Arrays - C Interview Questions IV

Q. Why can't constant values be used to define an array's initial size?

There are times when constant values can be used and there are times when they can't. A C program can use what C considers to be constant expressions, but not everything C++ would accept.

When defining the size of an array, you need to use a constant expression. A constant expression will always have the same value, no matter what happens at runtime, and it's easy for the compiler to figure out what that value is. It might be a simple numeric literal:

char a[ 512 ];

Or it might be a "manifest constant" defined by the preprocessor:

#define MAX     512

/* ... */

char    a[ MAX ];

Or it might be a sizeof:

char a[ sizeof( struct cacheObject ) ];

Or it might be an expression built up of constant expressions:

char buf[ sizeof( struct cacheObject ) * MAX ];

Enumerations are allowed too.

An initialized const int variable is not a constant expression in C:

int max = 512; /* not a constant expression in C */

char buffer[ max ]; /* not valid C */

Using const ints as array sizes is perfectly legal in C++; it's even recommended. That puts a burden on C++ compilers (to keep track of the values of const int variables) that C compilers don't need to worry about. On the other hand, it frees C++ programs from using the C preprocessor quite so much.

Q. What is the difference between a string and an array?

An array is an array of anything. A string is a specific kind of an array with a well-known convention to determine its length.

There are two kinds of programming languages: those in which a string is just an array of characters, and those in which it's a special type. In C, a string is just an array of characters (type char), with one wrinkle: a C string always ends with a NUL character. The "value" of an array is the same as the address of (or a pointer to) the first element; so, frequently, a C string and a pointer to char are used to mean the same thing.

An array can be any length. If it's passed to a function, there's no way the function can tell how long the array is supposed to be, unless some convention is used. The convention for strings is NUL termination; the last character is an ASCII NUL ('\0') character.

In C, you can have a literal for an integer, such as the value of 42; for a character, such as the value of '*'; or for a floating-point number, such as the value of 4.2e1 for a float or double.

There's no such thing as a literal for an array of integers, or an arbitrary array of characters. It would be very hard to write a program without string literals, though, so C provides them. Remember, C strings conventionally end with aNUL character, so C string literals do as well. "six times nine" is 15 characters long (including the NUL terminator), not just the 14 characters you can see.

There's a little-known, but very useful, rule about string literals. If you have two or more string literals, one after the other, the compiler treats them as if they were one big string literal. There's only one terminating NUL character. That means that "Hello, " "world" is the same as "Hello, world", and that

char    message[] =

  "This is an extremely long prompt\n"

  "How long is it?\n"

  "It's so long,\n"

  "It wouldn't fit on one line\n";

When defining a string variable, you need to have either an array that's long enough or a pointer to some area that's long enough. Make sure that you leave room for the NUL terminator. The following example code has a problem:

char greeting[ 12 ];

strcpy( greeting, "Hello, world" );    /* trouble */

There's a problem because greeting has room for only 12 characters, and "Hello, world" is 13 characters long (including the terminating NUL character). The NUL character will be copied to someplace beyond the greeting array, probably trashing something else nearby in memory. On the other hand,

char greeting[ 12 ] = "Hello, world"; /* not a string */

is OK if you treat greeting as a char array, not a string. Because there wasn't room for the NUL terminator, the NUL is not part of greeting. A better way to do this is to write

char greeting[] = "Hello, world";

to make the compiler figure out how much room is needed for everything, including the terminating NUL character.

String literals are arrays of characters (type char), not arrays of constant characters (type const char). The ANSI C committee could have redefined them to be arrays of const char, but millions of lines of code would have screamed in terror and suddenly not compiled. The compiler won't stop you from trying to modify the contents of a string literal. You shouldn't do it, though. A compiler can choose to put string literals in some part of memory that can't be modified—in ROM, or somewhere the memory mapping registers will forbid writes. Even if string literals are someplace where they could be modified, the compiler can make them shared.

For example, if you write

char    *p = "message";

char    *q = "message";

p[ 4 ] = '\0';  /* p now points to "mess" */

(and the literals are modifiable), the compiler can take one of two actions. It can create two separate string constants, or it can create just one (that both p and q point to). Depending on what the compiler did, q might still be a message, or it might just be a mess.