EN.601.220: C Arrays

C Arrays

Basic Usage

Hold multiple values all of the same base type
Use indices to access individual elements; like most languages these days, indices start at 0
Declare as type var[CONST_SIZE], e.g. int temps[3]
Can also declare w/initialization list: type var[] = {val1, val2, val3}, e.g. int temps[] = {75, 67, 83} and array will be as long as the list

Multiple dimension arrays

Two or more dimensions; the size of each must be specified
Declare as myarray[size1][size2], access individual element with myarray[i][j]
Initialize by nesting lists: int testr[2][3] = { {4, 3, 2}, {6, 8, 33} };
Does initialization one row at a time
myarray[i] refers to a one dimensional array of the base type (row i); myarray[size1][size2] can be used as an array of arrays

Example

  #include <stdio.h>
  int main (void) {
    char a[2][4] = { {'w','o','w','\0'}, {'h','i','\0','X'} };
    int i, j;
    for (i = 0; i < 2; i++)
        for (j = 0; j < 4; j++)
            printf("a[%d][%d] is %c\n", i, j, a[i][j]);
    for (i = 0; i < 2; i++)
            printf("row a[%d] is %s\n", i, a[i]);  // a[i] is a char array (i.e. a string)
    return 0;
  }

Arrays and Functions

Use [] to indicate an array variable in function definition and in function prototype, e.g.

      void doArray (int [], int); /* prototype version */
      void doArray (int myarray[], int size) /* on function itself */

Pass whole array to function with just name, no subscript: aray contents are passed by REFERENCE
Element values may be changed permanently within the function
Doesn't create new storage location (ie, doesn't copy array)
Array name is really address of first element (more later on this)
Array names are passed by VALUE (copied, changes won't reach caller)
Individual array elements passed to functions are handled like any variable of that base type

If declaring a multi-dimensional array as function argument, need sizes of subcripts for all dimensions but 1st <ul> <li>eg: void passarray(int a[][3][4]) receives a 3 dimensional array </li> <li>Sizes needed because arrays are stored sequentially - need dimensions to locate items </li> <li>Another example of low-level nature of C data structures. </li></ul>

Can use const in parameter list to make array unmodifiable: void doArray (const int ma[]) results in compile error if any attempt to assign or read new value into an array element

Command line options

To read what arguments if any were on a command line invocation, use the following main function declaration:
main(int argc, char *argv[])

argc is count of how many arguments, including program name
char *argv[] is a "pointer to an array of characters". We will explain pointers in detail later. For now, you can use it like char argv[][] - a 2d array of chars
argv[0] is program name (string)
argv[i] is ith argument as a string

Example: given ./a.out one two
argc is 3, argv[0] is “./a.out”, argv[1] is “one”, and argv[2] is “two”

CAUTION: review the below sections after learning pointers

Pointers and arrays

Welcome to C’s Wacky World

A declared array, say int a[10], is "really" just an address that starts a block of memory.
So, writing a is generally the same as writing &a[0]
a[3] is a synonym for *(a + 3) (offset three from pointer to start of array)
&a[3] is a synonym for a + 3

int *ptr and int a[10] variables are often interchangeable but not 100%:

a = ptr will be a compile error since size of a could get changed if this were allowed; sizeof(a) is always fixed
ptr = a is perfectly acceptable, but printf("%d", sizeof(ptr)); is only 4 or 8, the size of a pointer
locally declared array types (but not function parameters) have allocated space the size of the whole array:
char s[100]; printf("%d",sizeof(s)); prints 100 (bytes)
char s[10]; printf("%d",sizeof(&s[0])); prints the size of a pointer, 4 or 8 bytes

Arrays of pointers

Note writing main(int argc, char *argv[]) is same as main(int argc, char **argv) since more generally any function parameter mytype myvar[] is nearly identical to writing mytype * myvar.
Common use is array of strings
Each string is pointer to a string array, eg: char *suitptr[4] = {"Hearts", "Diamonds", "Clubs", "Spades"}
Note that this is not the same as a 2d array which is laid out sequentially in memory

See also the Pointers & Dynamic Memory Allocation notes resource.