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a newbie question.

 
 
John Bode
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      03-12-2008
On Mar 12, 1:27 am, Mike <(E-Mail Removed)> wrote:
> Hi
>
> #include<stdio.h>
> main()
> {
> char s[]="Taiwan University.";
> float a;
> scanf("%f",&a);
> printf("%f",a);
> printf("input string:");
> gets(s);
> puts("%s",s);
>
> }
>
> After compiled by Visual Studio, the above program can input a.
> But after it shows a, then I cannot enter s string.
> Why?
>
> Mike


When you type in your floating point number (3.14159, for example) and
hit Enter, the input stream looks like this:

{'3','.','1','4','1','5','9',\n}

After the call to scanf(), the newline character is still left in the
input stream:

{\n}

because scanf() with the %f conversion specifier stops reading at the
first character that isn't part of a valid floating-point constant.

gets() reads up to the next newline character; since there's already a
newline character in the input stream left over from the last input
operation, it returns immediately.

There are two ways around this. The easy way is to stick a getchar()
after the scanf() to consume the newline. A somewhat more involved
(but ultimately safer) way is to read your floating-point number in as
a string using fgets() (*NOT* gets()), and then convert using sscanf()
or strtod().

Do not use gets(); it *WILL* introduce a point of failure in your
code. Use fgets() instead:

fgets(s, sizeof s, stdin);

Unlike gets(), fgets() writes the trailing newline to your buffer.
 
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Keith Thompson
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      03-13-2008
Ralf Damaschke <(E-Mail Removed)> writes:
> Keith Thompson wrote:
>> Mike <(E-Mail Removed)> writes:
>>> char s[]="Taiwan University.";

>
>> it's a fairly concise way to say that
>> you want s to be big enough to hold that particular string value.
>>
>> An alternate way to do the same thing is:
>>
>> char s[sizeof "Taiwan University." + 1];
>>
>> but it's easy to forget the "+ 1" (needed to allow for the
>> terminating '\0').

>
> Oops, it seems as easy to confuse sizeof with strlen, too.


Absolutely; good catch. (sizeof "string literal" already includes the
trailing '\0'; the +1 is unnecessary.)

(As it happens, I realized I had made this error as I was driving to
work this morning; I haven't had a chance to post until now.)

--
Keith Thompson (The_Other_Keith) <(E-Mail Removed)>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
 
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David Thompson
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      03-24-2008
On Wed, 12 Mar 2008 06:59:36 GMT, Micah Cowan <(E-Mail Removed)>
wrote:

> Mike <(E-Mail Removed)> writes:

<snip other points>
> > printf("input string:");

>
> If stdout is line-bufered, you won't see this prompt before it starts
> expecting input.
>

Actually, you probably will; see 7.19.3p3 about "input is requested".
(Barring the UB earlier for an uninit float.) But you _might_ not.

> > gets(s);
> > puts("%s",s);

>
> Since gets() strips newlines, puts() won't print one, so you may not
> see this output either.
>

_printf_, which the OP apparently meant, wouldn't add a newline,
but _puts()_ would -- but would be called as puts(s) .

> Aside from that, gets() is unsafe.
>

That too.

- formerly david.thompson1 || achar(64) || worldnet.att.net
 
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