This function will be called by the menu.
void exponentiation()
{
int i, result = 0, first, second;
printf("\n%s\n%s\n\n%s",
"1) Exponentiation",
"------------------",
"Enter 1st integer: ");
scanf("%d", &first);
printf("Enter 2nd integer: ");
scanf("%d", &second);
printf("%d raised to %d equals %d\n", first, second, result);
main();
}
From this function I need to read the user input, if the user input is "enter" without any integer, it should be going back to the menu which is calling the main().
I already tried to get the input.
For example:
if(first == '\n')
{main();}
or
if(first == 10) /**which is 10 is ASCII code for enter**/
{main()}
Both ways it didn't work at all, any suggestions?
both ways it didn't work at all, any suggestion
The function scanf returns the number of items it successfully scanned. You should check its return and go back if it doesn't matches your expectations.
Also you should know %d ignores whitespace. So if the user hits return without entering an integer, scanf simply skips over it and waits for something else.
If you insist on not ignoring whitespace this way, you should avoid scanf and use other input methods such as fgets. Get input from the user line by line and use sscanf, strtoul and strtok to make sense of it.
Related
I was at first having trouble with a scanf() function being skipped, but I fixed that by adding in a space before %c in the scanf() function.
When trying to ask for input from the user as to whether the screen should be cleared, the scanf(" %c", cClear); conversion specifier gives an infinite loop, it is expecting a character, but responds to input as if not a character.
I believe it may have something to do with my input buffer.
I tried to use fflush(stdin) to no avail, I also used printf("%d", (int) cClear); to see the output, which was zero.
One other problem I have is trying to check user input for a digit.
I use:
if (isdigit(iSelection) == 0) {
printf("\nPlease select a valid numerical value.\n");
continue;
to check user input and restart the while loop, but anytime a character is entered and not an integer, I get an infinite loop.
My goal is to give the user the option to clear the screen after each calculation, and to also check input for being a digit.
Any help is appreciated.
//excluding code prior to main() and function definitions
int main(void) {
int iSelection = -1;
double foperand1 = 0, foperand2 = 0;
int ioperand1 = 0, ioperand2 = 0;
char cClear = '\0';
while (iSelection) {
printf("\n\nTHE CALCULATOR\n");
printf("\nCalculator menu:\n");
printf("\n1\tAddition");
printf("\n2\tSubtraction");
printf("\n3\tMultiplication");
printf("\n4\tDivision");
printf("\n5\tModulus (Integers only)");
printf("\n6\tTest if Prime (Integers only)");
printf("\n7\tFactorial (Integers only)");
printf("\n8\tPower");
printf("\n9\tSquare Root");
printf("\n0\tExit\n");
printf("\nPlease enter your selection: ");
scanf("%d", &iSelection);
//here we check for if input was a digit
if (isdigit(iSelection) == 0) {
printf("\nPlease select a valid numerical value.\n");
continue;
switch(iSelection) {
case 0:
break;
case 1:
printf("\nEnter the two numbers to add seperated by a space: ");
scanf("%lf %lf", &foperand1, &foperand2);
printf("\n%.5lf + %.5lf = %.5lf\n", foperand1, foperand2, addNumbers(foperand1, foperand2));
break;
}
//here we ask the user if they want to clear the screen
fflush(stdin)
if (iSelection != 0) {
printf("\nDo you want to clear the screen? ('y' or 'n'): ");
scanf("%c", cClear);
//printf("%d", (int) cClear); //used this to help debug
//scanf("%d", iSelection);
if (cClear == 'y')
system("cls");
}
}
printf("\nExiting\n");
return 0;
}
one error I get is "system" is declared implicitely. Could it possibly be the windows operating system not recognizing the pre defined function call?
Thanks to the people who commented to help me figure this out.
I had forgotten to add the (&) to the scanf() function call for the system"cls" function call, as well as didn't include the correct library (stdlib.h).
I was also able to make the program stop skipping the scanf() function by adding a space to the " %c" conversion specifier.
scanf Getting Skipped
I was able to make the isdigit() function work by changing the variable 'iSelection' to a character, but then I also had to change my case values to characters, not integers.
I need to take two int values from user. First value is using field specifier and second is normal integer value.
#include<stdio.h>
int main()
{
int num, num1;
printf("Enter first number: \n");
scanf("%2d", &num);
printf("First number is %2d\n", num);
printf("Enter second number: \n");
scanf("%d", &num1);
printf("Second number is %d\n", num1);
return 0;
}
and the output is
Enter first number:
12345
First number is 12
Enter second number:
Second number is 345
It won't give control to enter second number. I don't know why?
You see this behaviour because you have limited the size of input that the first scanf statement can consume. scanf("%2d", &num) says that scanf should read a field of width at most 2 and convert that into into num.
Change the scanf to scanf("%d", &num) and the entirety of 12345 will be processed.
you don't get to enter second num because your program gets it from first num.
First time, your scanf lets your num get first two digits of the number you entered, and rest remains in the buffer/stream.
next time scanf is executed, it reads the remaining digits from the stream till an enter stroke, hence you don't get to enter the second no.
if you want to read the second no.
try flushing the stream before using second scanf(), you will get what you want.
Why would you expect it to "give control"? The fist scanf() explicitly consumed at most two digits, leaving "345" unconsumed. The next scan begins with the unconsumed input. What else would you expect?
If you want to discard any unconsumed input before the next scan, use fpurge(stdin).
There is always a tradeoff with scanf. If you want to enter a whole number and then consume the trailing newline (left in the input buffer (stdin) as the result of pressing [enter]), you canappend a %*c to read and discard the trailing newline. This itself causes problems if an empty-string is entered.
However, limiting your scanf format string and specifier to %2d and then entering 123456, you intentially leave 3456\n in the input buffer which is taken as your input to the second scanf call. The only way to insure each of your scanf calls will only accept your expected input is to manually empty the input buffer after each read by scanf to insure there are no characters remaining prior to the next call. A simple way to do this is with either a do .. while or simply a while ; using getchar() to read each character in stdin until a '\n' is encountered or EOF:
#include<stdio.h>
int main()
{
int c, num, num1;
printf("Enter first number: \n");
scanf("%2d", &num);
while ((c = getchar()) && c != '\n' && c != EOF) ;
printf("First number is %2d\n", num);
printf("Enter second number: \n");
scanf("%d", &num1);
printf("Second number is %d\n", num1);
return 0;
}
output:
$ ./bin/scanf_tradeoff
Enter first number:
123456
First number is 12
Enter second number:
12345
Second number is 12345
The reason for second value is not getting from you, in first scanf you are mentioning that
get the two values from the input. You are giving more than two that time the remaining value is stored in the buffer. So second scanf will get the value from the buffer.
If you want to avoid this you can use this statement before the second scanf.
while ((c=getchar())!='\n' && c!=EOF);// clearing the buffer.
So now the second input will get from the user.
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.
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...
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;
}