I'm trying to implement a simple while loop to let the user enter multiple marks without having to reload the application, for some reason no matter what i seem to enter it always loops.
I've looked using the debugger and it doesn't seem to accept the final scanf() asking whether to repeat itself or not.
int mark = 0;
char grade;
char choice = 'y';
while(choice == 'y')
{
//Request input
printf("enter a mark: ");
scanf("%d", &mark);
//Assess mark
grade = assess(mark);
//Output result
printf("That equals ");
printf("%c", grade);
printf(" when graded\n");
//Repeat?
printf("Again?...\n");
fflush(stdin);
scanf("&c", &choice);
}
I've also tried it with a do - while loop and still no joy, any idea where the problem may lie?
At least two problems:
fflush(stdin);
is undefined - you can only flush output streams. And:
scanf("&c", &choice);
should be:
scanf("%c", &choice);
I think last line should be
scanf("%c", &choice);
instead of
scanf("&c", &choice);
fflush() is only defined for output streams. The comp.lang.c FAQ does not recommend using it for stdin.
Also, as others have noted, use scanf("%c", &choice); to read the choice.
Try scanf("%c", &choice);.
Note that scanf returns the number of inputs matched, so you should really check the return value. If the input does not, for som reason, map to a character, your variable might be unchanged. Before the call to scanf, set choice to something != 'y', so that you only continue if a y is input.
Related
If the user would enter an invalid input, the program will ask the question again until the user enters a valid input. The code below performs it however, it executes the command twice:
void error_checking(){
char input;
printf("Enter letters from a to e");
scanf("%c", &input);
if((input<102) || (input>96)){
printf("Valid input");
} else {
error_checking():
}
int main(){
error_checking();
}
Use
scanf(" %c", &input);
^^^
instead of
scanf("%c", &input);
^^
Otherwise the function scanf will read the new line character '\n' that corresponds to the pressed key Enter.
Also you firgot the closing brace of the function.
And it is a bad idea to use magic numbers like
if((input<102) || (input>96)){
And at least you have ti write
if((input<102) && (input>96)){
Maybe there is no problem in running the first code
However,it will always default after I enter comment's data
char answer;
do{
printf("Do you want to add new comment?Y/N: ");
scanf("%c",&answer);
fflush(stdin);
switch(tolower(answer))
{
case 'y':
comment();
break;
case 'n':
main();
break;
default:
printf("Wrong choice !\n\n");
break;
}
}while(tolower(answer)!=='y'||'n');
there is the code of comment(),I guess the problem at here.
FILE*fp=fopen("comment.txt","a+");
if(fp == NULL)
{
printf("FIle not Found");
exit(1);
}
else
{
printf("Please enter your name: ");
gets(c.name);
printf("Pleas enter the date: ");
gets(c.date);
printf("Please enter the movie name: ");
gets(c.movie);
printf("Please enter your comment in 100 words:\n");
printf("Please press [Tab]and[Enter] to submit your comment\n");
scanf("%[^\t]",c.comment);
fprintf(fp,"%s %s\n%s\n%s\n\n",c.name,c.date,c.movie,c.comment);
}
fclose(fp);
Can someone help me?Thx!!
try using strlwr() instead of tolower()
Compiler gives you no error for while condition?
Maybe you want to write:
while(tolower(answer)!='y'||tolower(answer)!='n');
Inside do{}while you are reading input buffer which has a character you entered and a enter key. You need to clear that. Try scanf("%c%*c",&answer); .which will read that extra character from the buffer
A couple of problems in your code.
scanf("%c",&answer);
When you enter a character for the first time and if the character if either y or n then in next iteration this scanf() will read the stray \n (newline) character from the input buffer. To overcome this problem, add a space character before %, like this:
scanf(" %c",&answer);
Another problem is this statement:
while(tolower(answer)!=='y'||'n');
^^ ^
The compiler must be giving you both error and warning in this statement.
Change this to:
while(tolower(answer)=='y'||tolower(answer)=='n');
With this the loop to be run till user give input either y or n and for any other character, the loop will exit.
If you don't want loop to exit for any input character other then y or n but just print the message Wrong choice ! message then you can do:
}while(tolower(answer)!='y'||tolower(answer)!='n');
I'm completing an assignment and after completing it, I have 1 bug, and 1 bug fix I made that I don't fully understand. Currently, as long as the user does what is asked, everything works fine. But I know that doesn't happen often, so I'd love to know how to stop these issues.
Would love any advice - I am a complete beginner with C.
I found many different pieces of advice here: C: Multiple scanf's, when I enter in a value for one scanf it skips the second scanf
I added a space to my scanf() statements which solved some of the bugs - and I understand that \n is added onto the end of the entered strings / chars, I'm just not sure how to check for it / handle it, and I tried using getchar() in place of the scanf() but I still get double print / loop problems.
Bug Issue
When the user is running through the game loop, if they enter more than 1 character (for example: 'oo', when prompted with the scanf() to enter 'y' or 'n') my printf statements run 1x per character entered, and connect to each other:
Example would be:
Welcome to Two doors.
Would you like to play? (y/n):Welcome to Two doors.
Would you like to play? (y/n):
This issue also shows up if the user enters 'y' to play the game but then enters a character other than 1,2 or 3 in the second section.
How can I limit the length of their response? Or is the best way to monitor the length of the play and choice variables prior to entering the if statements? Maybe checking to see if they are longer than 1 character and if so, only taking the first character?
Second issue - bug fix that I don't understand
In the scanf() functions I ran into a very similar problem to what I described above, but it happened when the user entered any character. The solution I found was to add a space before the character ->
scanf(" %c", &play);
vs
scanf("%c", &play);
Is this issue only a problem when using loops? Since I never found these bugs prior to looping back through the code.
Updated Code with 'while (getchar() != '\n');' suggestion from Sourav Ghosh
#include <stdio.h>
int main(void) {
char play;
int choice;
char answer[] = "No matter which one you choose the guards both tell you which door leads to death, and therefore you can pick the other door.\n";
int gameLoop = 1;
int timesPlayed = 0;
while (gameLoop == 1){
if (timesPlayed == 0) {
printf("Welcome to Two doors.\n");
printf("Would you like to play? (y/n):");
} else {
printf("Would you like to play again? (y/n):");
}
scanf(" %c", &play);
while (getchar() != '\n');
if (play == 'y') {
// == instead of =
printf("\nYou are a prisoner in a room with 2 doors and 2 guards.\n");
printf("One of the doors will guide you to freedom and behind the other is a hangman --you don't know which is which.\n");
printf("One of the guards always tells the truth and the other always lies. You don't know which one is the truth-teller or the liar either.\n");
printf("You have to choose and open one of these doors, but you can only ask a single question to one of the guards.\n");
printf("What do you ask so you can pick the door to freedom?\n\n");
printf("\t1.Ask the truth-guard to point to the door of doom.\n");
printf("\t2.Ask the liar-guard to point to the door of doom.\n");
printf("\t3.Doesn't matter which one you pick.\n");
scanf(" %d", &choice);
while (getchar() != '\n');
switch (choice) {
case 1:
printf("%s", answer);
timesPlayed++;
break;
case 2:
printf("%s", answer);
timesPlayed++;
break;
case 3:
printf("%s", answer);
timesPlayed++;
break;
default:
printf("The Troll Smasher comes out from the shadows and squeezes the stupid out of you until you pop. GAME OVER!\n");
break;
}
} else if(play == 'n') {
printf("Sorry to hear that, we at Two Doors hope you have a super duper day!\n");
gameLoop = 0;
break;
} else {
printf("That is not a valid input, please try again by entering either 'y' to start the game or 'n' to quit the game.\n");
}
}
return 0;
}
The problem with %c format specifier is that, it will read only one byte from the input buffer and if the input buffer has more in store and the call in encountered next time, it will not ask for user input, it will simply read the next byte from the available input stream.
So, to answer
How can I limit the length of their response?
well, there's no straightway approach that you can stop the user from entering only X characters/ digits, instead, swipe off the excess, (if any) and for the next call, start with an empty buffer is an easy approach.
So, the quick way out of this would be, to clean off the standard input of remaining inputs. You can do something like
int retval = scanf(" %c", &play);
//some code
while (getchar() != '\n'); //eat up the input buffer
//next call to scanf(), input buffer is empty now....
to stop scanf() from reading already existing unwanted inputs and force it to ask the input from user.
Also, don't forget to check the return value of scanf() to ensure the success of the call.
For the first issue the problem is caused because the execution of the program enters the loop again for example if the user types oo that means that after reading with scanf it is going all the way to the last else.
Inside that else none of the variables is modified so when it reenters the loop gameLoop is still 1 and timesPlayed is still 0 so it will print the statements in the first if, then scanf will read the second o and repeat the process. The problem is that scanf reads one character at the time.
Actually for entering one character you can use getchar() but in any case after char input you should clean standard input stream. Consider the following example, that forces the user to the correct input:
char name[11];
char answer = 0;
printf("Would you like to play again? (y/n): ");
while ((answer = getchar()) != 'y' && answer != 'n')
{
printf("You should answer 'y' or 'n'\n");
// clean the buffer from mess
while (getchar() != '\n');
}
// clean the buffer from mess
while (getchar() != '\n');
// next input
printf("Enter your name: ");
scanf("%10s", name);
// clean the buffer from mess
while (getchar() != '\n');
UPDATE:
Just for clarification, the code
while ((answer = getchar()) != 'y' && answer != 'n')
{
printf("You should answer 'y' or 'n'\n");
// clean the buffer from mess
while (getchar() != '\n');
}
can be be easier to understand while rewritten as
char name[11];
char answer = 0;
printf("Would you like to play again? (y/n): ");
while (1) // infinit loop
{
answer = getchar();
// clean the buffer from mess (immideatly after reading)
while (getchar() != '\n');
if (answer == 'y' || answer == 'n') // check the input
break; // stop the loop if condition is true
// or ask again
printf("You should answer 'y' or 'n'\n");
}
// next input
printf("Enter your name: ");
scanf("%10s", name);
// clean the buffer from mess
while (getchar() != '\n');
in my first example I just optimize the code combining reading and checking the data in parentheses after while: (answer = getchar()) != 'y' is like two actions - answer = getchar() and then answer != 'y'
In the last snippet condition answer != 'y' && answer != 'n' was intentionally replaced with answer == 'y' || answer == 'n' to show difference between "do while data is incorrect" and "stop when correct data get"
Hi this question might look kinda stupid, but I am facing some serious lack of fundamentals here which i cannot figure out. This is a very simple code for scanning four characters but it wont work properly. This is an extract from a larger program, but this is where I am facing the problem. Can anyone point out where the blunder is being made?
#include<stdio.h>
#include<stdlib.h>
{
char a, b, c, d;
printf("Enter the value of a\n");
scanf("%c", &a);
if(a == 'Y')
{
printf("Enter if this question is stupid or no Y/N\n");
scanf("%c", &b);
}
else
{
printf("This is such a big waste of time");
}
printf("Enter the value of c\n");
scanf("%c", &c);
if(c == 'Y')
{
printf("Enter if I am stupid or no? Y/N\n");
scanf("%c", &d);
}
else
{
printf("I will go mad soon\n");
}
}
I think it is possible that you have problems because of newline character. Try using
scanf(" %c", &var)
Note: My original answer was wrong but accepted, I copied this solution from #teppic.
The %c specifier tells scanf to read one character only. When you type a character and press enter, you're providing two. So the next call already has a character to read.
In order to get this work properly you need to skip any whitespace before you read a character. Change each call like this:
scanf(" %c", &a)
Now any previous newline you entered will be skipped.
You are not flushing your buffer after scanning the character, when You enter a character and press ENTER then in next scan your ENTER is treated as a character that's why it is skipping the second scan. Use any one of these __fpurge(stdin), fpurge(stdin) after each scan, whatever works on your system. It will work.
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.