Note: I would avoid having variable names that only differ by one being plural and singular unless one was just a scalar element of the plural container.
As per Walter's answer, it seems you have not shown all the code, since your first loop is useless if the map is not filled.
Assuming the map declaration you've shown is the actual you use:
variables = containers.Map();
then it uses 'char' as the key type. However, you then have:
for keys = 1:numel(variables)
if strcmp(variables(keys), 'Constant')
There are two issues here. For starter, since variables is a map, numel(variables) is just 1. Perhaps you meant, variables.Count. Secondly, keys will be a double. As said the key type is 'char', not 'double', so you can't actually use your keys as a key into the map. Hence the error message.
Perhaphs, you meant:
for elementidx = 1 : variables.Count
if strcmp(variables.values{elementidx}, 'Constant') %if you want to compare the value
%or maybe
if strcmp(variables.keys{elementidx}, 'Constant') %if you want to compare the actual key
