Defining varying constants in matlab gui

Hi! I'm writing a code for matlab gui using switch statements for popupmenus. For every switch statement, there are varying constants to be used in the formula that I am using. My problem is I don't know how to define 'varying constants' before the switch statements and that is what matlab wants me to do to run my code.

And also, when I try a single constant for the purpose of checking if the code is correct so far, my code works fine and shows correct results in matlab command window but won't show the result in the static box on my figfile it is only showing the value I assigned for the otherwise case. Please help me. This is my code so far:

valyearmenu = get(hObject,'Value');
stryearmenu = get(hObject,'String');
valvehiclemenu = get(hObject,'Value');
strvehiclemenu = get(hObject,'String');
L = str2num(get(handles.length,'String'));
V = str2num(get(handles.velocity,'String'));
Q = str2num(get(handles.flowrate,'String'));
c = varying constant;
C = num2str(Q*(L/V)*(c/60)*365);

switch stryearmenu(valyearmenu)
    case 2018
        switch strvehiclemenu(valvehiclemenu)
            case Private
                c = 524;
                set(handles.result, 'String', C);
            case Jeepneys
                c = 737;
                set(handles.result,'String',C);
            case Buses
                c = 2410;
                set(handles.result,'String',C);
            case Trucks
                c = 1381;
                set(handles.result,'String',C);
            otherwise
                set(handles.result,'String',0);
        end
    case '2019'
        switch vehiclemenu
            case 'Private'
                c = 545;
                set(handles.result,'String',C);
            case 'Jeepneys'
                c = 767;
                set(handles.result,'String',C);
            case 'Buses'
                c = 2509;
                set(handles.result,'String',C);
            case 'Trucks'
                c = 1473;
                set(handles.result,'String',C);
            otherwise
                set(handles.result,'String',0);
        end
    case '2020'
        switch vehiclemenu
            case 'Private'
                set(handles.result,'String',C);
            case 'Jeepneys'
                set(handles.result,'String',C);
            case 'Buses'
                set(handles.result,'String',C);
            case 'Trucks'
                set(handles.result,'String',C);
            otherwise
                set(handles.result,'String',0);
        end
    case '2021'
        switch vehiclemenu
            case 'Private'
                set(handles.result,'String',C);
            case 'Jeepneys'
                set(handles.result,'String',C);
            case 'Buses'
                set(handles.result,'String',C);
            case 'Trucks'
                set(handles.result,'String',C);
            otherwise
                set(handles.result,'String',0);
        end
    case '2022'
        switch vehiclemenu
            case 'Private'
                set(handles.result,'String',C);
            case 'Jeepneys'
                set(handles.result,'String',C);
            case 'Buses'
                set(handles.result,'String',C);
            case 'Trucks'
                set(handles.result,'String',C);
            otherwise
                set(handles.result,'String',0);
        end
    case '2023'
        switch vehiclemenu
            case 'Private'
                set(handles.result,'String',C);
            case 'Jeepneys'
                set(handles.result,'String',C);
            case 'Buses'
                set(handles.result,'String',C);
            case 'Trucks'
                set(handles.result,'String',C);
            otherwise
                set(handles.result,'String',0);
        end
    case '2024'
        switch vehiclemenu
            case 'Private'
                set(handles.result,'String',C);
            case 'Jeepneys'
                set(handles.result,'String',C);
            case 'Buses'
                set(handles.result,'String',C);
            case 'Trucks'
                set(handles.result,'String',C);
            otherwise
                set(handles.result,'String',0);
        end
    case '2025'
        switch vehiclemenu
            case 'Private'
                set(handles.result,'String',C);
            case 'Jeepneys'
                set(handles.result,'String',C);
            case 'Buses'
                set(handles.result,'String',C);
            case 'Trucks'
                set(handles.result,'String',C);
            otherwise
                set(handles.result,'String',0);
        end
    otherwise
        set(handles.result,'String',0);
end

fprintf('L= %.5f; V=%.5f; Q=%.5f; C=%s\n',L,V,Q,C);

guidata(hObject,handles);

Answer 1

[Aniket]

Hi @Lexa,

There are 2 issues with the code.

1. As you have mentioned, c needs to be defined before computing C
2. set(handles.result,'String',C) always uses an unupdated C.Even when c is later updated inside the switch block, you are using the old value of C that was computed with an undefined c. That’s why it always returns the value from the otherwise case.

The below code fixes these 2 issues and is also refractored for clarity and correctness:

valyearmenu = get(handles.yearmenu, 'Value');
stryearmenu = get(handles.yearmenu, 'String');
year = stryearmenu{valyearmenu};

valvehiclemenu = get(handles.vehiclemenu, 'Value');
strvehiclemenu = get(handles.vehiclemenu, 'String');
vehicle = strvehiclemenu{valvehiclemenu};

% Get numeric inputs
L = str2double(get(handles.length,'String'));
V = str2double(get(handles.velocity,'String'));
Q = str2double(get(handles.flowrate,'String'));

c = 0;  % Initialize constant

% Nested switch-case to assign correct constant `c`
switch year
    case '2018'
        switch vehicle
            case 'Private',   c = 524;
            case 'Jeepneys',  c = 737;
            case 'Buses',     c = 2410;
            case 'Trucks',    c = 1381;
        end
    case '2019'
        switch vehicle
            case 'Private',   c = 545;
            case 'Jeepneys',  c = 767;
            case 'Buses',     c = 2509;
            case 'Trucks',    c = 1473;
        end
    % You can add similar cases for 2020 to 2025 if needed
end

% Only compute C if c was set
if c ~= 0 && ~isnan(L) && ~isnan(V) && ~isnan(Q) && V ~= 0
    Cval = Q*(L/V)*(c/60)*365;
    C = num2str(Cval, '%.2f');
    set(handles.result, 'String', C);
else
    set(handles.result, 'String', '0');
end

fprintf('L= %.5f; V=%.5f; Q=%.5f; c=%.2f; C=%s\n', L, V, Q, c, get(handles.result, 'String'));
guidata(hObject, handles);

This ensure set(handles.result, 'String', C) uses latest C.

Hope this resolves the issue!