Common C Coding Mistakes

C Programming (COP 2220)

Here is a summary of common coding mistakes students have encountered in writing programs in this class. If you are having problems with your program, you might find a quick solution here.

• Input problems
• Output problems
• Problems with decision structures
• Problems with loops
• Problems with strings
• File handling problems

Input problems

1. Use the correct conversion specifier for the declared data type in a scanf(). A float is %f, an int is %d, and a double is %lf (lower case L). If you declare a variable to be an int, don't try to input it with a %f specifier.

2. Don't use a precision specification in a scanf() conversion specifier. For example, scanf("%.2f", &payRate) will not work (at least the ".2" part won't). Use scanf("%f", &payRate) instead. Width specifiers can be used, however.

3. Use the addressing operator (&) for your variables used as targets for input with scanf()'s, unless the target is already an address (such as the name of an array or a pointer variable).

4. The scanf() function can't be used with the %s specifier to input a string that contains white space (like "Electric sander"). Use another function, like gets().

5. When using getchar() or scanf() with a %c (character) conversion specifier, be aware that the last input operation (like a previous scanf() or getchar()) may have left characters in the keyboard buffer. 

Output problems

1. Do not use the addressing operator (&) for arguments in a call to printf( ).

2. When using puts( ), a newline character ('\n') is automatically appended, so don't use it for a prompt if you want the user's input on the same line.

Problems with decision structures

1. There is no semicolon after the condition:
    
      if (x > 0)     /* correct */
         ...
    
      if (x > 0);    /* wrong */
         ...

2. Use braces ( { } ) around the body of an if or else branch that contains multiple statements. You don't need them with a single statement in the body.

3. If you have a dual-alternative selection to implement, use the if...else rather than switch – it's cleaner.

4. There is no condition in the else branch – only in the if branch.

Problems with loops

1. There is no semicolon after the condition in a while:
    
      while (x > 0)     /* correct */
         ...
    
      while (x > 0);    /* wrong - infinite loop */
         ...

2. The loop index (loop control variable) is not updated inside the loop

Problems with strings

1. You can't compare two strings with with a relational or equality operator (like >= or ==). You must use one of the comparison functions from string.h, like strcmp( ) or strncmp( ), or write your own function.

2. To assign (copy, duplicate) one string to another, use one of the copy functions from string.h like strcpy( ) or strncpy( ), not the assignment operator (=).

File handling problems

1. You can fopen() a file more than once in a program, but a fclose() must be executed between fopen()'s. Review your algorithm to make sure you really need multiple file open and closes.