Cutting Fields From A CSV File

December 7, 2021

Over at Stack Overflow, I answered a question about cutting fields from a CSV file. Paraphrasing:

A file in CSV format has a header row containing field names followed by record rows containing actual data. The user wants to cut fields from the CSV file to create a new CSV file containing only the specified fields, specifying the fields by name instead of using their ordinal position.

Your task is to write a program to cut fields from a CSV file by name. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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Posted by programmingpraxis
Filed in Exercises
3 Comments »

3 Responses to “Cutting Fields From A CSV File”

  1. wisnuwidiarta said
    March 8, 2022 at 10:01 AM

    Using Pascal:

    program CSVSplitter;

    {$APPTYPE CONSOLE}
    {$R *.res}

    uses
    System.SysUtils, Classes;

    procedure Split(inputFileName, fieldName, outputFileName: String);
    var
    inputFile, outputFile: TextFile;
    header, line: String;
    columnList: TStringList;
    index: Integer;
    begin
    AssignFile(inputFile, inputFileName);
    columnList := TStringList.Create;
    try
    Reset(inputFile);
    if Not Eof(inputFile) then
    Readln(inputFile, header);

    columnList.CommaText := UpperCase(header);
index := columnList.IndexOf(UpperCase(fieldName));
if index >= 0 then
begin
  AssignFile(outputFile, outputFileName);
  try
    ReWrite(outputFile);
    writeln(outputFile, fieldName);

    while Not Eof(inputFile) do
    begin
      Readln(inputFile, line);
      columnList.CommaText := line;
      writeln(outputFile, columnList[index]);
    end;
  except
    on E: Exception do
    begin
      writeln('Cannot write to file ' + outputFileName + ' -> ' + E.Message);
    end;
  end;
end
else
  writeln('Field ' + fieldName + ' cannot be found in file ' + inputFileName);

    except
    on E: Exception do
    begin
    writeln('Exception occurs during reading file ' + inputFileName + ' -> ' + E.Message);
    end;
    end;
    columnList.Free;
    CloseFile(inputFile);
    CloseFile(outputFile);
    end;

    begin
    if ParamCount = 3 then
    begin
    if FileExists(ParamStr(1)) then
    begin
    Split(ParamStr(1), ParamStr(2), ParamStr(3));
    end
    else
    writeln('File ' + ParamStr(1) + ' cannot be found.');
    end
    else
    writeln('Usage: ' + ExtractFileName(ParamStr(0)) + ' ');
    end.

  2. Richard A. O'Keefe said
    April 6, 2022 at 10:18 AM

    I found the use of the word “cut” and “field” instead of, say, “select” and “column”, confusing.
    Using Smalltalk, with my own CSV library, I’ve lightly tested this:

      (CSVDecoder on: StdIn) bindOwn: [:input |
    |header inverse map row|
    header := input next.
    inverse := Dictionary new: header size.
    header keysAndValuesDo: [:index :name |
      inverse at: name put: index].
    map := arguments collect: [:name | inverse at: name].
    row := Array new: map size.
    (CSVEncoder on: StdOut) bindOwn: [:output |
      output nextPut: arguments.
      input do: [:each |
        map keysAndValuesDo: [:index :origin |
          row at: index put: (each at: origin)].
        output nextPut: row]]].
  3. Starlight said
    June 2, 2023 at 4:24 PM

    Hello!
    I’ve been a lurker of your blog for a few years now I was wondering why you suddenly stopped I hope we hear from you again.

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