Arrays and Pointers Prepared by Manuel E. Bermúdez, Ph.D. Associate Professor University of Florida Programming Language Principles Lecture 23

Arrays Most common composite data type. Semantically, viewed as a mapping from the index type to the element type. Some languages permit only integer as the index type; others allow any scalar.

Array Declaration Syntax C: char upper[26] ; /* array of 26 chars, */ Fortran: character(26) upper Pascal: var upper: array[‘a’.. ‘z’] of char; Ada: upper: array (character range ‘a’.. ‘z’) of character;

Arrays and functions in Ada In either case, upper(‘ a ’) returns ‘ A ’.

Multi-Dimension Arrays Ada: matrix: array (1..10, 1..10) of real; Modula-3: VAR matrix: ARRAY [1..10],[1..10] OF REAL; (same as) VAR matrix: ARRAY [1..10] OF ARRAY [1..10] OF REAL; and matrix[3,4] is the same as matrix[3][4].

Multi-Dimension Arrays (cont’d) In Ada, matrix: array(1..10,1..10) of real; is NOT the same as matrix: array(1..10) of array (1..10) of real; matrix(3)(4) not legal in first form; matrix(3,4) not legal in second form.

Multi-Dimension Arrays (cont’d) An array of arrays is a slice. In C, double matrix[10][10]. However, C integrates arrays and pointers, so matrix[3] is not an array of 10 doubles. It is (depending on context) either: A pointer to the third row of matrix, or the value of matrix[3][0]

Slices in Fortran

Array Dimensions, Bounds and Allocation Five cases: Global lifetime, static shape: static allocation (easy). Local lifetime, static shape: space allocated on a stack frame. Local lifetime, shape bound at elaboration: stack frame needs fixed-size part and variable-size part. Arbitrary lifetime, shape bound at elaboration: Java, programmer allocates space. Arbitrary lifetime, dynamic shape: array lives on the heap.

Array allocation in Ada (shape bound at elaboration time)

Conformant Arrays in Pascal Array shape determined at time of call. Pascal doesn’t allow local dynamic-shaped arrays. Ada DOES allow local dynamic-shaped arrays (see textbook)

Other Forms of Dynamic Arrays C: Arrays passed by reference, so bounds are irrelevant ! (programmer’s problem) Java strings: String s = “short”; s = s + “ and sweet”; // immutable

Resizing Arrays in Java Create a new array of proper length and data type: Integer[] a = new Integer[10]; Object[] newArray = new Object[newLength]; Copy all elements from old array into new one: System.arraycopy(a,0,newArray,0,a.length); Rename array: element = newArray; // old space reclaimed by garbage // collector.

Dynamic Arrays in Fortran 90 Arrays sized at run time, but can’t be changed once set.

Classic Array Memory Layouts

Memory Layout in C

Address calculation (static array bounds)

Virtual Location of Array With static array bounds, we’ve “moved” the array in 3Ds.

Dope Vectors A “run-time” descriptor for the array. Contains, for each dimension (except last one, always statically known): Lower bound Size Upper bound (if dynamic checks are required) Size of dope vector depends on # of dimensions (i.e. static). Typically placed next to the array pointer, in the fixed-size portion of the stack frame.

Strings Usually an array of characters. Many languages allow more flexibility with strings than with other types of arrays. Single-character string vs. single character: Pascal: no distinction. C: *very* different String constants: 'abc', ”abc”. Rules for embedding special characters: Pascal: double the character: ' ab''cd' C: escape sequence: ” ab \” cd ”.

Strings C, Pascal, Ada: string length bound no later than elaboration time (allocate in stack frame). Lisp, Icon, ML, Java: allow dynamically- bound strings, stored in the heap. Pascal supports lexicographically-ordered comparison of strings ('abc' < 'abd' ). Ada supports it on all 1D discrete-valued arrays. C: no string assignment, elements copied individually (library functions).

Strings in C

Sets Pascal supports sets of any discrete type: var a,b,c: set of char; d,e: set of weekday; a := b + c; (* union *) a := b * c: (* intersection *) a := b – c: (* difference *)

Set implementations Arrays, hash tables, trees. Bit-vectors: each entry true (element in the set), or false (element not in the set) Efficient operations: Union is inclusive bit-wise OR. Intersection is bit-wise AND. Difference is NOT, followed by AND. Won’t work for large base types: A set of 32-bit integers ~ 500MBs. A set of 64-bit integers ~ 2 41 MBs Usually limited to 128, or 512.

Pointers and Recursive Types Most recursive types are records. Reference model languages (Lisp, ML, Clu, Java): every field is a reference. A record of type f contains a reference to another record of type f. Value model languages (C, Pascal, Ada): need a pointer (a variable whose value is a reference). Don’t confuse pointer with address: an address may be segmented.

Storage Reclamation Explicit (C,C++, Pascal, Modula-2): programmer must reclaim unused heap space. Can be done efficiently. Easy to get wrong; if so, can lead to memory leaks. Implicit (Lisp, ML, Modula-3, Ada, Java): heap space reclaimed automatically. Not so efficient (but getting better) Simplifies programmer’s task a LOT.

Reference Model (ML) node (‘R’,node(‘X’,empty,empty), node(‘Y’,node(‘Z’,empty,empty), node(‘W’,empty,empty)))

Reference Model (Lisp) '(#\R(#\X()())(#\Y(#\Z()())(#\W()())))

Value Model Pascal: type chr_tree_ptr = ^chr_tree; chr_tree = record left, right:chr_tree_ptr; val: char end; C: struct chr_tree { struct chr_tree *left, *right; char val; } In C, struct names are not quite type names. Shorthand: typedef struct chr_tree chr_tree_type

Memory Allocation Pascal: new(my_ptr); Ada: my_ptr:=new chr_tree; C: my_ptr=(struct chr_tree *) malloc(sizeof (struct chr_tree)); C++, Java: my_ptr = new chr_tree(args);

Pointer References Pascal: my_ptr^.val := ‘X’; C: (*my_ptr).val = ‘X’; my_ptr->val = ‘X’; Ada: T: chr_tree; P: char_tree_ptr; T.val := ‘X’; P.val := ‘X’; good for record or pointer to one. T := P.all; if need to reference the record.

Pointers and Arrays in C int n; int *a; int b[10]; All are valid: a = b; n = a[3]; n = *(a+3); n = b[3]; n = *(b+3);

Pointers and Arrays in C (cont’d) Interoperable, but not the same: int *a[n] allocates n pointers int[n][m] allocates a full 2D array. In fact, assuming int a[n]; *(a+i) *(i+a) a[i] i[a] are all equivalent !

Pointers and Arrays in C (cont’d) In C, arrays are passed by reference: the array name is a pointer. It’s customary to pass the array name, and its dimensions: double det (double *M, int rows, int cols) { int i,j;... val = *(M+i*cols+j); /* M[i][j] */ }

Tombstones Technique for catching dangling references

Tombstones Advantages: Catch dangling references. Prevent memory leaks. Helpful in heap compaction. Disadvantages: Cheap on the heap, expensive on the stack (procedure entry/return). Tombstones themselves can dangle.

Locks and Keys

Advantage: No need to keep tombstones around. Disadvantages: Only work for heap objects. Significant overhead. Increase the cost of copying a pointer. Increase the cost of every access.

Reference Counts Set count to 1 upon object creation Upon assignment., Decrement count of object on left. Increment count of object on right. Upon subroutine entry, increment counts for local pointers. Upon subroutine return, decrement counts for local pointers. Need type descriptors for this: objects can be deeply structured. WILL FAIL ON CIRCULAR STRUCTURES !

Reference Counts Fail on Circular Structures

Garbage Collection System determines which memory is not in use and return the memory to the pool of free storage. Done in two or three steps: Mark nodes that are in use. Compact free space (optional). Move free nodes to storage pool.

Marking Unmark all nodes (set all mark bits to false). Start at each program variable that contains a reference, follow all pointers, mark nodes that are reached. caedb firstNode

Compaction Move all marked nodes (i.e., nodes in use) to one end of memory, updating all pointers as necessary. cbedb firstNode aed Free Memory

Lists in Lisp and ML

Equality Testing and Assignment Equality comparison is easy for scalars For complex or abstract data types, say, strings s and t, s = t could mean s and t are aliases s and t occupy the same storage s and t contain the same sequence of characters s and t print the same

Deep and Shallow Comparisons Shallow Comparison : Both expressions refer to the same object. Deep Comparison: Expressions refer to objects that are “equal” in content somehow. Most PLs use shallow comparisons, and shallow assignments.

Arrays and Pointers Prepared by Manuel E. Bermúdez, Ph.D. Associate Professor University of Florida Programming Language Principles Lecture 23