Need help on clarification of input dialog of matlab - matlab

Question

Suppose I have the following code:
%%input dialog box%%%
prompt = {'Enter gain:','Enter range:'};
dlg_title = 'Enter values';
num_lines= 1;
def = {'20','256'}; %default
answer = inputdlg(prompt,dlg_title,num_lines,def);
%%%to get the two entered values%%%%
A = getfield(answer,{1}); %first input field
A = str2double(A);
B = getfield(answer,{2}); %second input field
B = str2double(B);
What does it mean to use "dynamic field names with structures instead of getfield"?
How can I use a loop for the input values that are complex and smaller than zero, in the sense to request the user for another compatible input?
I've tried the following loop but it does not work. Why?
while isnan(A) || ~isreal(A) || A<0
prompt = {'Enter gain:'%'Enter range:'};
dlg_title = {'undefine!!'};
num_lines= 1;
def = {'',''}%{'20','256'}; %default
answer = inputdlg(prompt, dlg_title, num_lines, def);
A = getfield(answer,{1}); %first input field
A = str2double(A);
%A = str2double(input('Enter the value of module(mm) : ', 's'));
end

Answer 1

The calls to getfield aren't required and frankly don't make any sense. The variable answer is a cell array not a struct so you don't need to use getfield The second half of the pasted code could be replaced with:
A = str2double( answer{1} );
B = str2dobule( answer{2} );
With regards to looping until you get valid input. You can use a boolean flag, a while, loop and a function (lets call it areInputsValid()) that returns true if the inputs are valid
validInput = false;
while (~validInput)
% input dialog box
% code here
% get the two entered values
& code here
validInput = areInputsValid(A, B);
end