std.stdio is incomplete (missing scanf() replacement)

December 25, 2004
Posted by Tyro
Permalink
Tyro
Permalink
When I first started learning to program some three- years ago, with D, one of the major obstacles I faced was the lack of publication/documentation. So I decided to purchase books about C++ programming and use the examples in D. I soon found that almost none of the user interaction examples involving string input could be practiced because scanf() could not properly interact with D-strings (a.k.a. char[]).

With enough prodding, Pavel Minayev was able to bring us std.stream, which partially rectified the problem. Then along came Kris, who brought us Mango and in particular mango.io which, once again, partially rectifies the problem. Don't get me wrong; these are both great and necessary library and library facilities for which I am extremely grateful. But the problem still remains.

When I import std.stdio into my program, I expect to have two distinct functionalities at my disposal: 1) The ability to retrieve information (be it from file, strings, stdin, or what have you) and 2) The ability to output information. That by its very nature are the responsibilities of stdio (a standardized way of performing input and output).

To some extent the current stdio module accomplishes both of these responsibilities. It provides input functionality through the new designed writef() family of functions. It also provides for output; albeit through C linkage to the rickety old scanf() function in std.c.stdio.

scanf(), though a versatile function for C programming, does not cut the mustard for D. We need better. Now how can this be rectified?

Simple: allow out and inout parameters for use in conjunction with variadic functions.

private import std.stdio;
private import std.stream;

extern (C) int fflush(FILE *stream);

alias std.c.stdio.stdin stdin;
alias std.c.stdio.stdout stdout;

void main() {
 char[] name = "";
 double d = 0.0;
 readf(name,d,name,d);
 writefln(name);
 writefln(d);
}

void readf (out char[] v1,out double v2, ...) {
 fflush(stdout);
 readx (stdin, _arguments, _argptr);
 _argptr -= double.sizeof;
 v2 = *cast(double*)_argptr;
 _argptr -= (char[]).sizeof;
 v1 = *cast(char[]*)_argptr;
 fflush(stdin);
}

void freadf (FILE* fp, ...) {
 readx (fp, _arguments, _argptr);
}

void readx (FILE* buf, TypeInfo[] args, inout void* argptr)
{
 foreach(TypeInfo ti; args)
 {
   if (ti is typeid(char))
   {
     writef("Enter a char: ");
     scanf("%c", &*cast(char*)argptr);
     writefln("done: char(",*cast(char*)argptr,")");
     argptr += int.sizeof;
   }
   else if (ti == typeid(double))
   {
     writef("Enter a double: ");
     scanf("%lf", &*cast(double*)argptr);
     writefln("done: double(",*cast(double*)argptr,")");
     argptr += double.sizeof;
   }
   else if (ti == typeid(char[]))
   {
     writef("Enter a string: ");
     char[256] tmp;
     scanf("%s",&tmp);
     int i;
     foreach(int ndx, char c; tmp)
     { if(c == '\0') { i = ndx; break; } }
     *cast(char[]*)argptr = tmp[0 .. i];
     writefln("done: char[](",*cast(char[]*)argptr,")");
     argptr += (char[]).sizeof;
   }
 }
}

As is demonstrated, new values can be passed back trough _argptr, and with the out and inout, it would make it that much easier. This will help us to develop a more more robust, D-centric scanf function. Thereby completely invalidating the statement:

"Who D is Not For
  As a first programming language - Basic or Java is more suitable for beginners."

Like it or not. Beginners are using the language. And noting you do or say will stop them. So how about we accommodate and more quickly acclimatize the to the community?

Andrew Edwards
[acedwards] (at) [ieee] (dot) [org]
Forums
