Does the scanf function automatically add the NULL character at the end of the user's input? And if so, does it also "consume" the character of new line or tab as you click on enter or space? I was wondering about that because as far as I know, gets reads the input, until the user presses enter and then adds the NULL character and throws away the character of new line caused by enter. So what led me to ask this question were programs that include something like this:
This is not a complete program.
scanf("%s", array1);
gets(array2);
Now in this case, gets will read the character of new line and stop there, terminating the program without the user actually giving any input. So basically what I am asking, is what's the difference between scanf and gets as for the details mentioned above.
Note: I've recently learnt that using gets should be completely avoided but since my book and my teachers use it constantly, I'll have to stick to it for now.
Related
Environment:
Windows 7 Enterprise SP1
gcc version 6.3.0 (MinGW.org GCC-6.3.0-1)
I'm new to C language (not new to programming).
I'm migrating a big program to C language. All works fine except for this.
I create this small program to reproduce the problem:
int AAA ()
{
const char *p = "Enter to proceed.. X to Exit.. : ";
const char *ok = "OK" ;
char proceed1 ;
char proceed2 ;
printf(p) ;
fflush(stdin) ;
scanf("%c",&proceed1);
if (proceed1 == 'X'){
return 0 ; }
sleep(3) ;
fflush(stdin) ;
printf("\n%s",p) ;
scanf("%c",&proceed2);
if (proceed2 == 'X'){
return 0 ; }
printf("\n%s",ok) ;
return 0 ;
}
All worls fine, BUT if the user (wrongly) hits twice the enter key at the proceed1 then scanf of proceed2 automatically reads the newline character '\n' that remain in the buffer...
I tried all: rewind(stdin), fseek stdin, a getchar before the second prompt to bypass the problem, but let the scanf hang if the user correctly hit enter key once.
I repeat I'm new to C language. I understand the problem can be bypassed writing a scanf that does not accpet the \n alone.
But before proceeding I ask here this question:
How to completely clear the stdin buffer?
Just because this is a common enough question, notwithstanding the links to other like questions and/or duplicates, I think this bears repeating:
If you are trying to fflush(stdin), you are almost certainly doing something wrong
An input buffer flush says, “I don’t care what you typed so far. I’m throwing it away and we’re starting over.”
Such behavior is exceedingly frustrating to users of a program. Users expect their inputs to be recognized and acted on — even if those inputs are wrong! By throwing it away, your program tells the user that their wishes are not respected, leaving them feeling like they have less control of their computer than they should.
It’s an XY problem
The usual suspect behind desires to clear the input is a scanf failure. The user typed something wrong, input is stuck with invalid characters, and the program needs to get past that.
But the program does not need to get any further than the next newline, which is to say, it only needs to discard stuff that was input before the user pressed Enter. This comes from one very simple principle:
The user will always press Enter after every prompted input
The solution comes in three forms:
Crash and burn. This is a strong signal that the user needs to fix his input to work, and actually provides him (or her) with a greater sense of control: the program does only what it is supposed to, accepting only input it should, and does nothing unless the input is correct!This also affords the user greater control in how he or she uses your program, because he can now supply prepared input to the program without having to babysit the keyboard.
Skip to the EOL and ask for the user to try again. This presupposes a human is frobbing the keyboard. But if you are going to go through this grief, then:
Read all input as a string, one line at a time, and endeavor to process that string (using sscanf, for example). This keeps your input and processing pointers all lined up and you will never have need to try to resynchronize with the input buffer.
Obviously, my opinion is that crash and burn is the most correct method for your usual console programs.
This is, in fact, part of the core design of Unix. This behavior enables individual utility programs to combine to perform useful scripted tasks.
The exception is a TUI program.
A text-based user interface is for a program that requires a human to operate it*, and takes over the terminal to behave in a different way, not requiring the user to press Enter to receive and act on input.
Games and text editors are examples of this kind of program.
* It is still possible to automate a TUI, of course, but the point is that the expected interaction with the program is no longer via newline-delimited streams, but rather via direct, individual key presses as input.
The C Standard says that fflush(stdin) has undefined behavior. You should not use it to flush pending input even on platforms where it seems to have the expected effect. There is no portable way to flush pending input, ie: all input that was typed before the prompt was issued.
If you just want to consume the characters typed by the user for a previous entry parsed with scanf(), up to and including the newline, you can use this simple function:
#include <stdio.h>
// Read and discard the rest of the current line of input.
// Returns EOF at end of file, otherwise returns a newline character
int flush_input(FILE *fp) {
int c;
while ((c = getc(fp)) != EOF && c != '\n')
continue;
return c;
}
If you are required to use scanf(), you can use these 2 calls:
scanf("%*[^\n]"); // read the remaining characters on the line if any.
scanf("%1[\n]"); // read the newline character if present.
Note however that you should not combine these 2 as a single call to scanf() because the first conversion will fail if the pending byte is a newline, and this newline will not be consumed by the second conversion.
char keyin, buffer[1024];
do
{
keyin=gets(buffer);
}
while (keyin != "\n");
I have been trying to write a pause function in C where a user exits a paused state by pressing the Enter key. Where the user writes "pause" and this function gets executed. I have been working on this function for a while and it eludes me. I have implemented the code in several different ways and none of them work. I suspect it's because the comparison of "\n" with keyin. I think "\n" does not directly translate to the enter key.
You do not need a while loop to wait for a single Enter key press. It will wait (and you can press any key) until you press Enter: http://en.cppreference.com/w/c/io/gets
But you need to reserve a lot of space -- what if someone keeps pressing any other key "until something happens"? The buffer will overflow, and your program will (most likely) crash.
You probably want to use getchar -- this will send one keypress at a time back.
Note: the Enter key typically sends the ASCII code 13 (0x0D), not 10 (0x0A). You could use '\r' instead (minding others' notes on 'character' vs. "string"!), or prevent all confusion and use the hex or dec value.
This is different from the behavior of '\n' you are used to when outputting, because only certain functions will expand or convert the code '\n' in text to the required end-of-line sequence for your OS.
You cannot compare strings with == or != in C. You must compare the contents of the strings with a function like strcmp (or, preferably, one of the safer variants), e.g.:
while (strcmp(keyin, "\n") != 0);
As an aside, you should never ever actually use gets. It is impossible to use safely and securely.
Im writing this program in C and Im having a big problem when I compile it, once I get to the part were I ask the user if hes under 21, if the answer is yes, I ask more questions about that, but when I compile it, the program basicly answers its self. how can I fix this plz?
heres a screenshot, the orange box, i did not answer any of those questions.
http://i.imgur.com/4H3nl.png
You have forgotten to ask the user for input after each question except the first one. Just do scanf("%c", &a); after each question.
In the branch you're having trouble with, you're forgetting to prompt the user for input. Hence, it's reusing whichever answer was last given by the user.
You are reusing the same variable,a, without actually asking for the user input to put into a.
You only read information from the user once. You need to do a scanf before each
if (a == 'y')
only one scanf() function was using which means only once input is acceptable from the stdin stream. for scanf(), when press "enter", the stream stack to 'a'. then stdin stream is NULL. so before each question, should use scanf() once.
I'm working on a final project for a C programming course. The project is to create a database to store drug information. I've completed all of the elements and now it's down to fine tuning everything.
There's a requirement in the project in the function where the user can change the drugs information. In the requirement, the user should be able to skip a field by hitting enter. For example, the user can change the producer of the drug and the quantity. If the user didn't want to change the producer, they'd hit enter and move onto the quantity.
I've looked around the internet and was able to let the user skip entering a string for the producer. However, I cannot get it work with an integer.
This is what I used so that the user can skip entering a string:
scanf("%30[^\n]", fentry[found].producer);
For clarity sake, fentry.producer is a string with 30 characters and found is an integer variable.
I've tried doing something similar with the integer input (EDIT: By integer input, I meant the one to enter the quantity, not the 'found' varible). It will let you skip entering something, but if you do enter something, it stores a random value.
Anyone know how to go about this?
Rather than using scanf(), a better way to get interactive input from a user is to use fgets(). This function gets a complete line of input up to where the user presses Enter, and returns it so you can parse it.
You can then use sscanf() to read the information out of the line, if you like. sscanf() works almost exactly like scanf() except it reads data from a string rather than from standard input.
Have you initialized the integer to 0 before the scanf?
What about this?
char s[50];
fgets(s, 50, stdin);
if(s[0] != '\n') {
fentry[found].producer = atoi(s);
}
You have to remove all spaces from s though.
scanf("%29s", fentry[found].producer); while('\n'!=getchar());
This works at all, is strictly ANSI C conform, max. portable.
Input buffer is cleared, chararray-length is supported, chararray contains ever a (terminating) '\0' (no initialization needed) and your array contains never the '\n' from user input.
I suspect the keyboard buffer is causing the problems in your code. The first time you call the code, it will read a maximum of 30 characters - so long as they are not the NEWLINE character. They will terminate when they read the NEWLINE. The NEWLINE will remain in the buffer.
However... if you call scanf in the same way again, the NEWLINE from the previous call will still be in the keyboard buffer. This will be read before requesting more input from the keyboard. The NEWLINE will immediately fail the scanf pattern, the char array will not be updated, and you will be left to a char pointer that points to an uninitialized area of memory.
I'm it really sure how to explain this question but i'll give it a try.
I have this line of code in my program:
scanf (" %c", &character);
Notice the space before %c. This is supposed to keep the scanf from interpreting the last ENTER key as a character. This works, however after this line and its resulting printf output the program will not continue to execute until I provide another character and then press the ENTER key. After that it ignore all scanf and prints all printf output with the absence of any user input. Does that make sense. Im new to C so im sure im missing something simple but I couldn't find any solutions.
Thanks!
The standard input is buffered.
Your code will not see the buffer until it (the standard input) is flushed.
This happens when the buffer becomes full or the '\n' (Enter Key) is inserted.
Because scanf() is a blocking I/O call. The application will wait for input (indefinitely). Thus making the code look like it has stalled.
IMHO
You should not try and interpret what has happend behind you in previous parts of the code. This will couple your code to other input that you may have no control over. Your input should read everything it need to and no more.
For example: If you ask the user to enter a character then hit enter. You should remove the character then enter from the input stream. You should not expect the next user of the stream to tidy up for you (or compensate for your code).
printf("Enter (Y/N)\n");
scanf("%c", &character);
scanf("%*[^\n]"); // Ignore everything to the end of line.
// Assuming you have told the user you will ignore
// extraneous characters. The next input should now
// expect only what they need.
Try this:
scanf ("\n%c", &character);
It should "consume" the \n character in buffer and then wait for a new char.
In your previous scanf call, use something like scanf("%c\n", &character); to eat the newline as it is entered. This is typically less error-prone than waiting to handle it the next time you need input.