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Re: Recursion: getting input from the user

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Alla _ <modelling.data@gmail.com> writes:

> I am trying to write a function that gets an input from the user.
> If the user has not provided a correct input, then the function
> should start all over again, prompting the user for another input.
> I wonder why it doesn't perform the recursion. If I enter only new
> line and then press ctrl D to indicate EOF, I get back the printf
> message for the second time and the program stops.

Here's what's happening...

The "top" call reads lines (rather short ones!) until EOF (or an error,
but let's forget about those).  You typed \nCtrl-D so fgets is called
twice.  The first time it puts \n\0 in the buffer, the second time it
puts nothing there (leaving \n\0 there).  buf[0] != '\n' is false so the
top call recurses and returns industry_rate().

This second call sets up a new buffer full of nulls and tries to read a
line.  That fails (the input is at EOF) but this time buf[0] != '\n' is
true (buf[0] is 0) so strtod is called.  No number is seen and
not_converted will be left pointing at the first null in the buffer.
Thus the test *not_converted == '\n' || *not_converted == '\t' ||
*not_converted == '\0' is also true and rate will be returned.

When I run this code I get a different effect because my Linux system
breaks C's rules for interactive input: once EOF has been seen, further
input is possible, but you really should no rely on that possibility
(and it seems your system does not do that).

It's a little odd to read all the input and then look to see if the last
line had a number on it (which seems to be the plan).  It's usually
better to read until you get some acceptable data or you find that the
stream as ended.

> When I enter a number, it correctly stores the number.

Only in some situations.  If you type \n\n100\n\nCtrl-D you won't get
100 returned.  I'm not saying that's a problem, but you should give some
thought to what you will accept as input and you won't.

> Here is how the function is called:
> av.industry_rate = industry_rate();
>
> The function:
> double industry_rate()
> {
>     printf("Please, enter the projected industry growth rate beyond 5 year horizon; maximum 3 numbers after the decimal point\n");
>     char buf[7] = {0};

That's a little small.  If someone inputs a few spaces before the number
it could get truncated.

>     while(fgets(buf, sizeof(buf), stdin) != NULL);
>     
>     if(buf[0] != '\n')
>     {
>         char *not_converted = NULL;
>         double rate = strtod(buf, &not_converted);
>         
>         if(*not_converted == '\n' || *not_converted == '\t'
>            || *not_converted == '\0')
>         return rate;

Odd indentation!

>     }
>     return industry_rate();
> }

-- 
Ben.

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Thread

Recursion: getting input from the user Alla _ <modelling.data@gmail.com> - 2017-01-30 02:06 -0800
  Re: Recursion: getting input from the user Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-30 12:11 +0000
    Re: Recursion: getting input from the user Alla _ <modelling.data@gmail.com> - 2017-01-30 04:46 -0800
      Re: Recursion: getting input from the user David Brown <david.brown@hesbynett.no> - 2017-01-30 14:35 +0100
        Re: Recursion: getting input from the user Alla _ <modelling.data@gmail.com> - 2017-01-30 05:55 -0800
      Re: Recursion: getting input from the user Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-30 17:16 +0000
  Re: Recursion: getting input from the user Barry Schwarz <schwarzb@dqel.com> - 2017-01-30 10:07 -0800
    Re: Recursion: getting input from the user Alla _ <modelling.data@gmail.com> - 2017-01-30 11:13 -0800
    Re: Recursion: getting input from the user "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-30 15:07 -0500
    Re: Recursion: getting input from the user Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-30 20:16 +0000
    Re: Recursion: getting input from the user Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-02 01:47 -0800
    Re: Recursion: getting input from the user supercat@casperkitty.com - 2017-02-02 12:38 -0800

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