Assuming that H1 contains only one value, you could use
n = H1{1} % to access that value in the cell
if c <=0
c = inputdlg('Positive number of rows');
else
numRows = (c); %note that numRows is stored as 'char'
end
On a side note, is it really necessary to use inputdlg? You could store it as a double if you had use input instead.
H1 = input('Number of rows: ')
