Technical Briefs

ROHAN help Files

C mistakes

A few of Dr. Doom's list of C mistakes, condensed considerably from the originals:

1. EOF is an int, not a char; getchar(), despite its character-sounding name, RETURNS AN INT! In fact, all functions in the C language return either an int or a double; it is not possible to write a function which returns a char. Besides, the compiler generates code which takes the same amount of space for (scalar) chars and ints. So: there is NEVER any reason to say, for example: char c;. You should ALWAYS declare c to be an int, especially when it receives the return value from getchar, which is then compared to EOF.

   Usage Examples:

   /* this is WRONG (WRONG!, WRONG!, WRONG! ...*/

           char c;
           while(  (c=getchar()) != EOF){ ... }

   /* this is RIGHT */

           int c;
           while(  (c=getchar()) != EOF){ ... }

2. Beware the distinction between = and ==. The single-equal-sign (we'll call it SE) causes the things on the two sides of it to BECOME equal; the resultant value is then whatever value was originally on the right-hand-side of the SE operator. The double-equal sign (DE) first tests the two sides to see if they are equal; the resulting value of the DE operator is zero if they are not equal, and one if they are equal. No other results are possible.

   Usage Examples:

   /* this is (probably) WRONG */

           int i,j;
           if(i=j){ ... }  /*will execute if j is nonzero */

   /* this is (probably) RIGHT */

           int i,j;
           if(i==j){ ... } /*will execute if i is equal to j */

3. Even experienced programmers sometimes forget the &s which are required on the arguments to scanf. All arguments after the first must have & in front of it.

   Usage Examples:

   /* WRONG ... guaranteed to dump core */

           int i;
           scanf("%d",i);

   /* RIGHT */

           int i;
           scanf("%d",&i);

   /* for advanced students only: */

           int j, *i = &j;
           scanf("%d",i);
           printf("%d %d\n",i,j); /* what does it print? */

4. Beware the extraneous semicolon.

   Usage Examples:

   /* (probably) WRONG */

           for(i=0; i<10; i++);
                   printf("%d\n",i);

   /* (probably) RIGHT */

           for(i=0; i<10; i++)
                   printf("%d\n",i);

5. Again, even experienced programmers sometimes get bit by the dangling else bug:

           if(i==5)
                   k=7;
                   if (line==27) d = 39;
           else
                   d = 41;

   and variations too numerous to show here. The else goes with the nearest preceding if(well, actually, the nearest preceding if that doesn't already have an else associated with it). The compiler does not know indentation.

6. Scanf() from the terminal can produce some perplexing results until you learn about the terminating newline, and what scanf considers to be whitespace for %d, %s, and %c formats. Primarily try to remember that for a %d format, scanf IGNORES leading whitespace, and PUTS BACK the whitespace which caused the termination of a %d scan. Moral of the story: do not mix %c and %s formats with %d or %f formats, nor mix scanf with getchar, unless you are very careful AND know what you're doing.