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digitalmars.D.learn - Reading a line from stdin

reply =?ISO-8859-1?Q?Ali_=C7ehreli?= <acehreli yahoo.com> writes:
I am going over some sample programs in a text of mine and replacing 
std.cstream references with std.stdio. There are non-trivial differences 
with formatted input.

The following program may be surprising to the novice:

import std.stdio;

void main()
{
     write("What is your name? ");
     string name = readln();
     writeln("Hi ", name, "!");
}

The newline character is read as a part of the input:

What is your name? Ali
Hi Ali
!       <-- this is outputted on the next line
             because of the newline character

A solution is to strip the line after reading:

import std.string;
// ...
     string name = strip(readln());

Right? Is there a better way that I am missing?

Thank you,
Ali
Mar 15 2011
next sibling parent reply Jesse Phillips <jessekphillips+D gmail.com> writes:
Ali «ehreli Wrote:

 Right? Is there a better way that I am missing?
 
 Thank you,
 Ali

No better way, the stated reason IIRC is that it is easier to remove the new line then to append it back on.
Mar 15 2011
next sibling parent reply spir <denis.spir gmail.com> writes:
On 03/16/2011 06:41 AM, Jesse Phillips wrote:
 Ali Çehreli Wrote:

 Right? Is there a better way that I am missing?

 Thank you,
 Ali

No better way, the stated reason IIRC is that it is easier to remove the new line then to append it back on.

May be stated, but it is very wrong! I guess: s = s ~ '\n'; versus if ((str[$-1] == '\n') || (str[$-1] == '\r')) { str = str[0..$-1]; if ((str[$-1] == '\n') || (str[$-1] == '\r')) { str = str[0..$-1]; } } And it's not what programmers want in most cases, anyway. Actually, when does one need it? Denis -- _________________ vita es estrany spir.wikidot.com
Mar 16 2011
parent Jesse Phillips <jessekphillips+D gmail.com> writes:
spir Wrote:

 On 03/16/2011 06:41 AM, Jesse Phillips wrote:
 Ali Çehreli Wrote:

 Right? Is there a better way that I am missing?

 Thank you,
 Ali

No better way, the stated reason IIRC is that it is easier to remove the new line then to append it back on.

May be stated, but it is very wrong! I guess:

Sorry what I believe it refers to is that it is easier for the computer. There is no need to allocate or reallocate any memory.
 And it's not what programmers want in most cases, anyway. Actually, when does 
 one need it?

I do not know of an actual use case for needing the new line. In fact you could argue that splitlines should keep the end line character, but no one would see that.
Mar 16 2011
prev sibling parent "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
On Wed, 16 Mar 2011 11:20:43 +0100, spir wrote:

 On 03/16/2011 06:41 AM, Jesse Phillips wrote:
 Ali Çehreli Wrote:

 Right? Is there a better way that I am missing?

 Thank you,
 Ali

No better way, the stated reason IIRC is that it is easier to remove the new line then to append it back on.

May be stated, but it is very wrong! I guess: s = s ~ '\n'; versus if ((str[$-1] == '\n') || (str[$-1] == '\r')) { str = str[0..$-1]; if ((str[$-1] == '\n') || (str[$-1] == '\r')) { str = str[0..$-1]; } }

That comparison seems a bit biased. :) This one is more fair: import std.path; ... s ~= linesep; versus import std.string; ... s = s.chomp; -Lars
Mar 16 2011
prev sibling next sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Tuesday 15 March 2011 22:05:37 Ali =C7ehreli wrote:
 I am going over some sample programs in a text of mine and replacing
 std.cstream references with std.stdio. There are non-trivial differences
 with formatted input.
=20
 The following program may be surprising to the novice:
=20
 import std.stdio;
=20
 void main()
 {
      write("What is your name? ");
      string name =3D readln();
      writeln("Hi ", name, "!");
 }
=20
 The newline character is read as a part of the input:
=20
 What is your name? Ali
 Hi Ali
 !       <-- this is outputted on the next line
              because of the newline character
=20
 A solution is to strip the line after reading:
=20
 import std.string;
 // ...
      string name =3D strip(readln());
=20
 Right? Is there a better way that I am missing?

I don't think that it's all that odd from the newline to be left in. It _wa= s_ in=20 the original input, after all. I don't recall what the typical thing to do = is in=20 other languages though, since I don't do a lot with input from stdin. I can= =20 understand why you would think that it's odd, but I think that it's fine as= is.=20 It's a matter of preference really, and as Jesse points out, it's easier to= just=20 leave it in and allow the programmer to remove it than to remove it and mak= e the=20 programmer put it back if they want it (especially since that would require= a=20 reallocation whereas the other does not). =2D Jonathan M Davis
Mar 15 2011
prev sibling next sibling parent spir <denis.spir gmail.com> writes:
On 03/16/2011 06:05 AM, Ali Çehreli wrote:
 I am going over some sample programs in a text of mine and replacing
 std.cstream references with std.stdio. There are non-trivial differences with
 formatted input.

 The following program may be surprising to the novice:

 import std.stdio;

 void main()
 {
 write("What is your name? ");
 string name = readln();
 writeln("Hi ", name, "!");
 }

 The newline character is read as a part of the input:

 What is your name? Ali
 Hi Ali
 ! <-- this is outputted on the next line
 because of the newline character

This is a design bug. 99% of the time one does not want the newline, which is not part of the string data, instead just a terminator. Even more on stdin where it is used by the user to say "I"m done!". If the text is written back to the output /and/ newline is needed, it's easy to add it or use writeln. Also, to avoid using strip --which is costly and may remove other significant whitespace at start and end of line, one would have to manually check for CR and/or LF, and remove it, *twice*. A solution may be to have a boolean param "keepNewLine" beeing false in standard.
 A solution is to strip the line after reading:

 import std.string;
 // ...
 string name = strip(readln());

 Right? Is there a better way that I am missing?

Dunno. Denis -- _________________ vita es estrany spir.wikidot.com
Mar 16 2011
prev sibling next sibling parent reply Kagamin <spam here.lot> writes:
Ali «•hreli Wrote:

 The following program may be surprising to the novice:
 
 import std.stdio;
 
 void main()
 {
      write("What is your name? ");
      string name = readln();
      writeln("Hi ", name, "!");
 }

What if the user typed leading spaces? Will the program operate as you expect?
Mar 16 2011
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 03/16/2011 05:49 AM, Kagamin wrote:
 Ali «•hreli Wrote:

 The following program may be surprising to the novice:

 import std.stdio;

 void main()
 {
       write("What is your name? ");
       string name = readln();
       writeln("Hi ", name, "!");
 }

What if the user typed leading spaces? Will the program operate as you expect?

I would not like the leading spaces either, and that's another issue: contrary to readln(), readf() leaves the newline character in the input. I was about to start adopting the guideline of using " %s" (note the space) when reading formatted unless there is a reason not to. Most of the time the newline left from the previous input has nothing to do with the next read. Otherwise the following program gets stuck: import std.stdio; void main() { int i, j; readf("%s%s", &i, &j); } As a result, my current guideline is "put a space before every format specifier": readf(" %s %s", &i, &j); This is a departure from C's scanf but is more consistent. I don't want to accept and teach buggy behavior and that's why I asked on the D forum. Unfortunately I failed to attract interest there. After accepting the above, I wanted to readf() lines too: import std.stdio; void main() { string s; readf(" %s", &s); writeln(s); } As another departure from C, readf() does not stop at the first whitespace. It reads until the end of the input. Ok, I like that behavior but it's not useful for "What is your name? " like inputs. So it led me to readln(). I don't have a problem with whitespace being left in the line, I just want to know whether that's the intended or accepted behavior. Ali
Mar 16 2011
next sibling parent Kagamin <spam here.lot> writes:
Ali Çehreli Wrote:

 I don't have a problem with whitespace being left in the line, I just 
 want to know whether that's the intended or accepted behavior.

AFAIK, it is. It was intended to preserve eols while reading and writing lines.
Mar 16 2011
prev sibling parent reply Kai Meyer <kai unixlords.com> writes:
On 03/16/2011 07:54 AM, Ali Çehreli wrote:
 On 03/16/2011 05:49 AM, Kagamin wrote:
 Ali «•hreli Wrote:

 The following program may be surprising to the novice:

 import std.stdio;

 void main()
 {
 write("What is your name? ");
 string name = readln();
 writeln("Hi ", name, "!");
 }

What if the user typed leading spaces? Will the program operate as you expect?

I would not like the leading spaces either, and that's another issue: contrary to readln(), readf() leaves the newline character in the input. I was about to start adopting the guideline of using " %s" (note the space) when reading formatted unless there is a reason not to. Most of the time the newline left from the previous input has nothing to do with the next read. Otherwise the following program gets stuck: import std.stdio; void main() { int i, j; readf("%s%s", &i, &j); } As a result, my current guideline is "put a space before every format specifier": readf(" %s %s", &i, &j); This is a departure from C's scanf but is more consistent. I don't want to accept and teach buggy behavior and that's why I asked on the D forum. Unfortunately I failed to attract interest there. After accepting the above, I wanted to readf() lines too: import std.stdio; void main() { string s; readf(" %s", &s); writeln(s); } As another departure from C, readf() does not stop at the first whitespace. It reads until the end of the input. Ok, I like that behavior but it's not useful for "What is your name? " like inputs. So it led me to readln(). I don't have a problem with whitespace being left in the line, I just want to know whether that's the intended or accepted behavior. Ali

I think there are two issues here. First, I think is perfectly reasonable to let the programmer use a simple mechanism like "string.chomp(stdin.readline())" or simply "chomp(readln())" when they don't want the new line. Second, D doesn't seem to have a graceful way of reading an endless stream of <your favorite data type> delimited by <your favorite delimiting character>. I think readf is rigid, and works great in some cases. I would greatly appreciate something more flexible like C++'s extraction operator (operator>>) though. For example: int i = 0; while(cin >> i) { //Do something } // Done doing something
Mar 16 2011
parent Jesse Phillips <jessekphillips+D gmail.com> writes:
Kai Meyer Wrote:

 Second, D doesn't seem to have a graceful way of reading an endless 
 stream of <your favorite data type> delimited by <your favorite 
 delimiting character>. I think readf is rigid, and works great in some 
 cases. I would greatly appreciate something more flexible like C++'s 
 extraction operator (operator>>) though. For example:

There is interest in getting a CSV parser in D. Of which I was hoping to be able to make use of a ForwardRange with Slicing... but could not find a way turn a buffered input range into a forward range. This made my second attempt useless for input ranges. https://github.com/he-the-great/JPDLibs/tree/csv https://github.com/he-the-great/JPDLibs/tree/separator The benefit of the second is that you can using string to make a separator, or would be if startsWith worked as I want. But started working on the first to make it closer to what should be available in the standard library.
Mar 16 2011
prev sibling parent Gerrit Wichert <gwichert yahoo.com> writes:
Am 16.03.2011 11:09, schrieb spir:
 This is a design bug. 99% of the time one does not want the newline,
 which is not part of the string data, instead just a terminator. Even
 more on stdin where it is used by the user to say "I"m done!".
 If the text is written back to the output /and/ newline is needed,
 it's easy to add it or use writeln.

experience. I think It is never a design bug for a standart function to forward as much information as possible. Suppressing unwanted information is very easy, but getting back some information that has been cut off by a courteous tool is an awful job. That data comes in via stdin doesn't mean is is typed in by the user. It's perfectly possible that it is piped in and that the line endings should not be touched. You must be a very lucky person if you have never been bitten by such a courteous piece of over engineered software. I hope you can continue being lucky for a long time. :-) Gerrit
Mar 16 2011