Can't we store the character in the first element (index:0) of the character array?

Question

0.00/5 (No votes)

See more:

Can't we store the element in the first element (index:0) of the character array?

When the program was debugged,
at i=0, it is not accepting any input. But from i=1 onwards it is accepting the input

and this is not happening in the case of normal array(array of type int)

What I have tried:

<pre lang="C++">
#include<stdio.h>
int main()
{
    int n;
    scanf("%d",&n);
    char a[n];
    int i;
    for(i=0;i<n;i++)
    scanf("%c",&a[i]);
}

Posted 1-May-21 2:12am

_-_-_-me

Updated 1-May-21 2:49am

Add a Solution

Comments

Richard MacCutchan 1-May-21 8:28am

~~That code will not even compile so you cannot debug it. The line char a[n]; is not valid in C, as already mentioned in a previous question.~~

Well, it appears that gcc allows it.

OriginalGriff 1-May-21 8:46am

Are you sure about that? It works fine for me with the GDB compiler:
https://www.onlinegdb.com/online_c_compiler

Richard MacCutchan 1-May-21 8:58am

Microsoft's cl compiler doesn't accept it:

test.cpp(18): error C2131: expression did not evaluate to a constant

OriginalGriff 1-May-21 10:04am

That's because it's C90 compliant, not C99.

Gawd, how do I remember this kind of trivia? :D

Richard MacCutchan 1-May-21 10:17am

"how do I remember this kind of trivia?"
I'd like to know the answer to that. I struggle to remember most things these days.

2 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

OriginalGriff · Answer 1 · 2021-05-01T02:45:00

Is does work: it just probably doesn't do what you expect it to.
When you use scanf for a number, it waits until the user enters the number and presses ENTER, then parses the input and returns the numeric value - but the ENTER character is "left on the shelf" so when you subsequently use scanf for the individual characters, the first one returned is the ENTER character: '\n'
Then the remaining characters are read, so it looks like you end up one short...
Try this and see if it helps:

C

scanf("%d\n", &n);

Richard MacCutchan · Answer 2 · 2021-05-01T02:49:00

Solution 2

Adding the \n specifier to the format strings appears to resolve it:

C++

int n;
scanf("%d\n",&n); // tell scanf to consume the newline
char a[n];
int i;
for(i=0;i<n;i++)
{
    scanf("%c\n",&a[i]);
    printf("a[%d]: %c\n", i, a[i]); // print the stored character
}

[edit]
I should have mentioned that this was using gcc under Linux. Microsoft's C/C++ compiler does not accept dynamic stack allocations.
[/edit]

Posted 1-May-21 2:49am

Richard MacCutchan

Updated 1-May-21 4:31am

v2

Comments

k5054 1-May-21 10:21am

Did you try this? For me, this just hangs in scanf.

Richard MacCutchan 1-May-21 10:30am

Yes, but using gcc under Linux. But see also my comment to OG above.

k5054 1-May-21 11:11am

Interesting. I've hit enter about 10 times after entering a number, and gdb says I'm still in scanf. eg:

[k5054@localhost]$ cat example.c
#include <stdio.h>
#include <stdlib.h>

int main()
{
    int n;
    printf("enter a number: ");
    scanf("%d\n", &n);
    printf("n = %d\n", n);

    return 0;
}
[k5054@localhost]$ gcc example.c -o example
[k5054@localhost]$ ./example.c
bash: ./example.c: Permission denied
[k5054@localhost]$ ./example
enter a number: 10





^C
[k5054@localhost]$ gdb example
GNU gdb (GDB) Fedora 10.1-4.fc33
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from example...
(No debugging symbols found in example)
(gdb) r
Starting program: /home/ebacon/tmp/example 
enter a number: 10



^C
Program received signal SIGINT, Interrupt.
0x00007ffff7ec4442 in __GI___libc_read (fd=0, buf=0x4056b0, nbytes=1024) at ../sysdeps/unix/sysv/linux/read.c:26
26        return SYSCALL_CANCEL (read, fd, buf, nbytes);
(gdb) bt
#0  0x00007ffff7ec4442 in __GI___libc_read (fd=0, buf=0x4056b0, nbytes=1024) at ../sysdeps/unix/sysv/linux/read.c:26
#1  0x00007ffff7e55122 in _IO_new_file_underflow (fp=0x7ffff7f96800 <_IO_2_1_stdin_>) at libioP.h:948
#2  0x00007ffff7e563a6 in __GI__IO_default_uflow (fp=0x7ffff7f96800 <_IO_2_1_stdin_>) at libioP.h:948
#3  0x00007ffff7e2ed5b in __vfscanf_internal (s=<optimized out>, format=<optimized out>, argptr=argptr@entry=0x7fffffffd930, 
    mode_flags=mode_flags@entry=2) at vfscanf-internal.c:3023
#4  0x00007ffff7e2c712 in __isoc99_scanf (format=<optimized out>) at isoc99_scanf.c:30
#5  0x0000000000401163 in main ()
(gdb) quit
A debugging session is active.

        Inferior 1 [process 195134] will be killed.

Quit anyway? (y or n) y
[k5054@localhost]$ 
[k5054@localhost]$ lsb_release -ir
Distributor ID: Fedora
Release:        33
[k5054@localhost]$

Richard MacCutchan 1-May-21 11:21am

Same here. No matter how often I try I never get scanf to do what it is supposed to do. Your code sample (and mine incidentally) is still waiting for another input value.

_-_-_-me 1-May-21 11:38am

Why it is waiting for other input value?
Thank you !

Richard MacCutchan 1-May-21 11:39am

Ha ha, if I knew that ...

k5054 1-May-21 12:03pm

Lets try this again: I think this has to do with how scanf handles whitespace, in that given a format string "%d %d", scanf will consume any and all whitespace between integer values. So if your format string ends with whitespace, (tab, space, newline, etc), then scanf will try to consume all trailing newlines to satisfy the format string. The only way to proceed, then, is to enter a non whitespace character.

Richard MacCutchan 1-May-21 11:47am

I just ran another test, and discovered that the order and structure of the input values is significant. If I enter values like this:

3
a
b
c
// it hangs here

But if I enter them like this:

3
abc

It works correctly.

OriginalGriff 1-May-21 10:35am

It will "hang in scanf" becuas ethis is line based input: scanf does nothing until the user has pressed the ENTER key - giving him a chance to correct his mistakes!

There isn't really a "standard way" of receiving key-by-key data but getch shoudl work for Windows (you'll need conio.h)

_-_-_-me 1-May-21 11:29am

why it is hanging in scanf()?
But In the case of not using new line character in first scanf() function , and using new line character in scanf() of for loop . It is not hanging up
why is that so?
Thank you !

OriginalGriff 1-May-21 11:59am

Because scanf processes a stream - it doesn't "restart" each time to call it, the underlying stream buffers data up and only "releases" the user input to scanf once ENTER is entered. (That's actually bollocks, what really happens is more complicated that that, but it'll do as a rough guide)

The first scanf waits for you to press enter before scanf even starts seeing character that might be a number: it "hangs" there too.
The second and subsequent scanf calls do exactly the same thing: wait for the user to press ENTER before looking for the input you have specified.

Richard MacCutchan 1-May-21 12:10pm

"That's actually bollocks"
Well I think that's a fair description of scanf. :)) :))

OriginalGriff 1-May-21 12:29pm

:D

Richard MacCutchan 1-May-21 11:38am

What is even more weird is that the example at scanf - C++ Reference[^] works fine. And I just noticed in my example that it always waits for an extra character input after the Nth has been accepted.