Parsing a string in D

manpreet Best Answer 2 years ago

I'm trying to learn D but am struggling with lack of documentation (or my understanding of it), so I came here. I already asked a different but unrelated question earlier today.

Anyway, here goes:

I would like to parse a string for different things.

String format is something like:

[]  [parameters]

If there is no label, there is mandatory whitespace. Parameters can be comma-delimited. Parameter types are dependent on the mnemonic.

I would like to use std.conv: parse from the Phobos library to aid me, but I fail to understand the documentation on how to parse a "word", as in, some characters seperated by whitespace on either end. It works fine for integers and the like as int i = parse!int(line). But if I were to do string s = parse!string(line) it would grab the entire line.

I cold parse this by hand, using char** (or, ref string) as a datatype, just like I did when I wrote this in C. But I'm learning D to not have to.

I tried something like this to do it manually:

string get_word(ref string s)
{
        int i = 0;
        while (i < s.length && isAlphaNum(s[i]))
                i++;

        string word = s[0 .. i];
        s = s[i+1 .. $];
        return word;
}

Is this a good way to do it? Is there a cleaner way? A faster way? A safer way, perhaps? I'm not sure the i+1 index always exists.

Thanks for the help!

My faith in D is slightly dwindling already, as I've run into all sorts of problems. But the path is surely going to be worth it.

manpreet 2 years ago

import std.string;
import std.stdio;
import std.algorithm;
import std.math;

enum string[] separators = [ " ", "\t", ",", ";", "\n", "\r\n" ];

string get_word( ref string s ){
    string token;
    sizediff_t storePositions[separators.length + 1]; // set size array to the number of separator in array "separators" and latest field for current string lenght 
    foreach( i, separator; separators ){             // compute position for each separator
        sizediff_t position = countUntil( s, separator );
        if( position == -1 ) position = sizediff_t.max;
        storePositions[i] = position;
    }
    storePositions[ $ -1 ] = s.length;
    sizediff_t end    = reduce!min( storePositions );
    token             = s[0 .. end].idup;
    writefln( "%s | %d", s, end );
    return token;
}

void main( string[] args ){
    string s        = "a long;string\tyeah\n strange; ok";
    bool   isRunning= true;
    size_t start    = 0;
    writefln( "parse: %s", s ); 
    while( isRunning ){
        string result = get_word( s[ start .. $] );
        if( result == "" )
            isRunning = false;
        else{
            start  += result.length + 1;
            result = get_word( s[ start .. $] );
        }
        writefln( "token: %s, position: %d", result, start );
        writeln( "----" );
    } 
}

parse: a long;string yeah
 strange; ok
a long;string yeah
 strange; ok | 1
long;string yeah
 strange; ok | 4
token: long, position: 2
----
long;string yeah
 strange; ok | 4
string yeah
 strange; ok | 6
token: string, position: 7
----
string yeah
 strange; ok | 6
yeah
 strange; ok | 4
token: yeah, position: 14
----
yeah
 strange; ok | 4
 strange; ok | 0
token: , position: 19
----
 strange; ok | 0
token: , position: 19