Ah, that is very helpful. Thank you.
I create a sample text file containing a single line
{{1,2,3},{4,5,6},{"ciao",e,f}}
I import using
data=Import["data.txt"];
Mathematica will often think it is being helpful by hiding many important details so I use
FullForm[data]
That shows me]
"{{1,2,3},{4,5,6},{\"ciao\",e,f}}"
Using Fullform on an expression when something does not work is a very powerful trick and it will help you often if you remember it.
This shows that Mathematica read the file not as a matrix of numbers, but as a long string in quotes.
I try to turn the string into a matrix of numbers
FullForm[ToExpression[data]]
which gives me
List[List[1,2,3],List[4,5,6],List["ciao"]]
and
MatrixForm[ToExpression[data]]
shows you your matrix.
I caution you, MatrixForm and FullForm and other "Form"s in Mathematica are good to look at the data,
but each of those has placed the information inside a "container" and new users are often confused
when they try to then do math operation on the output from FullForm or MatrixForm. If you want to do
operations on your matrix I would suggest you use
newdata=ToExpression[data]
which will give you your data in a two dimensional list, but not contained inside FullForm formatting.
