Default value for user input
in [Learn C or C++]
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From: Greg Comeau on 7 Oct 2005 12:57 In article <3qn797Ffnii1U1(a)individual.net>, Jochen Rundholz <jochen_r(a)gmx.de> wrote: >Karl Heinz Buchegger wrote: >> Jochen Rundholz wrote: >>> I want to ask for several values and enter should go for the default. I >>> tried the following: >>> >>> int a = 0; >>> cout << "Enter a value, hit enter for default [0]: "; >>> cin >> a; >>> >>> It doesn't work... any idea what I can do? >> >> Yep. Don't read as int, read the user input as string. >> If the user enters an empty string, you know that you have >> to apply the default value. If the user did not enter an empty >> string, convert that string to a number. > >Okay, I'll do it that way. This way I won't have a backspace problem either. >Google told me two different versions to convert an integer to int: > >int a; >string s="5"; >a = atoi(s) // doesnt work Right, it won't. >a = atoi(s.c_str()); //this works > >Is the first call just something old and outdated? Nope, never worked with a std::string argument. At least not as far as Standard C or Standard C++ is concerned. >Why doesn't it work? Because it is not specified to take std::strings. Its origin is in C. With C++ there are other choices. Furthermore, even if C atoi should be used cautiously, if at all. See http://www.comeaucomputing.com/techtalk/#atoi >I thought the s.c_str() converts a string to a char pointer It does, that why the second one works, or can work. >and furthermore >I've read in some documentation that atoi() needs a string as an argument. >I am puzzeled... This is a terminology problem. Formally, C does not string types, although we like to talk about "...." and things that char *'s point to (char[]s) a `strings', especially as the library section of Standard C refers to them in that way. So a C programmer might refer to a string meaning a `C string', being one of those things just mentioned. However, a C++ programmer might also mention a string and will probably be referrring to a `std::string' (say "standard string", though this is not officialese either). So, when you read on some google link that atoi take a string, it was referring to a so-called C string. -- Greg Comeau / Celebrating 20 years of Comeauity! Comeau C/C++ ONLINE ==> http://www.comeaucomputing.com/tryitout World Class Compilers: Breathtaking C++, Amazing C99, Fabulous C90. Comeau C/C++ with Dinkumware's Libraries... Have you tried it? |