What is Array Initialization
Array Initialization
C/C++ allows the initialization of arrays at the time of their declaration. The general form of array initialization is similar to that of other variables, as shown here:
type_specifier array_name [size1]…[sizeN]={value_list};
The value_list is a comma-separated list of values whose type is compatible with type_specifier. The first value is placed in the first position of the array, the second value in the second position, and so on. Note that a semicolon follows the }.
In the following example, a 10-element integer array is initialized with the numbers 1 through 10:
int i[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
This means that i[0] will have the value 1 and i[9] will have the value 10. Character arrays that hold strings allow a shorthand initialization that takes the form:
char array_name [size]=”string”;
For example, this code fragment initializes str to the phrase “I like C++”.
char str[11] = “I like C++”;
This is the same as writing
char str[11] = {’I', ‘ ‘, ‘l’, ‘i’, ‘k’, ‘e’,’ ‘, ‘C’,
‘+’, ‘+’, ‘\0′};
Because null-terminated strings end with a null, you must make sure that the array you declare is long enough to include the null. This is why str is 11 characters long even though “I like C++” is only 10. When you use the string constant, the compiler automatically supplies the null terminator.
Multidimensional arrays are initialized the same as single-dimension ones. For example, the following initializes sqrs with the numbers 1 through 10 and their squares.
int sqrs[10][2] = {
1, 1,
2, 4,
3, 9,
4, 16,
5, 25,
6, 36,
7, 49,
8, 64,
9, 81,
10, 100
};
When initializing a multidimensional array, you may add braces around the initializers for each dimension. This is called subaggregate grouping. For example, here is another way to write the preceding declaration.
int sqrs[10][2] = {
{1, 1},
{2, 4},
{3, 9},
{4, 16},
{5, 25},
{6, 36},
{7, 49},
{8, 64},
{9, 81},
{10, 100}
};
When using subaggregate grouping, if you don’t supply enough initializers for a given group, the remaining members will be set to zero automatically.
Unsized Array Initializations
Imagine that you are using array initialization to build a table of error messages, as shown here:
char e1[12] = “Read error\n”;
char e2[13] = “Write error\n”;
char e3[18] = “Cannot open file\n”;
As you might guess, it is tedious to count the characters in each message manually to determine the correct array dimension. Fortunately, you can let the compiler automatically calculate the dimensions of the arrays. If, in an array initialization statement, the size of the array is not specified, the C/C++ compiler automatically creates an array big enough to hold all the initializers present. This is called an unsized array. Using this approach, the message table becomes
char e1[] = “Read error\n”;
char e2[] = “Write error\n”;
char e3[] = “Cannot open file\n”;
Given these initializations, this statement
printf(”%s has length %d\n”, e2, sizeof e2);
will print
Write error has length 13
Besides being less tedious, unsized array initialization allows you to change any of the messages without fear of using incorrect array dimensions.
Unsized array initializations are not restricted to one-dimensional arrays. For multidimensional arrays, you must specify all but the leftmost dimension. (The other dimensions are needed to allow the compiler to index the array properly.) In this way, you may build tables of varying lengths and the compiler automatically allocates enough storage for them. For example, the declaration of sqrs as an unsized array is shown here:
int sqrs[][2] = {
{1, 1},
{2, 4},
{3, 9},
{4, 16},
{5, 25},
{6, 36},
{7, 49},
{8, 64},
{9, 81},
{10, 100}
};
The advantage of this declaration over the sized version is that you may lengthen or shorten the table without changing the array dimensions.
Popularity: 1% [?]
