How to accept only positive numbers on textbox ?

Hello;

I need to put only positive number in textbox. It's a equation and about time so it's cant be negative. Which codes help me ?

S = str2num(char(get(handles.edit1 , 'String')));
if (isempty(S))
     set(hObject,'String','0'); 
     warndlg('Insert positive value','!! Warning !!');
end

I tried someting, If I put letters in textbox that give me a warning (working) but I need to change that with positive numbers.

Answer 1

[Walter Roberson]

S = abs(S);

[Murat Piri]

I need this code give me warning with warndlg if I input negative value.

[Walter Roberson]

if S < 0
 warndlg('Value was negative, you shouldn't do that!');
 S = -S;
end

[Murat Piri]

It's working Thank you :)

Answer 2

[Matas Mendelis]

It's best to use a while loop after every imput. This is my code to force the user only imput possitive spring stiffness values. I also added a warning message box.

promt='front spring stiffness(N/mm)';
F_Kspring=input(promt);

if F_Kspring<=0 %warning message box
f = warndlg('Insert only POSITIVE values');
end 

while F_Kspring<=0 %While loop for making user insert only possitive values
promt='front spring stiffness(N/mm)';
F_Kspring=input(promt);
end

[Rik]

I would avoid having the same code twice:

promt='front spring stiffness(N/mm)';
while true %While loop for making user insert only possitive values
    F_Kspring=input(promt);
    if F_Kspring<=0 %warning message box
        f = warndlg('Insert only POSITIVE values');
    else
        break
    end
end

[Matas Mendelis]

Yes, this is better, then my original. Neater and the warning box appears every time when negative value is inserted.