Input output c

C Programming

  

Handling User Input In C
scanf() has problems, in that if a user is expected to type an integer, and types a string instead, often the program bombs. This can be overcome by reading all input as a string (use getchar()), and thenconverting the string to the correct data type.

/* example one, to read a word at a time */
#include <stdio.h>
#include <ctype.h>
#define MAXBUFFERSIZE   80

void cleartoendofline( void );  /* ANSI function prototype */

void cleartoendofline( void )
{
 char ch;
 ch = getchar();
 while( ch != '\n' )
  ch = getchar();
}

main()
{
 char    ch;                     /* handles user input */
 char    buffer[MAXBUFFERSIZE];  /* sufficient to handle one line */
 int     char_count;             /* number of characters read for this line */
 int     exit_flag = 0;
 int     valid_choice;

 while( exit_flag  == 0 ) {
  printf("Enter a line of text (<80 chars)\n");
  ch = getchar();
  char_count = 0;
  while( (ch != '\n')  &&  (char_count < MAXBUFFERSIZE)) {
   buffer[char_count++] = ch;
   ch = getchar();
  }
  buffer[char_count] = 0x00;      /* null terminate buffer */
  printf("\nThe line you entered was:\n");
  printf("%s\n", buffer);

  valid_choice = 0;
  while( valid_choice == 0 ) {
   printf("Continue (Y/N)?\n");
   scanf(" %c", &ch );
   ch = toupper( ch );
   if((ch == 'Y') || (ch == 'N') )
    valid_choice = 1;
   else
    printf("\007Error: Invalid choice\n");
   cleartoendofline();
  }
  if( ch == 'N' ) exit_flag = 1;
 }
}

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Another Example, read a number as a string

/* example two, reading a number as a string */
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#define MAXBUFFERSIZE   80

void cleartoendofline( void );          /* ANSI function prototype */

void cleartoendofline( void )
{
 char ch;
 ch = getchar();
 while( ch != '\n' )
  ch = getchar();
}

main()
{
 char    ch;                     /* handles user input */
 char    buffer[MAXBUFFERSIZE];  /* sufficient to handle one line */
 int     char_count;             /* number of characters read for this line */
 int     exit_flag = 0, number, valid_choice;

 while( exit_flag  == 0 ) {
  valid_choice = 0;
  while( valid_choice == 0 ) {
   printf("Enter a number between 1 and 1000\n");
   ch = getchar();
   char_count = 0;
   while( (ch != '\n')  &&  (char_count < MAXBUFFERSIZE)) {
    buffer[char_count++] = ch;
    ch = getchar();
   }
   buffer[char_count] = 0x00;      /* null terminate buffer */
   number = atoi( buffer );
   if( (number < 1) || (number > 1000) )
    printf("\007Error. Number outside range 1-1000\n");
   else
    valid_choice = 1;
  }
  printf("\nThe number you entered was:\n");
  printf("%d\n", number);

  valid_choice = 0;
  while( valid_choice == 0 ) {
   printf("Continue (Y/N)?\n");
   scanf(" %c", &ch );
   ch = toupper( ch );
   if((ch == 'Y') || (ch == 'N') )
    valid_choice = 1;
   else
    printf("\007Error: Invalid choice\n");
   cleartoendofline();
  }
  if( ch == 'N' ) exit_flag = 1;
 }
}

Other validation examples

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©Copyright B Brown. 1984-1999. All rights reserved.