See http://saml.rilspace.org/node/248

gc_mischi.pas

program gc;
{$mode objfpc} // Do not forget this ever
 
uses
    Sysutils;
 
var
    FastaFile: TextFile;
    CurrentLine: String;
    GCCount: Integer;
    ATCount: Integer;
    TotalBaseCount: Integer;
    i: Integer;
    c: Char;
    GCFraction: Single;
 
begin
    GCCount := 0;
    TotalBaseCount := 0;

    AssignFile(FastaFile, 'Homo_sapiens.GRCh37.67.dna_rm.chromosome.Y.fa'); 
    {$I+} //use exceptions
    try  
        Reset(FastaFile);
        repeat
            Readln(FastaFile, CurrentLine); 
            for i := 0 to length(CurrentLine) - 1 do
            begin
                c := CurrentLine[i];
                if c in ['A', 'T'] then
                    inc(ATCount)
                else if c in ['G', 'C'] then
                begin
                    inc(GCCount);
                end;
            end;
        until(EOF(FastaFile)); 
        CloseFile(FastaFile);
    except
        on E: EInOutError do
        begin
            Writeln('File handling error occurred. Details: '+E.ClassName+'/'+E.Message);
        end;    
    end;
    TotalBaseCount := GCCount + ATCount;
    GCFraction := GCCount / TotalBaseCount;
    Writeln(FormatFloat('00.0000', GCFraction * 100));
end.

Compile flags used:

fpc -Ur -O3 -Xs- -OWall -FWgc_mischi -XX -CX -ogc_fpc_mischi gc_mischi.pas
fpc -Ur -O3 -Xs- -Owall -Fwgc_mischi -XX -CX -ogc_fpc_mischi gc_mischi.pas