
                        D = input(prompt1, 's');  
                        if any(D == W)
                            break;  
                            
                        end
                    end
                end
            end
    end
end
end

[Stephen23]

Because square brackets are a concatenation operator, your code:

W = ['A', 'B']; 
X = ['b', 'r', 'm', 'c', 'y', 'g'];

is equivalent to writing this:

W = 'AB'; 
X = 'brmcyg';

Note that using EQ on character vectors performs an element-wise comparison, which therefore throws an error if the two character vectors have incompatible sizes. If you want to write robust code use cell arrays and STRCMP or ISMEMBER or similar instead.

Answer 1

[dpb]

Because you didn't have a place to return the values from the function when you called it...so they were thrown away.

[C,D]=GetUserInput();
filename = ['ENGR131_Lab4_CatMap_', D];  
load(filename, '-mat');  
PlotMap(C,D)

Answer 2

[Walter Roberson]

function [C,D]=GetUserInput()

That code does not mean that variables C and D are to be set in the calling context. MATLAB outputs are strictly positional. Your code is equivalent to

function varargout=GetUserInput()
W = ['A', 'B']; 
X = ['b', 'r', 'm', 'c', 'y', 'g'];
varargout{1} = '';  
varargout{2} = '';
for i=1:2
    switch i
        case 1
            %unable to get it too work without error W=options
            prompt = 'Enter head marker body color (b, r, m, c, y, g): '
            
        case 2
            %unable to get it too work without error X=options
            prompt1 = 'File (A,B): '
            for I=1:2 
                if I == 1
                    while true  
                        varargout{1} = input(prompt, 's');  
                        if any(varargout{1} == X)
                            
                            break;  
                            
                        end
                    end
                else I == 2
                    
                    while true  
                        varargout{2} = input(prompt1, 's');  
                        if any(varargout{2} == W)
                            break;  
                            
                        end
                    end
                end
            end
    end
end
end

The names given to the output variables are strictly for local convenience -- they are strictly aliases for varargout (except the named variables are individually tracked as to whether they are defined or not, whereas varargout elements could in theory be skipped and get default output values.)