Copy input from user and put it on screen - c

Question

I have been going through The C Programming Language by Brian W. Kernighan and Dennis M. Ritchie and am at Character Input and Output, specifically on File Copying. The example in the book
#include <stdio.h>
int main(void)
{
int c;
c = getchar();
while (c != EOF)
{
putchar(c);
c = getchar();
}
return 0;
}
works perfectly.
But I decided to put my own spin on things and rewrite it a bit:
#define <stdio.h>
int main(void)
{
int c;
printf("Please enter a digit: ");
c = getchar();
while (c != EOF)
{
printf("The digit entered was: ");
putchar(c);
printf("Please enter a digit: ");
c = getchar();
}
return 0;
}
After compiling and executing, the result of my code is:
Please enter a digit: 9
The digit entered was: 9Please enter a digit: The digit entered was:
Please enter a digit: *cursor is here*
The input should be:
Please enter a digit: 9
The digit entered was: 9
Please enter a digit: *the cursor should be here*
Also, I have a little problem understanding EOF. It would be great if someone could help me with these issues.
Thank you!

Answer 1

Most 'terminals' do not send keyboard input to the program 'one key at at time'. Rather, the terminal will wait for the enter key, and then send all keys pressed (up to that point) to the target program. Hence, the question code suffers from getting newlines \n as input, as well as digits.
The code below throws away all non-digit values (but also allows for the EOF value):
#include <stdio.h>
#include <ctype.h>
int main(void)
{
int c;
do {
c=0;
/* Get user input */
printf("Please enter a digit: ");
while(!isdigit(c) && EOF != c)
c = getchar();
/* Produce output. */
if(c == EOF)
printf("\n");
else
printf("The digit entered was: %c\n", c);
} while (c != EOF);
return 0;
}
Compile the above (Linux GCC example: gcc -Wall -o test *.c)
Then execute: (Linux example: ./test)
Please enter a digit: 5
The digit entered was: 5
Please enter a digit: 6
The digit entered was: 6
Please enter a digit: <CTRL-D>
<CTRL-D> usually generates an EOF condition.

Answer 2

Because the ENTER value "\n" (ASCII value 10) will be get by
getchar(c);
so it will do second time of
printf("The digit entered was: ");
printf("Please enter a digit: ");

Answer 3

Well, it's the trouble about the new-line character that you feed the standard input with while handing your piece into the console screen.
Let me ask you this:
With the code that you have said to work perfectly, you probably have tried some input like "Hello!" let's say, or maybe just a single character like "9" as in your example. You have typed down 'H' then 'e' then 'l' ... then '!' and then hit the enter-key, or just '9' and then hit the enter-key.
After that, you had the following on your screen:
Hello!
Hello!
_
The last underscore there is for indicating the flashing cursor. My question is: Why is it there? Why is it not right next to the last '!'?
Same story with the 9, this would be the things on your screen:
9
9
_
Well, here's why: It is because your programme has printed one more character that you cannot see directly, a new-line character '\n'. Proof: If it hadn't printed that, the underscore would flash next to the '!' or '9'.
When you hit your enter-key, you feed the standard input with a new-line character '\n' in addition to whatever you had typed down so far in that session. Standard input gets fed with all that and getchar(); consumes them one by one.
In your code which doesn't work as you had expected, you feed the standard input with '9' '\n', getchar(); first gets the '9' and then '\n', and then it looks for more, but there isn't any more. Only then (when there isn't more) it asks from you for more.
And that's the story...
To get what you were expecting, you should do something else, I won't tell you that directly, it's a good exercise. I can hint you that you should be using another while loop within the one you already have with a condition checking for a '\n' encounter.
EOF is just a special character, or rather a value indicating a special case of being at the End Of File. For standard input, I guess it can be issued with the CTRL + D key combination on Linux, it is CTRL + Z on Windows. Using that combination and then hitting enter should get you out of that outer loop.

Answer 4

You're being tripped up by a trailing newline in your input; when you type 9 and hit Enter, the input stream that your code reads from will contain the encodings for two characters, the 9 and a newline (in ASCII, it would be the sequence of values [57,10]) . Here's how it breaks down:
Code prompts you to enter a digit
You type 9 and hit Enter
Input stream now contains the characters {'9', '\n'};
getchar returns the next available character from the input stream, which is '9'
'9' is not equal to EOF, so you print the digit and prompt for the next character
getchar returns the next available character from the input stream, which is '\n'
'\n' is not equal to EOF, so you print the character and prompt for the next character
The input stream is now empty, so getchar blocks until you type something else.
You can fix this relatively easily:
#include <ctype.h>
...
while (c != EOF)
{
printf("The digit entered was: ");
putchar(c);
printf("Please enter a digit: ");
do
{
c = getchar();
} while ( isspace( c ));
}
That last little do loop will read characters from the input stream until it sees something that isn't a newline or other whitespace character. You can add an additional check to make sure the character entered really was a digit [0-9]:
while (c != EOF)
{
printf("The digit entered was: ");
putchar(c);
do
{
printf("Please enter a digit: ");
do
{
c = getchar();
} while ( isspace( c ));
if ( !isdigit( c ))
printf( "%c is not a digit!\n" );
} while ( !isdigit( c ));
}
Now, something to be aware of; what you're storing in c is not the value 9, but the encoding for the character '9', which in ASCII is the integer value 57. If you want to store the value 9 in c, you'll have to do something different.