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digitalmars.D.learn - readf with strings

reply GreatEmerald <pastas4 gmail.com> writes:
This should be a very elementary question. How do you get a string off stdin?
Or an integer, or a boolean, for that matter? If I use this:

  float MyFloat;
  string MyString;
  readf("%f", &MyFloat);
  writeln(MyFloat);
  readf("%s", &MyString);
  writeln(MyString);

I get the float printed correctly, but when it asks to input the string,
whatever I input gets ignored - I can't reach the writeln part in any way and
I have to forcibly close the program. The same thing is with ints - if I enter
an int, it acts as if I didn't enter anything at all. But floats work fine for
some reason. Any thoughts about what is happening there?

I'm using openSUSE 11.4 and DMD 2.053.
Jun 22 2011
parent reply Ali =?iso-8859-1?q?=C7ehreli?= <acehreli yahoo.com> writes:
On Wed, 22 Jun 2011 14:57:57 +0000, GreatEmerald wrote:


 This should be a very elementary question. How do you get a string off
 stdin? Or an integer, or a boolean, for that matter? If I use this:
 
   float MyFloat;
   string MyString;
   readf("%f", &MyFloat);
   writeln(MyFloat);
   readf("%s", &MyString);
   writeln(MyString);
 
 I get the float printed correctly, but when it asks to input the string,
 whatever I input gets ignored - I can't reach the writeln part in any
 way and I have to forcibly close the program. The same thing is with
 ints - if I enter an int, it acts as if I didn't enter anything at all.
 But floats work fine for some reason. Any thoughts about what is
 happening there?
 
 I'm using openSUSE 11.4 and DMD 2.053.


Reading from an input stream is sometimes confusing.

Things to remember:

- Use %s for any type unless there is reason not to

- The line terminator from the previous entry is still in the input. (You 
may call readf(" ") to flush those white space characters. (I've just 
discovered this.))

- string can hold any character including space and the line terminator. 
That's why pressing the Enter doesn't terminate reading a string.

- Use a space character before any format specifier to ignore zero or 
more whitespace characters before the previous input: " %s".

- To read a string (actually a line), use chomp(readln())

- I don't know whether this is intended and I don't think that we should 
routinely use this: The EOF (Ctrl-D on Unix consoles, Ctrl-Z on Windows) 
terminates reading the string but strangely not the entire input.

import std.stdio;
import std.string;

void main()
{
    float MyFloat;
    readf(" %s", &MyFloat);
    writeln(MyFloat);

    readf(" ");

    string MyString = chomp(readln());
    writeln(MyString);
}

Ali
Jun 22 2011
next sibling parent reply "Dainius (GreatEmerald)" <pastas4 gmail.com> writes:
I see. Using %s does indeed work with ints, and chomp(readln()) works
with strings. It's rather counter-intuitive, though.
I wonder why there isn't a simpler way to do this, something like
writeln() - you could input the variables as parameters and it would
automatically read them...
Jun 22 2011
parent Ali =?iso-8859-1?q?=C7ehreli?= <acehreli yahoo.com> writes:
On Wed, 22 Jun 2011 20:59:46 +0300, Dainius (GreatEmerald) wrote:


 I see. Using %s does indeed work with ints, and chomp(readln()) works
 with strings. It's rather counter-intuitive, though.


There are many counter intuitive bits and imperfect modelling in stream I/
O. For example "42a" is perfectly fine when reading an int because the 
reader decides that the '4' and '2' characters make up the int and 'a' 
must be for something else. Why? :) What if I really meant that "42a" 
should be read as int. It should be an error!

I think strings are special because they are powerful to take any 
character.

Additionally output and input are not symmetrical: The ints 1 and 2 would 
be written as 12 back to back but they can't be read as two ints.


 I wonder why there
 isn't a simpler way to do this, something like writeln() - you could
 input the variables as parameters and it would automatically read
 them...


I know that the insistence on the space character to munch white space is 
intentional. Andrei commented on a bug that this is more consistent than 
scanf()'s implicit munching.

There is <cstream> that is being (is?) deprecated. It reads simpler like 
writeln:

import std.cstream;

void main()
{
    double d;
    int i;
    din.readf(&d, &i);
    dout.writefln("d: %s, i: %s", d, i);
}

It would still be problematic with strings. How many characters should be 
a part of it?

Ali
Jun 22 2011
prev sibling next sibling parent reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
This library has some nice user-input methods for D2:
https://github.com/he-the-great/JPDLibs/tree/cmdln
Jun 22 2011
parent reply Ali =?iso-8859-1?q?=C7ehreli?= <acehreli yahoo.com> writes:
On Wed, 22 Jun 2011 20:17:39 +0200, Andrej Mitrovic wrote:


 This library has some nice user-input methods for D2:
 https://github.com/he-the-great/JPDLibs/tree/cmdln


Thanks! :)

The Turkish D community has experimented with a similar solution:

  http://ddili.org/forum/post/2960

"oku" means "read":

    // Read into an existing variable
    double d;
    oku("Please enter a double: ", &d);

    // Read and return a value
    int i = oku!int("Please enter an int: ");

Ali
Jun 22 2011
next sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
I didn't know there was a Turkish D community. :)
Jun 22 2011
prev sibling next sibling parent reply Jimmy Cao <jcao219 gmail.com> writes:
On Wed, Jun 22, 2011 at 1:31 PM, Ali =C7ehreli <acehreli yahoo.com> wrote:


 On Wed, 22 Jun 2011 20:17:39 +0200, Andrej Mitrovic wrote:


 This library has some nice user-input methods for D2:
 https://github.com/he-the-great/JPDLibs/tree/cmdln


 Thanks! :)

 The Turkish D community has experimented with a similar solution:

  http://ddili.org/forum/post/2960

 "oku" means "read":

    // Read into an existing variable
    double d;
    oku("Please enter a double: ", &d);

    // Read and return a value
    int i =3D oku!int("Please enter an int: ");

 Ali


The design of this community site is quite splendid.  So D has three major
language sub-communities:  German, Japanese, and Turkish?
Jun 22 2011
parent Ali =?iso-8859-1?q?=C7ehreli?= <acehreli yahoo.com> writes:
On Wed, 22 Jun 2011 13:45:56 -0500, Jimmy Cao wrote:


 On Wed, Jun 22, 2011 at 1:31 PM, Ali Çehreli <acehreli yahoo.com> wrote:
 

 On Wed, 22 Jun 2011 20:17:39 +0200, Andrej Mitrovic wrote:


 This library has some nice user-input methods for D2:
 https://github.com/he-the-great/JPDLibs/tree/cmdln


 Thanks! :)

 The Turkish D community has experimented with a similar solution:

  http://ddili.org/forum/post/2960





 The design of this community site is quite splendid.


Thank you! :) We have three books too:

- D Programming Language (complete, except special memory management like 
emplace(), the 'new' and 'delete' operators, interacting with the garbage 
collector, etc. That chapter is being written as we speak)

- GtkD (needs work)

- Game Programming with SDL (freshly started; moving on pretty well)

  http://ddili.org/ders/index.html


 So D has three
 major language sub-communities:  German, Japanese, and Turkish?


The wiki lists others:

  http://www.prowiki.org/wiki4d/wiki.cgi

Ali
Jun 22 2011
prev sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
Ali Çehreli:


 "oku" means "read":
 
     // Read into an existing variable
     double d;
     oku("Please enter a double: ", &d);
 
     // Read and return a value
     int i = oku!int("Please enter an int: ");


I have a related enhancement request since lot of time:
http://d.puremagic.com/issues/show_bug.cgi?id=4716

Bye,
bearophile
Jun 22 2011
parent reply "Dainius (GreatEmerald)" <pastas4 gmail.com> writes:
 I have a related enhancement request since lot of time:
 http://d.puremagic.com/issues/show_bug.cgi?id=4716

 Bye,
 bearophile


That's exactly what I'd like to see. Voted.

After all, D is created with practicality in mind, and doing all that
parsing is the opposite of what it's trying to achieve.
Jun 23 2011
parent Kai Meyer <kai unixlords.com> writes:
On 06/23/2011 02:27 AM, Dainius (GreatEmerald) wrote:

 I have a related enhancement request since lot of time:
 http://d.puremagic.com/issues/show_bug.cgi?id=4716

 Bye,
 bearophile


 That's exactly what I'd like to see. Voted.

 After all, D is created with practicality in mind, and doing all that
 parsing is the opposite of what it's trying to achieve.


The goal here is to have semantics for "Read an int from STDIN." I think 
the difficulty here is this really *should* be "Read a T from a stream." 
In C++, we have the stream operators ">>" and "<<". I really really 
really really miss these. I would love to have my stream operators back 
(and then implement the operators for classes and structs.) I think that 
"a = stdin.type_read!int();" or "std.type_read!int(&a);" would go a long 
way in getting there, as long as we could extend the template. I don't 
like the way python takes input from STDIN with raw_input() and input(). 
I'm not opposed to having a language support those semantics, but I 
would rather have a generic stream reader that does the conversion to a 
type for me that allows me to extend the conversion to include my own types.
Jun 23 2011
prev sibling parent Kai Meyer <kai unixlords.com> writes:
On 06/22/2011 09:30 AM, Ali Çehreli wrote:

 On Wed, 22 Jun 2011 14:57:57 +0000, GreatEmerald wrote:


 This should be a very elementary question. How do you get a string off
 stdin? Or an integer, or a boolean, for that matter? If I use this:

    float MyFloat;
    string MyString;
    readf("%f",&MyFloat);
    writeln(MyFloat);
    readf("%s",&MyString);
    writeln(MyString);

 I get the float printed correctly, but when it asks to input the string,
 whatever I input gets ignored - I can't reach the writeln part in any
 way and I have to forcibly close the program. The same thing is with
 ints - if I enter an int, it acts as if I didn't enter anything at all.
 But floats work fine for some reason. Any thoughts about what is
 happening there?

 I'm using openSUSE 11.4 and DMD 2.053.


 Reading from an input stream is sometimes confusing.

 Things to remember:

 - Use %s for any type unless there is reason not to

 - The line terminator from the previous entry is still in the input. (You
 may call readf(" ") to flush those white space characters. (I've just
 discovered this.))

 - string can hold any character including space and the line terminator.
 That's why pressing the Enter doesn't terminate reading a string.

 - Use a space character before any format specifier to ignore zero or
 more whitespace characters before the previous input: " %s".

 - To read a string (actually a line), use chomp(readln())

 - I don't know whether this is intended and I don't think that we should
 routinely use this: The EOF (Ctrl-D on Unix consoles, Ctrl-Z on Windows)
 terminates reading the string but strangely not the entire input.

 import std.stdio;
 import std.string;

 void main()
 {
      float MyFloat;
      readf(" %s",&MyFloat);
      writeln(MyFloat);

      readf(" ");

      string MyString = chomp(readln());
      writeln(MyString);
 }

 Ali


Remember that readf is reading characters, and converting them to types 
for you. I've just gotten in the habbit of reading in the string, and 
then parsing the string on my own. Readf doesn't grant me anything 
special that I can't do on my own. It's easy enough to do something like 
this:

[kai.meyer kai-rhel6 sandbox]$ cat d_read.d
import std.stdio;
import std.string;
import std.conv;

void main()
{
     string[] buffer;
     int a;
     float b;
     string c;
     buffer = chomp(readln()).split(" ");
     a = to!(int)(buffer[0]);
     b = to!(float)(buffer[1]);
     c = buffer[2..$].join(" ");
     writef("Read in: '%d' '%f' '%s'\n", a, b, c);
}


If I type:
1 1.3 this is a string

On the command line after the execution, I get this back:
Read in: '1' '1.300000' 'this is a string'

It's not very stream-ish, because readln breaks on a new line. You could 
call the "buffer = chomp..." line again if (buffer.length == 0) before 
you attempt another conversion.
Jun 22 2011