How to print an integer with n leading zeros in Mathematica?

Question

I tried to do the following:

Do[ f1 = StringReplace[ "obsxxxx.out", {"xxxx" -> ToString[i]}]; Print[f1]; , {i, 200}]; 

and get

 obs0001.out obs0002.out ... obs0010.out ... obs0100.out ... 

etc.

I tried:

 ToString[Flatten[IntegerDigits[20, 10, 4]]] 

but I still have a list ...

Answer 1

You might need something like:

 Table[IntegerString[i, 10, 4], {i, 1, 10}] 

gives

 {"0001", "0002", "0003", "0004", "0005", "0006", "0007", "0008", "0009", "0010"} 

or

 Table["obs" <> IntegerString[i, 10, 4] <> ".out", {i, 1, 10}] 

gives

{"obs0001.out", "obs0002.out", "obs0003.out", "obs0004.out", "obs0005.out", "obs0006.out", "obs0007.out", "obs0008.out", " obs0009.out "," obs0010.out "}