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Top Forums Programming Counting duplicate chars in C Post 302416193 by achenle on Sunday 25th of April 2010 12:33:21 PM
Old 04-25-2010
Lots of problems in that code.

This is one:
Code:
while ( (c = getchar()) != EOF)
       count ++;
       lastChar += c;
{
  if(c = lastChar)
    duplicate ++;
  else( duplicate == duplicate);
}

What that really does is this:
Code:
while ( (c = getchar()) != EOF)
{
       count ++;
}

       lastChar += c;
{
  if(c = lastChar)
    duplicate ++;
  else( duplicate == duplicate);
}

That's a big difference.

Note also that getchar() returns an int, not a char. So you can't read the valid character input that corresponds to the truncated integer value of EOF, whatever that value happens to be on your system.

Also, you're not properly initializing your lastChar variable, so you never know what you're comparing it too when you start. Because you could read any character from the stream, your lastChar variable should probably be an int instead of a char, and you could initialize it to something like EOF to make sure it starts with a value that isn't a valid char value.

Not only that, you're doing an assignment instead of a comparison in your if statement. This:

Code:
  if(c = lastChar)
    duplicate ++;

should be:

Code:
  if(c == lastChar)
    duplicate ++;

I'd do something like this:
Code:
int main( void )
{
    int c;
    int last = EOF;
    int dupes = 0;
    int count = 0;

    while ( 1 )
    {
        c = getchar();
        if ( EOF == c )
        {
            break;
        }

        count++;

        if ( last == c )
        {
            dupes++;
        }

        last = c;
    }

    printf( "%d %d %d\n", count, dupes, count - dupes );

    return( 0 );
}

You will note that I deliberately avoided placing ANY assignment statement inside a conditional clause. Most compilers in my experience provide for a warning when they encounter an assignment in a conditional clause. If you make a point of never putting assignment statements in a conditional clause, you can then use that warning capability to find and fix mistakes in your code.

Because you will make mistakes when you code. Anyone who says they write mistake-free and bug-free code is lying or deluded, plain and simple.

Note also that I always used { and } after my conditional clauses. Always using braces even for one-line conditional code makes mistakes like the one you made with your while-loop impossible to make.

Don't pay attention to how code is laid out in textbooks. In textbooks, whitespace and extra lines cost the publisher money.
 

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GETS(3) 						     Linux Programmer's Manual							   GETS(3)

NAME
fgetc, fgets, getc, getchar, gets, ungetc - input of characters and strings SYNOPSIS
#include <stdio.h> int fgetc(FILE *stream); char *fgets(char *s, int size, FILE *stream); int getc(FILE *stream); int getchar(void); char *gets(char *s); int ungetc(int c, FILE *stream); DESCRIPTION
fgetc() reads the next character from stream and returns it as an unsigned char cast to an int, or EOF on end of file or error. getc() is equivalent to fgetc() except that it may be implemented as a macro which evaluates stream more than once. getchar() is equivalent to getc(stdin). gets() reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF, which it replaces with ''. No check for buffer overrun is performed (see BUGS below). fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A '' is stored after the last character in the buffer. ungetc() pushes c back to stream, cast to unsigned char, where it is available for subsequent read operations. Pushed - back characters will be returned in reverse order; only one pushback is guaranteed. Calls to the functions described here can be mixed with each other and with calls to other input functions from the stdio library for the same input stream. For non-locking counterparts, see unlocked_stdio(3). RETURN VALUE
fgetc(), getc() and getchar() return the character read as an unsigned char cast to an int or EOF on end of file or error. gets() and fgets() return s on success, and NULL on error or when end of file occurs while no characters have been read. ungetc() returns c on success, or EOF on error. CONFORMING TO
ANSI - C, POSIX.1 BUGS
Never use gets(). Because it is impossible to tell without knowing the data in advance how many characters gets() will read, and because gets() will continue to store characters past the end of the buffer, it is extremely dangerous to use. It has been used to break computer security. Use fgets() instead. It is not advisable to mix calls to input functions from the stdio library with low - level calls to read() for the file descriptor associ- ated with the input stream; the results will be undefined and very probably not what you want. SEE ALSO
read(2), write(2), ferror(3), fopen(3), fread(3), fseek(3), puts(3), scanf(3), unlocked_stdio(3) GNU
1993-04-04 GETS(3)
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