What's wrong with my code (HELP)

hi, can you guys help me out with this portion of code. I don't know what's wrong with it.

Code:

#include <stdio.h>
#include <math.h>

main()
{
  int count;
  float ctr,a;
  char x,s,S,c,C,q,Q;

  do{
    printf("(S)ine, (C)osine, or (Q)uit: ");
    scanf("%c", &x);
    if(x!='s' && x!='S' && x!='c' && x!='C' && x!='q' && x!='Q')
      printf(" -- Illegal Input!\n");
  } while(x!='s' && x!='S' && x!='c' && x!='C' && x!='q' && x!='Q');
}

it is supposed to print out this:

Code:

(S)ine, (C)osine, or (Q)uit: x
 -- Illegal Input!
(S)ine, (C)osine, or (Q)uit:

but it prints this out:

Code:

 (S)ine, (C)osine, or (Q)uit: x
 -- Illegal Input!
(S)ine, (C)osine, or (Q)uit:  -- Illegal Input!
(S)ine, (C)osine, or (Q)uit:

it prints out an extra line of sine, cosine, or quit with the illegal input statement after that. What did i do wrong? Help!!

What did i do wrong?

you used scanf()
FAQ

i still don't get it. what can i do/change/add to the code?

Well, a quick fix would be:

Code:

 {
   printf(" -- Illegal Input!\n");
   while (getchar() != '\n');
 }

Also, check the return from scanf() to ensure if read something OK:

Code:

 if (scanf("%c", &x) != 1)
 {
   /* Error */
 }

Read the FAQ that viaxd linked for you. The first scanf() is leaving \n on the buffer so the next time through the loop it thinks you entered a blank line.

The problem is not necessarily with scanf, but character input in general. When you ask an interactive user to enter a single character, two are actually placed on the input stream. The first is the character that the user typed and the second is the newline character created when the user pressed Enter. Your code does not account for the newline character and simply treats it as normal input.

A quick and dirty solution is to simply ask for another character from the stream and discard it.

Code:

#include <stdio.h>
#include <math.h>

int main()
{
  char x;

  do{
    printf("(S)ine, (C)osine, or (Q)uit: ");
    scanf("%c", &x);
    getchar();
    if(x!='s' && x!='S' && x!='c' && x!='C' && x!='q' && x!='Q')
      printf(" -- Illegal Input!\n");
  } while(x!='s' && x!='S' && x!='c' && x!='C' && x!='q' && x!='Q');

  return 0;
}

However, this simply fixes the symptom and not the problem. A decidedly better solution is to work at the line level rather than the character level when dealing with streams.

Code:

#include <stdio.h>
#include <math.h>

int main()
{
  char x[BUFSIZ];

  do{
    printf("(S)ine, (C)osine, or (Q)uit: ");
    if (fgets(x, sizeof x, stdin) == NULL) {
      break;
    }
    if(x[0]!='s' && x[0]!='S' && x[0]!='c' && x[0]!='C' && x[0]!='q' && x[0]!='Q')
      printf(" -- Illegal Input!\n");
  } while(x[0]!='s' && x[0]!='S' && x[0]!='c' && x[0]!='C' && x[0]!='q' && x[0]!='Q');

  return 0;
}

This way you avoid leaving unwanted characters in the stream and you can also improve error handling by saving an entire input line in memory for multiple parsing.

to make the large if() and while() statements easier to read and work with, you may want to change them to a switch statement for x.

Originally Posted by Draco

to make the large if() and while() statements easier to read and work with, you may want to change them to a switch statement for x.

Or better yet, modularize each operation into a specialized function so a proper solution can be used without cluttering the main function.

Code:

#include <ctype.h>
#include <math.h>
#include <stdio.h>

#define PROMPT "(S)ine, (C)osine, or (Q)uit: "

int getOpt(const char *prompt);
int isValid(char item, const char *options);

int main()
{
  int x;

  while ((x = getOpt(PROMPT)) != EOF && !isValid(x, "SCQ")) {
    printf(" -- Illegal Input!\n");
  }

  return 0;
}

int getOpt(const char *prompt)
{
  char buffer[BUFSIZ];

  fputs(prompt, stdout);
  fflush(stdout);
  if (fgets(buffer, sizeof buffer, stdin) == NULL) {
    return EOF;
  }

  return buffer[0];
}

int isValid(char item, const char *options)
{
  while (*options != '\0') {
    if (*options == toupper((unsigned char)item)) {
      return 1;
    }
    ++options;
  }

  return 0;
}

Code:

#include <stdio.h>
#include <math.h>

main()
{
  int count;
  float ctr,a;
  char x,s,S,c,C,q,Q;

  do{
    do{
      printf("(S)ine, (C)osine, or (Q)uit: ");
      scanf("%c", &x);
      if(x!='s' && x!='S' && x!='c' && x!='C' && x!='q' && x!='Q')
        printf(" -- Illegal Input!\n");
      while(getchar() != '\n');
    } while((x!='s') && (x!='S') && (x!='c') && (x!='C') && (x!='q') && (x!='Q'));


    switch(x){
    case 's':
    case 'S':
      printf("   Enter starting value: ");
      scanf("%f", &a);
      for(count = 0; count < 10; count++)
        {
          for(ctr = a; ctr < a+.09; ctr += 0.01)
            printf("%1.3f ", sin(ctr));
          a += .1;
          printf("\n");
        }
      printf("%1.3f\n", sin(a));
      break;
    case 'c':
    case 'C':
      printf("   Enter starting value: ");
      scanf("%f", &a);
      for(count = 0; count < 10; count++)
        {
          for(ctr = a; ctr < a+.09; ctr += 0.01)
            printf("%1.3f ", cos(ctr));
          a += .1;
          printf("\n");
        }
      printf("%1.3f\n", cos(a));
      break;
    }
  } while(x!='q' && x!='Q');
  printf(" -- Goodbye\n");
}

here is all my code. i tried using the things you guys posted but i still got the same error.

Try using the same solution after every call to scanf and see if the problem persists. scanf is a very complex function that is difficult to use properly even for experienced programmers. Once again I highly recommend tossing it in favor of fgets and a parsing scheme such as sscanf.

Code:

  if(x!='s' && x!='S' && x!='c' && x!='C' && x!='q' && x!='Q')
  {
    printf(" -- Illegal Input!\n");
    while(getchar() != '\n');
  }

but the other solutions are better...

[edit]
Actually, that doesn't make a lot of difference. Here's the output I get from your code:

Code:

 (S)ine, (C)osine, or (Q)uit: x
  -- Illegal Input!
 (S)ine, (C)osine, or (Q)uit: x
  -- Illegal Input!
 (S)ine, (C)osine, or (Q)uit: a
  -- Illegal Input!
 (S)ine, (C)osine, or (Q)uit: q
  -- Goodbye

What's wrong with that?

Originally Posted by Hammer

Code:

  if(x!='s' && x!='S' && x!='c' && x!='C' && x!='q' && x!='Q')
  {
    printf(" -- Illegal Input!\n");
    while(getchar() != '\n');
  }

but the other solutions are better...

[edit]
Actually, that doesn't make a lot of difference. Here's the output I get from your code:

Code:

 (S)ine, (C)osine, or (Q)uit: x
  -- Illegal Input!
 (S)ine, (C)osine, or (Q)uit: x
  -- Illegal Input!
 (S)ine, (C)osine, or (Q)uit: a
  -- Illegal Input!
 (S)ine, (C)osine, or (Q)uit: q
  -- Goodbye

What's wrong with that?

if you input "s" or "c" and then input a number, it prints out the table but then it also prints out "(S)ine, (C)osine, or (Q)uit: -- Illegal Input" and then it waits for you to press enter and then it finally re-enters the loop. i have to find a way to get rid of the first message. It should restart the loop without the message that i just wrote.

Do this after every scanf():
while(getchar() != '\n');

The problem is not necessarily with scanf, but character input in general. When you ask an interactive user to enter a single character, two are actually placed on the input stream. The first is the character that the user typed and the second is the newline character created when the user pressed Enter. Your code does not account for the newline character and simply treats it as normal input

Not trolling. but isn't that a problem with scanf then? thats why you clear the buffer if you use it. if you put a string in it would it not keep the '\n' in it as well? thats the problem with scanf. it keeps garbege in the buffer no. ex:

PHP Code:

#include <stdio.h>
#define SIZE 10
int main(void){
    char word[SIZE];
    scanf("%s", word);
    printf("entered %s", word);
    getchar(); //this one handles the '\n'
    getchar(); //this one waits for input to exit
    return 0;
    } 


also to the original poster. along with the FAQ here is another answer as to why not to use scanf or at least if your going to use it. use it properly. like checking its return value and such.
cheers.

Originally Posted by n00by

hi, can you guys help me out with this portion of code. I don't know what's wrong with it.

Code:

#include <stdio.h>
#include <math.h>

main()
{
  int count;
  float ctr,a;
  char x,s,S,c,C,q,Q;

  do{
    printf("(S)ine, (C)osine, or (Q)uit: ");
    scanf("%c", &x);
    if(x!='s' && x!='S' && x!='c' && x!='C' && x!='q' && x!='Q')
      printf(" -- Illegal Input!\n");
  } while(x!='s' && x!='S' && x!='c' && x!='C' && x!='q' && x!='Q');
}

it is supposed to print out this:

Code:

(S)ine, (C)osine, or (Q)uit: x
 -- Illegal Input!
(S)ine, (C)osine, or (Q)uit:

but it prints this out:

Code:

 (S)ine, (C)osine, or (Q)uit: x
 -- Illegal Input!
(S)ine, (C)osine, or (Q)uit:  -- Illegal Input!
(S)ine, (C)osine, or (Q)uit:

it prints out an extra line of sine, cosine, or quit with the illegal input statement after that. What did i do wrong? Help!!

Hi, this might be another alternative...

Code:

#include<stdio.h>

int main()
{
	char ch;
	do
	{
		printf("(S)ine (C)os (Q)uit : ");
		scanf("%c%*c",&ch);
		switch(ch)
		{
			case 's':
			case 'S':
				printf("Sine Function\n");
				break;
			case 'c':
			case 'C':
				printf("Cos Function\n");
				break;
			case 'q':
			case 'Q':
				exit(0);
			default:
				printf("Invalid Input\n");
		}
	}while(1);
	return 0;
}

Try out.....

-Harsha.