COMS 3157 Advanced Programming

05 - arrays


Array basics

Arrays and pointers

int a[10];  // 10 integers in contiguous memory, from 0th to 9th
int *p = &a[0];  // p points to the 1st element of a
  x = *p;      // *p     is same as a[0]
  x = *(p+1);  // *(p+1) is same as a[1]
  x = *(p+9);  // *(p+9) is same as a[9]

  for (int i = 0; i < 10; i++)
      printf("%d\n", *p++);
  x = *a;      // *a     is same as a[0]
  x = *(a+1);  // *(a+1) is same as a[1]
  x = *(a+9);  // *(a+9) is same as a[9]

In fact, the compiler automatically converts a[b] to *(a+b).

  int foo(int a[10]);
  int foo(int a[]);
  int foo(int *a);

To summarize:

These are the same:


And these are the same:

a + 5
&a[0] + 5;

And all these are same:


How about these? Try them!


char array, aka the string


char c[] = "abc";  // short-hand for: char c[] = {'a','b','c','\0'};

Everywhere else, “abc” is an expression whose value is a pointer:

char *p = "abc";   // p points to the 1st element of 4-char array

String literals such as “abc” are stored in code section or static data section of the process memory, depending on compiler and OS.

*p = 'A';  // result undefined - probably segmentation fault

Different ways to implement strcpy from K&R2, p105-106:

while ((s[i] = t[i]) != 0) i++;

while ((*s = *t) != 0) { s++; t++; }

while ((*s++ = *t++) != 0) ;

Heap memory allocation


We want dynamic, yet persistent arrays.

malloc(n) allocates n bytes of memory on the heap, and returns a pointer to the beginning of the memory.

int *p = (int *) malloc(100 * sizeof(int));

// malloc returns NULL if it cannot allocate the requested memory
if (p == NULL) { 
    perror("malloc failed"); 

// initialize all elements to 0
for (int i = 0; i < 100; i++)
    p[i] = 0;

// another way to do the same thing
memset(p, 0, 100 * sizeof(int));

free() deallocates the memory block previously returned by malloc.


Pointer to pointer

Array of pointers:

char *a[] = { "hello", "world" };

char **p = a;
printf("%s %s\n", p[0], p[1]);

// the following is a little different
// see K&R2, p114 for an illuminating picture
char a[][10] = { "hello", "world" };

Command line arguments are passed to main() as an array of char pointers. For example, when you run

echo hello world

the following data structure is passed to main(int argc, char **argv):

          +-------+       +-------+
    argv  |     --|-----> |     --|----> "echo"
          +-------+       +-------+
                          |     --|----> "hello" 
                          |     --|----> "world"
                          |   0   |

argc is set to 3, and argv[argc] is set to NULL.

Different ways to implement ‘echo’ program (K&R2, p115):

for (i = 1; i < argc; i++)
    printf("%s\n", argv[i]);

while (--argc > 0)
    printf("%s\n", *++argv;);

while (*argv)
    printf("%s\n", *argv++);