C Compatibility Between Integers and Characters

Question

How does C handle converting between integers and characters? Say you've declared an integer variable and ask the user for a number but they input a string instead. What would happen?

Answer 1

The program would crash, you need to call atoi function.

Answer 2

Uh? You always input a string. Then you parse convert this string to number, with various ways (asking again, taking a default value, etc.) of handling various errors (overflow, incorrect chars, etc.).

Answer 3

The user input is treated as a string that needs to be converted to an int using atoi or another conversion function. Atoi will return 0 if the string cannot be interptreted as a number because it contains letters or other non-numeric characters.

You can read a bit more at the atoi documentation on MSDN - http://msdn.microsoft.com/en-us/library/yd5xkb5c(VS.80).aspx

Answer 4

Another thing to note is that in C, characters and integers are "compatible" to some degree. Any character can be assigned to an int. The reverse also works, but you'll lose information if the integer value doesn't fit into a char.

char foo = 'a'; // The ascii value representation for lower-case 'a' is 97
int bar = foo; // bar now contains the value 97 
bar = 255; // 255 is 0x000000ff in hexadecimal
foo = bar; // foo now contains -1 (0xff) 
unsigned char foo2 = foo; // foo now contains 255 (0xff)

Answer 5

As other people have noted, the data is normally entered as a string -- the only question is which function is used for doing the reading. If you're using a GUI, the function may already deal with conversion to integer and reporting errors and so in an appropriate manner. If you're working with Standard C, it is generally easier to read the value into a string (perhaps with fgets() and then convert. Although atoi() can be used, it is seldom the best choice; the trouble is determining whether the conversion succeeded (and produced zero because the user entered a legitimate representation of zero) or not.

Generally, use strtol() or one of its relatives (strtoul(), strtoll(), strtoull()); for converting floating point numbers, use strtod() or a similar function. The advantage of the integer conversion routines include:

• optional base selection (for example, base 10, or base 10 - hex, or base 8 - octal, or any of the above using standard C conventions (007 for octal, 0x07 for hex, 7 for decimal).
• optional error detection (by knowing where the conversion stopped).

The place I go for many of these function specifications (when I don't look at my copy of the actual C standard) is the POSIX web site (which includes C99 functions). It is Unix-centric rather than Windows-centric.