char again;
do {
counter = 0;
while (counter < 3) {
printf("Please enter a number: ");
scanf("%d", &num);
counter++;
sum += num;
}
if (counter == 3) {
printf("Would you like to continue? [Y]Yes [N]No:");
scanf("%c", &again);
}
}while (again == 'Y');
I can't seem to figure out why this won't work. If i enter Y it breaks, if I enter N it breaks. I need to loop until the user enters "N" to exit the program and no other letter.
Change the scanf call to:
scanf(" %c", &again);
The trick is in the space before the %c: it instructs the scanf function to ignore any whitespace character before returning your N or Y. Otherwise you will be reading the return carriages from the previous scanf calls.
From man 3p scanf (the POSIX one):
A directive composed of one or more white-space characters shall be executed by reading input until no more valid input can be read, or up to the
first byte which is not a white-space character, which remains unread.
That is a complex function. I recommend reading the man page with care: it can do much more than most people think.
The problem is that the scanf("%c", &again) reads the newline after the last number, which is why the loop always breaks. It's also why people shun scanf() in favour of fgets() plus sscanf(); you tend to get better diagnostics and fewer nasty gotchas like this.
How could you have debugged this for yourself?
You could have printed the data that is read as it is read — add:
printf("Read: %d\n", again);
after the scanf(). Also, you should be checking each scanf() call to ensure you are getting the data you expect:
if (scanf("%c", &again) != 1)
...oops — error handling...
Related
I'm trying to make a program where the user inputs value to an array. What is actually required is that the program should validate against a char character. So if the user inputs a random char such as 'n' the program should tell him "You introduced a char, please input an integer: ".
How is that possible to make that without using a char variable?
for (i = 1; i <= size; i++) {
printf("Introduce the value #%d of the list: ", i);
scanf("%d", &list[i]);
if () { // I'm blocked right in this line of code.
printf("What you tried to introduce is a char, please input an integer: ");
scanf("%d", &list[i]);
}
Thanks in advance.
As #MFisherKDX says, check the return value of scanf. From the scanf man page:
These functions return the number of input items successfully matched
and assigned, which can be fewer than provided for, or even zero in
the event of an early matching failure.
The value EOF is returned if the end of input is reached before either
the first successful conversion or a matching failure occurs. EOF is
also returned if a read error occurs, in which case the error
indicator for the stream (see ferror(3)) is set, and errno is set
indicate the error.
So capturing the return value of scanf in an int variable and then comparing that variable to 1 (in your case, because you are only attempting to read 1 item) should tell you if scanf successfully read an integer value.
However, there is a nasty pitfall when using scanf that you should be aware of. If you do type n at the prompt, scanf will fail and return 0, but it will also not consume the input you typed. Which means that the next time you call scanf, it will read the same input (the n character you typed), and fail again. And it will keep doing so no matter how many times you call scanf. It always amazes me that computer science educators continue to teach scanf to students, given not only this potential pitfall, but several other pitfalls as well. I wish I had a nickel for every hour that some CS student somewhere has spent struggling to get scanf to behave the way their intuition tells them it should. I'd be retired on my own private island by now. But I digress.
One way around this particular pitfall is to check if scanf failed, and if so, to purposely consume and discard all input from stdin up to and including the next newline character or EOF, whichever comes first.
First let's look at some unfixed code that causes an infinite loop if you enter a non-integer as input:
// Typing the letter 'n' and hitting <Enter> here causes an infinite loop:
int num, status;
while (1) {
printf("Enter a number: ");
status = scanf("%d", &num);
if (status == 1)
printf("OK\n");
else
printf("Invalid number\n");
}
The above code will (after you type n and hit <Enter>), will enter an infinite loop, and just start spewing "Invalid number" over and over. Again, this is because the n you entered never gets cleared out of the input buffer.
There are a few possible ways to get around this problem, but the consensus seems to be that the most portable and reliable way to do so is as follows:
// Fixed. No more infinite loop.
int num, status;
while (1) {
printf("Enter a number: ");
status = scanf("%d", &num);
if (status == 1)
printf("OK\n");
else {
printf("Invalid number\n");
// Consume the bad input, so it doesn't keep getting re-read by scanf
int ch;
while ((ch = getchar()) != '\n' && ch != EOF) ;
if (ch == EOF) break;
}
}
The function scanf() will returns the number of elements read, so in this case it will return 1 every time it reads an int and 0 when it reads a char, so you just need to verify that return value.
Keep in mind that after reading a character it will remain in the buffer so if you use the scanf() command again it will read the character again and repeat the error. To avoid that you need to consume the character with while(getchar() != '\n');
With that in mind I modified your code so that it works properly printing an error message if a character is introduced and asking for a new int.
for (int i = 1; i <= size; i++) {
printf("Introduce the value #%d of the list: ", i);
while (!scanf("%d", &list[i])) { //verifies the return of scanf
while(getchar() != '\n'); //consumes the character in case of error
printf("What you tried to introduce is a char\n");
printf("please introduce the value #%d of the list: ", i);
}
}
I need to use scanf to get a character and a string which would store the user's answer. (yes/no)
The code below skips scanf("%c", &elem).
while ( !strcmp ("yes", option))
{
printf("enter the elements \n\n");
elem=getchar();
printf("you have entered %c\n",elem);
enqueue(st, elem);
printf("please enter yes or no ");
scanf("%s[^\n]",option);
}
./out
enter the elements
a
you have entered a
enqueue elem= a
please enter yes or no yes
enter the elements
you have entered
enqueue elem=
You don't have any scanf("%c", &elem) in your code... btw the problem is with the enter for scanf. When you get an input by scanf, an enter character stays in the input buffer which will be read by your getchar() function in the second round. one simple way to solve it is to add a dummy getchar after your scanf line:
while ( !strcmp ("yes",option))
{
printf("enter the elements \n\n");
elem=getchar();
printf("you have entered %c\n",elem);
enqueue(st,elem);
printf("please enter yes or no ");
scanf("%s[^\n]",option);
getchar();
}
You can find more information about how to clear your input buffer here: How to clear input buffer in C?
I can recommend you consider two things:
For getting only a character, I personally found it much more easier to use getch and getche function in Windows, and equivalent of them for GCC-compatible environments. You can find samples of it online or on this line [What is Equivalent to getch() & getche() in Linux?
Always flush the input buffer after you read your input to prevent any similar problems to happen.
The input functions check the input buffer, which you can find at 0xb8000000, and check the first input there. If the buffer is empty, they wait for the user to enter the input, otherwise, they check the first element in the buffer and then examine that to what they expect to read. If they succeed, they read it and remove it from buffer. Otherwise, they fail to give you your input and depending on the function, the result is different.
For Example, consider the following line:
scanf("%d %d %f", &a, &b &c);
and give the input as:
a 2 4
The scanf will return 0, which means it reads zero inputs so 'a', 2, and 4 remains in your buffer. So your buffer looks like: [a, 2, 4]. As a result if you add the following line:
scanf("%c", &ch);
scanf will try to get a character from the buffer, and it reads character 'a' and put it in variable ch. So it doesn't get any input from user. And you end up with having 2 and 4 on your buffer again.
When you are pressing Enter/Return key to enter the element then a \n character is also passed to the buffer along with the element. This \n is read by your getchar on next call.
To consume this \n place this line after the getchar();
int ch;
while((ch = getchar()) != EOF && ch != '\n');
Take care mixing scanf() format specifiers "%c", "%s" and "%[]".
Correct usage of "%[^\n]": there is no s. If leading whitespace in not wanted to be saved, include a leading space as in " %[^\n]"
char option[100];
// scanf("%s[^\n]", option);
scanf(" %[^\n]", option);
// or better
scanf(" %99[^\n]", option);
// or pedantic
switch (scanf(" %99[^\n]", option)) {
case EOF: HandleEOForIOError(); break;
case 0: HandleNoData(); break; // might not be possible here.
case 1: HandleSuccess();
Correct usage of "%c". If leading whitespace in not wanted to be save, include a leading space as in " %c". This may be the case in OP's code so the preceding inputs Enter or '\n' is consumed.
char elem;
scanf(" %c", &elem);
Correct usage of "%s". Leading whitespace is not saved with or without a leading space.
char option[100];
scanf("%99s", option);
// Following works the same.
scanf(" %99s", option);
My program which finds prime factors is all set...the only thing left that I need to do is this type of output:
Do you want to try another number? Say Y(es) or N(o): y
//asks for another number (goes through the program again)
Do you want to try another number? Say Y(es) or N(o): n
//Thank you for using my program. Good Bye!
I have my attempt at this below...When I type n it does the correct output. But if I type 'y' it just says the same thing n does....How can I loop the entire program without putting the code for the program inside this while loop I have? So when I press y it goes through the program again?
int main() {
unsigned num;
char response;
do{
printf("Please enter a positive integer greater than 1 and less than 2000: ");
scanf("%d", &num);
if (num > 1 && num < 2000){
printf("The distinctive prime facters are given below: \n");
printDistinctPrimeFactors(num);
printf("All of the prime factors are given below: \n");
printPrimeFactors(num);
}
else{
printf("Sorry that number does not fall within the given range. \n");
}
printf("Do you want to try another number? Say Y(es) or N(o): \n");
response = getchar();
getchar();
}
while(response == 'Y' || response == 'y');
printf("Thank you for using my program. Goodbye!");
return 0;
} /* main() */
The problem is probably, that you're getting something that isn't y from getchar and the loop exits, as the condition is not matched.
getchar() may use a buffer, so when you type 'y' and hit enter, you will get char 121 (y) and 10 (enter).
Try the following progam and see what output you get:
#include <stdio.h>
int main(void) {
char c = 0;
while((c=getchar())) {
printf("%d\n", c);
}
return 0;
}
You will see something like this:
$ ./getchar
f<hit enter>
102
10
What you can see is that the keyboard input is buffered and with the next run of getchar() you get the buffered newline.
EDIT: My description is only partially correct in terms of your problem. You use scanf to read the number you're testing against. So you do: number, enter, y, enter.
scanf reads the number, leaves the newline from your enter in the buffer, the response = getchar(); reads the newline and stores the newline in response, the next call to getchar() (to strip the newline I described above) gets the 'y' and your loop exits.
You can fix this by having scanf read the newline, so it doesn't linger in the buffer: scanf("%d\n", &number);.
When reading input using scanf (when you enter your number above), the input is read after the return key is pressed but the newline generated by the return key is not consumed by scanf.
That means your first call to getchar() will return the newline (still sitting in the buffer), which is not a 'Y'.
If you reverse your two calls to getchar() - where the second one is the one you assign to your variable, your program will work.
printf("Do you want to try another number? Say Y(es) or N(o): \n");
getchar(); // the newline not consumed by the scanf way above
response = getchar();
just put getchar() after scanf statement of yours that will eat the unnecessary '\n' from buffer...
As others have stated, there is a single '\n' character in the input stream left over from your earlier call to scanf().
Fortunately, the standard library function fpurge(FILE *stream) erases any input or output buffered in the given stream. When placed anywhere between your calls to scanf() and getchar(), the following will rid stdin of anything left in the buffer:
fpurge(stdin);
Beginner with C here. I am trying to run a loop where strings and ints are entered into various fields of a struct. When prompted for a 'last name', the user can press enter with no other input and the loop should end.
The problem is that with this code, the loop doesnt end (last name and first name entry requests run together on the same line) and the value for salary always comes out wrong (0 or some large number)
while (employee_num <= 2)
{
printf("Enter last name ");
fgets(employee[employee_num].last_name, sizeof(employee[employee_num].last_name), stdin);
if(strlen(employee[employee_num].last_name) == 0)
break;
printf("Enter first name ");
fgets(employee[employee_num].first_name, sizeof(employee[employee_num].first_name), stdin);
printf("Enter title ");
fgets(employee[employee_num].title, sizeof(employee[employee_num].title), stdin);
printf("Enter salary ");
fgets(strng_buffer, 1, stdin);
sscanf(strng_buffer, "%d", &employee[employee_num].salary);
++employee_num;
getchar();
}
If I try this code instead, I am able to exit the loop properly after the first run through it, but cannot exit after that (by pressing enter at the last name portion - perhaps a \n I cant seem to clear?):
char strng_buffer[16];
while (employee_num <= 5)
{
printf("Enter last name ");
fgets(strng_buffer, sizeof(strng_buffer), stdin);
sscanf(strng_buffer, "%s", employee[employee_num].last_name);
if(strlen(employee[employee_num].last_name) == 0)
break;
printf("Enter first name ");
fgets(strng_buffer, sizeof(strng_buffer), stdin);
sscanf(strng_buffer, "%s", employee[employee_num].first_name);
printf("Enter title ");
fgets(strng_buffer, sizeof(strng_buffer), stdin);
sscanf(strng_buffer, "%s", employee[employee_num].title);
printf("Enter salary ");
scanf("%d", &employee[employee_num].salary);
++employee_num;
getchar();
}
I am curious as to how to make this work as intended and what best practice would be for entries like this (ie use of sscanf, fgets, etc)
Thanks in advance!
The Loop breaks prematurely when it encounters the break statement
if(strlen(strng_buffer) == 0)
break;
The uninitialized character buffer strng_buffer, coincidently has null as the first character causing strlen to return 0
I believe you may have intended
if(strlen(employee[employee_num].last_name) == 0)
break;
as the loop terminatorm, and it was a typo in your part causing premature loop exit.
Assuming the fix mentioned by Abhijit, why transform the first into the second? Are you aware that the second behaves differently to the first, because of the addition of sscanf? If your intention was to shorten the first, the second seems quite bulky. Rather than adding sscanf to the situation, why not shorten the first by declaring a struct employee *e = employee + employee_num; and using that repetitively, instead of employee[employee_num]?
One "best practise" regarding fgets is to check it's return value. What do you suppose fgets might return, if it encounters EOF? What do you suppose fgets would return if it's successful?
One "best practise" regarding scanf is to check it's return value. In regards to the return value of scanf, I suggest reading this scanf manual carefully and answering the following questions:
int x = scanf("%d", &employee[employee_num].salary); What do you suppose x will be if I enter "fubar\n" as input?
Where do you suppose the 'f' from "fubar\n" will go?
If it's ungetc'd back onto stdin, what would your next employee's last name be?
int x = scanf("%d", &employee[employee_num].salary); What do you suppose x will be if I run this code on Windows and press CTRL+Z to send EOF to stdin?
int x = scanf("%d %d", &y, &z); What would you expect x to be, presuming scanf successfully puts values into the two variables y and z?
P.S. EOF can be sent through stdin in Windows by CTRL+Z, and in Linux and friends by CTRL+D, in addition to using pipes and redirection to redirect input from other programs and files.
The problem is that fgets returns the string with the line break (\n) included. So, even the user presses return without entering info, the string won't be empty. Also, your buffer size for salary is too small.
So, either you strip out the \n on every fgets or you change your check to:
if(strlen(employee[employee_num].last_name) == 1) break;
Also, when you're getting the buffer, change 1 to something bigger, like
fgets(strng_buffer, 10, stdin);
However, if you do want to strip out the \n from each fgets, you can do something like:
employee[employee_num].last_name[strlen(employee[employee_num].last_name)-1] = 0;
You can do this for every string or, better yet, create a function that does it.
EDIT: if you can guarantee that the user will press enter after each input then you can safely assume this. However if it's not always the case it's possible that the last character is not \n and just stripping this way might cause problems.
int flag = 0;
int price = 0;
while (flag==0)
{
printf("\nEnter Product price: ");
scanf("%d",&price);
if (price==0)
printf("input not valid\n");
else
flag=1;
}
When I enter a valid number, the loop ends as expected. But if I enter something that isn't a number, like hello, then the code goes into an infinite loop. It just keeps printing Enter Product price: and input not valid. But it doesn't wait for me to enter a new number. Why is that?
When you enter something that isn't a number, scanf will fail and will leave those characters on the input. So if you enter hello, scanf will see the h, reject it as not valid for a decimal number, and leave it on the input. The next time through the loop, scanf will see the h again, so it just keeps looping forever.
One solution to this problem is to read an entire line of input with fgets and then parse the line with sscanf. That way, if the sscanf fails, nothing is left on the input. The user will have to enter a new line for fgets to read.
Something along these lines:
char buffer[STRING_SIZE];
...
while(...) {
...
fgets(buffer, STRING_SIZE, stdin);
if ( sscanf(buffer, "%d", &price) == 1 )
break; // sscanf succeeded, end the loop
...
}
If you just do a getchar as suggested in another answer, then you might miss the \n character in case the user types something after the number (e.g. a whitespace, possibly followed by other characters).
You should always test the return value of sscanf. It returns the number of conversions assigned, so if the return value isn't the same as the number of conversions requested, it means that the parsing has failed. In this example, there is 1 conversion requested, so sscanf returns 1 when it's successful.
The %d format is for decimals. When scanf fails (something other a decimal is entered) the character that caused it to fail will remain as the input.
Example.
int va;
scanf("%d",&va);
printf("Val %d 1 \n", val);
scanf("%d",&va);
printf("Val %d 2 \n", val);
return 0;
So no conversion occurs.
The scanf function returns the value of the macro EOF if an input failure occurs before
any conversion. Otherwise, the scanf function returns the number of input items
assigned, which can be fewer than provided for, or even zero, in the event of an early
matching failure
7.19.6. The scanf function - JTC1/SC22/WG14 - C
So you should note that scanf returns its own form of notice for success
int scanf(char *format)
so you could have also did the following
do {
printf("Enter Product \n");
}
while (scanf("%d", &sale.m_price) == 1);
if(scanf("%d", &sale.m_price) == 0)
PrintWrongInput();
Also keep in the back of your head to try to stay away from scanf. scanf or scan formatted should not be used for interactive user input. See the C FAQ 12.20
After the first number, a '\n' will be in the input buffer (the return you pressed to input the number), so in the second iteration the scanf call will fail (becouse \n isn't a number), scanf will not remove that \n from the buffer, so in the next iteration it will fail again and so on.
You can fix that by reading the '\n' with a getchar() call after scanf.
The "answers" that say it will because there is a '\n' in the buffer are mistaken -- scanf("%d", ...) skips white space, including newlines.
It goes into an infinite loop if x contains 0 and scanf encounters a non-number (not just whitespace) or EOF because x will stay 0 and there's no way for it to become otherwise. This should be clear from just looking at your code and thinking about what it will do in that case.
It goes into an infinite loop because scanf() will not consumed the input token if match fails. scanf() will try to match the same input again and again. you need to flush the stdin.
if (!scanf("%d", &sale.m_price))
fflush(stdin);
Edit: Back when I first wrote this answer, I was so stupid and ignorant about how scanf() worked.
First of all let me clear something, scanf() is not a broken function, if I don't know how scanf() works and I don't know how to use it, then I probably haven't read the manual for scans() and that cannot be scanf()'s fault.
Second in order to understand what is wrong with your code you need to know how scanf() works.
When you use scanf("%d", &price) in your code, the scanf() tries to read in an integer from the input, but if you enter a non numeric value, scanf() knows it isn't the right data type, so it puts the read input back into the buffer, on the next loop cycle however the invalid input is still in the buffer which will cause scanf() to fail again because the buffer hasn't been emptied, and this cycle goes on forever.
In order to tackle this problem you can use the return value of scanf(), which will be the number of successful inputs read, however you need to discard the invalid inputs by flushing the buffer in order to avoid an infinite loop, the input buffer is flushed when the enter key is pressed, you can do this using the getchar() function to make a pause to get an input, which will require you to press the enter key thus discarding the invalid input, note that, this will not make you press the enter key twice whether or not you entered the correct data type, because the newline character will still be in the buffer. After scanf() has successfully finished reading the integer from input, it will put \n back into the buffer, so getchar() will read it, but since you don't need it, it's safe to discard it:
#include <stdio.h>
int main(void)
{
int flag = 0;
int price = 0;
int status = 0;
while (flag == 0 && status != 1)
{
printf("\nEnter Product price: ");
status = scanf("%d", &price);
getchar();
if (price == 0)
printf("input not valid\n");
else
flag = 1;
}
return 0;
}