Question

我有以下代码来读取文件，该文件删除文件中的每个注释。

let s_read_all line =
    if line = "" then
        raise Pbm_format_error
    else if line.[0] = '#' then
        ""  
    else
        line ^ "\n"
;;

let read_all flec =
    let rec loop accum_ref =
        let line = input_line flec in
        accum_ref := (!accum_ref) ^ (s_read_all line);
        loop accum_ref
    in  

    let accum_ref = ref "" in
    try 
        loop accum_ref
    with
        End_of_file -> !accum_ref
;;

对于180k线路（约2分钟），我的代码非常慢。我在解释器模式下执行它。是否会使我的代码变得如此缓慢？

Answer 1

问题是字符串连接很慢。更准确地说，它重复的字符串连接很慢。您应该使用Buffer而不是string来累积行：

let read_all flec =
    let rec loop buffer =
        let line = input_line flec in
        Buffer.add_string buffer (s_read_all line);
        loop buffer
    in  

    let buffer = Buffer.create 180 in
    try 
        loop buffer
    with
        End_of_file -> Buffer.content buffer
;;

Answer 2

您正在使用line ^ "\n"和(!accum_ref) ^ (s_read_all line);。

与Java类似，^是直接连接，并将不断创建新的字符串。所以我想这就是180K线路速度慢的原因。

你应该使用Buffer，就像Java中的StringBuilder一样。

另外，如果你给Buffer.create一个好的初始长度，它会稍快一点。

exception Pbm_format_error

let s_read_all line =
    if line = "" then
        raise Pbm_format_error
    else if line.[0] = '#' then
        ""  
    else
        line


let read_all flec =
    let rec loop accum_buf =
        let line = s_read_all (input_line flec) in
        Buffer.add_string accum_buf line;
        if line <> "" then Buffer.add_string accum_buf "\n" else ();
        loop accum_buf
    in  
    let accum_buf = Buffer.create (180 * 1000 * 128) in
    try 
        loop accum_buf
    with
        End_of_file -> Buffer.contents accum_buf

Ocaml相当慢的文件阅读器

2 个答案: