From 3603585680348889783 X-Google-Thread: 7894ca11fe,d47b345505a72a70 X-Google-Attributes: gid7894ca11fe,public,usenet X-Google-NewGroupId: yes X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news1.google.com!news.glorb.com!news.alt.net!frodo.cs.rpi.edu!not-for-mail From: Daphne Pfister Newsgroups: comp.std.c++ Subject: Re: for-else: adding else to for loops Date: Sat, 22 Aug 2009 12:37:18 CST Organization: unknown Lines: 136 Sender: cppmods@cs.rpi.edu Approved: stephen.clamage@sun.com Message-ID: <2009082201262116807-aw9jxzk02@sneakemailcom> References: <5d82cd6c-2a22-4dd7-8c7d-790b579533ea@d15g2000prc.googlegroups.com> <9e9c2ca5-78eb-460c-adec-2ec051c54210@r18g2000yqd.googlegroups.com> NNTP-Posting-Host: netlab.cs.rpi.edu Content-Type: text/plain; charset=ISO-8859-1; format=flowed To: (Usenet) Return-Path: X-Original-Date: Sat, 22 Aug 2009 01:26:21 -0400 X-Submission-Address: std-c++@netlab.cs.rpi.edu Xref: g2news2.google.com comp.std.c++:1315 On 2009-08-17 09:18:48 -0400, Hyman Rosen said: > A language construct has a defined meaning. Why would a > reader be in the dark about it? My suggestion to augment > loops was > for (init; test; next) SL or SO and SA > while (test) SL or SO and SA > do SL while(test); and SA > where a missing 'or' or 'and' part is taken to be there > with a null statement. SO is executed if 'test' is false > the first time it is evaluated. SA is executed if 'test' > is ever false when evaluated. A 'break' in SL, SO, or SA > moves control past the entire construct. Variables in the > 'init' statement are in scope in SO and SA. Because of the > semantics of the 'or' and 'and' parts, execution of SO > falls into execution of SA, which is a nice touch. Sounds almost like a switch... why not something like? switch while (test) { case 1: // some code break; // exit the loop case 2: // some code // fall though to next case case 3: // some code continue; // go back to the top of the loop default: // some code // fall through, and since last case would exit loop } Likewise with for: switch for( init; test; next ) SL Not only do they allow for something akin to original suggestion they can simplify some scanning loop flows. In particular it eliminates having to worry about adding a done bool to handle a common case of having a switch directly inside a for/while loop. For example: // Skip whitespace but keep track of line numbers char* c_str_ptr = ...; int line_num = 0; switch for ( ; *c_str_ptr; ++c_str_ptr ) { case '\n': ++line_num; case ' ': case '\t': continue; case 0: // end of string reached before non-white space // do something fancy if desired break; // This default does not really need to be here. // For exhibition only default: // non-white found break; } Combine with support for "case if (test):" and you have even more power. switch for ( auto my_iter = x.begin(); *my_iter; ++my_iter ) { case if ( my_iter == x.end() ): // Note *my_iter would not have been evaluated yet // since only cases seen so far are case ifs. std::cout << "Needle not found" << std::endl; break; case NEEDLE1: std::cout << "Needle1 found" << std::endl; break; case NEEDLE2: std::cout << "Needle2 found" << std::endl; break; default: continue; } My thoughts on case if would be that the switch test would not be evaluated until the first case constant: or default: (including the implicit default: at the bottom of a switch if not present) was seen. And any case if would only be evaluated if none of the cases before matched. Compilers would still be able to take emit jump tables, but would have to split them for crossing an "case if" boundary. This would mean that the form switch for( auto i : seq ) { // ... } Could be the same as switch for( auto i = begin( seq ); *i; ++i ) { case if ( i == end( seq ) ): break; // ... } I'm aware that this suggestion does not completely cover the "or SO" case of Hyman Rosen's suggestion. Daphne Pfister -- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@netlab.cs.rpi.edu] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ]