How to properly input a range in a vector

I have a code in this style:

range=a:b;
A(range);

The a and b are set with the user's input via a prompt box (with inputdlg). They could be i.e.

a=1;
b=37;

This works fine. But I'd like to be able to not have a range as well; that is, a range that covers all rows. Setting

a=1;
b=end;

does not work. This other answer says that instead of end, the size of the vector should be retrieved:

a=1;
b=numel(A);

The problem with this is that numel isn't variable. I am setting the range outside a loop, while inside the loop the range is used to retrieve the same rows from each looped column (in several datafiles). If no range is set, or if the default is chosen, then all rows should be retrieved. The columns may have different lengths, so I will have to iterate through each column and set a new end-of-range b in each loop. This will cause some delay, so I am searching and asking here to find out if there is a simpler way to simply say "choose all", just like when writing

A(:)

but with the : stored in a variable range.

Thank you.

Answer 1

[Stephen23]

The simplest is to use the character ':'

>> A = [1,2,3;4,5,6]
A =
   1   2   3
   4   5   6
>> range = ':';
>> A(range,1)
ans =
   1
   4

[Jos (10584)]

+1

Answer 2

[Jos (10584)]

A = magic(4) 
range = ':' % in a variable
B = subsref(A,substruct('()',{range, 1}))

But i do think there are easier ways around your problem ... perhaps you can explain more what you really want to accomplish in the end?

[Jos (10584)]

See Stephen's answer!