How to remove an expression from a file?

Question

To read in expressions from a file, one can do ReadList[file, Expression, 3]. How to do the opposite, i.e. remove a number of expressions from a file, modifying the file on the disk?

Answer 1

Functions like Read, Skip and Find usually operate on streams in an entirely sequential fashion. Each time one of the functions is called, the current point in the stream moves on.

In a Sequential file you can´t delete entries. The only option is shifting the "tail" of the file upwards.

Answer 2

A filtered stream reading function:

Clear[MakeFilteredReader];
Options[MakeFilteredReader] = {Omit -> None};
MakeFilteredReader[stream_InputStream, OptionsPattern[]] := 
 Module[{omitList = OptionValue@Omit, position = 0},
  Function[s, position += 1;
   If[MemberQ[omitList, position],
    Skip[stream, s];,
    Read[stream, s]]
   ]
  ]

Define a stream to read.

str = StringToStream["103 123456 234 456 34"];

Create a reader on that stream.

fr = MakeFilteredReader[str, Omit -> {2, 3}];

Read from the filtered stream reader.

{fr[Number], fr[Number], fr[Number], fr[Number], fr[Number],  fr[Number]}

Which returns:

{103, Null, Null, 456, 34, EndOfFile}