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CMSC 212 โ F05 (lect 19)^1
Announcements
l Program #5 posted
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Representing Arrays
l Arrays (C style)
- sequence of consecutive memory locations
- address of array is the address of the starting point for the array
- subscript defines offset into array
- location = baseAddr + (subscript * sizeof(object))
- no further information about array stored for access at runtime
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Arrays in Other Languages
Typical Implementations
l Fortran
- sequence of logically consecutive bytes
- array descriptor (runtime data) stores additional information
including the number of dimensions and
- distance between array elements (for each dimension)
- number of elements (for each dimension)
- lower bound of array dimension (for each dimension)
l Java
- sequence of objects
- array descriptor (runtime data) stores
- type information about what this is an array of
- length array
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Comparison of Representations
l Runtime Descriptors
- require space
- allow checking of array overflow
- permit passing multi-dimensional arrays as parameters
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Examples of Using Array Descriptors
l Can Use Additional information to Check array bounds
l Traditional C:
int strcmp(char *a, char *b) {
int i;
for (i=0; a[i] && b[i]; i++) {
if (a[i] < b[i]) return -1;
else if (b[i] < a[i]) return 1;
if (a[i] && !b[i]) return 1;
else if (b[i]) return -1;
else return 0;
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With Array Descriptors
int strcmp(aRef *a, aRef *b) {
int i;
for (i=0; i < a->len && i < b->len && a->data[i] && b->data[i]; i++)
if (a->data[i] < b->data[i]) return -1;
else if (b->data[i] < a->data[i]) return 1;
if (i >= a->len || i >= b->len) {
printf("Array subscript error\n");
exit(-1);
if (a->data[i] && !(b->data[i])) return 1;
else if (b->data[i]) return -1;
else return 0;
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Structures
l Store values in a sequence of memory locations
l Compiler remembers where the fields are stored
- generates code to compute the field
- fieldAddr = baseAddress + fieldOffset
l Example:
struct location {
int x, y;
struct location spot = { 25, 36 };
main() {
printf("%d\n", spot.x);
printf("%d\n", spot.y);
main: Load R3 R0 spot
Output R
Load R2
Load R3 R2 spot
Output R
Halt
spot: Data 25
Data 36
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Objects
l Very Similar to structures
l Include additional information
- type of the object
- functions to invoke on the object
- sometime might know this from type info
