Initializing Arrays

C only has two built-in types of “compound” data storage:

• Structures
• Arrays

Examples:

// Example demonstrating array initialization in C

int main() {
	char a[3] = {'c', 'a', 't'};
	char b[3] = "cat";
	int c[5] = {1, 2, 3} // this would work as the remaining becomes 0
}

the address of an array is always the same as the address of the first element~

// This program demonstrates how the value of an array
// is the same as the address of the first element

#include "cs136-trace.h"

int main(void) {
  int a[6] = {4, 8, 15, 16, 23, 42};
  
  trace_ptr(a);
  trace_ptr(&a);
  trace_ptr(&a[0]);
  
  trace_int(a[0]);
  trace_int(*a);
}

Expected output

0x7fff5fbff690
0x7fff5fbff690
0x7fff5fbff690
4
4

Pointer Arithmetics

<aside> 💡 The array indexing syntax ([]) is an operator that performs pointer arithmetic. In other words, a[i]is equivalent to *(a + i).

</aside>

So, the following are equivalent!

int array_function(int a[], int len) {...} // a[]
int array_function(int *a, int len) {...} // *a

When adding an integer i to a pointer p, the address computed by (p + i) in C is given in “normal” arithmetic by: p + i * sizeof(*p).

int sum_array(const int *a, const int len) {
  assert(a);
  assert(len > 0); 
  int sum = 0;
  for (const int *p = a; p < a + len; ++p) {
    sum += *p;
  }
  return sum;
}

Sorting Algorithms