Mathematica importing issue - import

Question

I have exported matrix from Mathematica
Export["all.txt", all]
i.e.
Matrix is s.t
{{1,2,3,4},{1,2,3,4}}
I tried to import it back, but it is not usable..
Import["text.txt"]
{1, 2, 3}
{1, 2, 3}
,
Import["text.txt", "Data"]
{{"{1,", "2,", "3}"}, {"{1,", "2,", "3}"}}
or
Import["text.txt", "String"]
{1, 2, 3}
{1, 2, 3}
I cannnot use it because it is not matrix. How can I import it back as matrix? so I can get do some command like %[[1]][[1]] to obtain rows and column

Answer 1

There's nothing wrong with the file extension .txt, but when Mathematica exports and imports a text file, it assumes it is a string. You can see:
Export["C:\\test.txt", {{1, 2, 3}, {4, 5, 6}}]
Head[Import["C:\\test.txt"]]
(* Output: String *)
Instead, you can change the file extension as Eugene suggests:
Export["C:\\test.dat", {{1, 2, 3}, {4, 5, 6}}]
Head[Import["C:\\test.dat"]]
(* Output: List *)
Notice that it won't even use a file extension it doesn't understand:
Export["C:\\test.xyz", {{1, 2, 3}, {4, 5, 6}}]
Head[Import["C:\\test.xyz"]]
(* Output: Export::type: "List cannot be exported to the "XYZ" format." *)
Really, what's happening is Import and Export are trying to infer the type you want to use from the file extension. You can always specify that type manually instead:
Export["C:\\test.txt", {{1, 2, 3}, {4, 5, 6}},"List"]
Head[Import["C:\\test.txt"]]
(* Output: String *)
The output files for type "List" and type "String" are going to be the same, so you need to specify it when you import the file too:
Export["C:\\test.txt", {{1, 2, 3}, {4, 5, 6}},"List"]
Head[Import["C:\\test.txt","List"]]
(* Output: List *)
Note that this would work even if you forgot to Export with the type "List" specified, since the output file would still be compatible for importing as a list.
Worst case scenario, you can always try this:
Export["C:\\test.txt", {{1, 2, 3}, {4, 5, 6}}]
ToExpression[Import["C:\\test.txt"]]
(* Output: {4,5,6} *)
That doesn't quite work and only gives you bottom row. You'd have to be a bit pedantic:
Export["C:\\test.txt", {{1, 2, 3}, {4, 5, 6}}]
ToExpression[StringSplit[Import["C:\\test.txt"], "\n"]]
But of all these possibilities, the best and simplest I've mentioned is to simply tell the Import command that what you have is a list:
Import["C:\\test.txt","List"]
That's why that optional argument exists for Import in the first place. The reason it's optional is so we can be lazy and let Mathematica decide that .txt files are strings and .dat files are lists, but if you don't want Mathematica to do that (i.e. Mathematica is doing it wrong), you just specify "List" or whatever type of file you are importing.
You almost got this right but you used the type "Data" which is actually meant to indicate (from the documentation):
"data in a generic Wolfram Language form (list, string, etc.)"
which means you won't get much help there -- Mathematica will still decide this is a string. The type you want is "List".

Answer 2

I've noticed long time ago (since 6th versions) that Mathematica has some issues with *.txt files. Use suffix .dat. It has been proven issue-proof for me with matrices or anything.