Data Types

Primitives

• Integer

• Float

• Character

• Boolean

• Pointers

Aggregates

• Strings

• Records

• Enumerated

• Arrays

• Objects

Strings

• Fixed length.

• Null terminated.

• Length field.

• Heap allocated.

B e t s y b b b

B e t s y 0

5 B e t s y

B e t s y

String Allocation

• Static length.– Blank fill as in Fortran, Pascal, etc.

• Limited Dynamic length– grows to a limit

• Dynamic length– no length restriction– reallocates from heap

char x[4];strcpy(x,”abc”); //OKstrcat(x,”def”); // NO

x=“abc”;x=“abcdefghij”;

Implementation

• Can be viewed as primitive type– some machine language supports string

operations at a level which treats them as primitives even though operations are slower

• Sometimes requires both– compile-time descriptors– run-time descriptors– know the difference

Enumerated types

• Usually implemented as integers.

• Implied size limitation which is not a problem– (red, green, blue) red is 0, green is 1, etc

• Strong typing sometimes creates ambiguity– desire types to be distinguished but for

• weekday = (Mon, Tue, Wed, Thur, Fri);

• classday = (Mon, Wed, Fri)

– assignment ok one direction, but not other

• I/O sometimes allowed, others not

Subrange

• Sequence of an ordinal type– Mon..Fri

• Used for tighter restriction of values than primitive types provide– subtype age is integer 0..150;

• Sometimes compatible, others not– EXAMPLE: is age compatible with integer?– type age is new integer range 1..150; NO– type age is integer range 1..150; YES

Array Operations

• ARRAY operations are infrequent except APL

• Examples– elements (common)– entire array (as parameters/pointers)– slice (a row, column, or series of rows/columns)

• APL– matrix multiplcation– vector dot product– add a scalar to each element

Allocation strategies

• Static array

• Fixed stack-dynamic– int x[20]; compile-time decision of size allocation

• Stack dynamic– int x[n]; once allocated, size can’t change, but determined by n

• Heap dynamic– array can grow dynamically and change subscript– ever been frustrated by the MAX size of array?

Subscript/subrange errors

• Subscript bounds problems for arrays are one of our biggest programming nuisances

• Checking for them at run-time is expensive• Even if within the range -> no assurance they

are correct• Some languages such as c do NO checking• Consequence in programs is

difficult/impossible to trace

Addressing

• Storage is row-major or column-major order

(1,2)(1,1)

(2,2)(2,1)

(3,2)(3,1)

(1,2)

(1,1)

(2,2)

(2,1)

(3,2)

(3,1)

(1,2)

(1,1)

(2,2)

(2,1)

(3,2)

(3,1)

int A[2,3];

Determining location

Location (a[I]) = base address (a)+ (I- lowerbound)*element size

[1]

[2]

[3]

[4]

[5]

[6]

Assume size 4 bytes each starting at 100

100

104

108

112

116

120

integer a[6];

Loc(a[3])= 100 + (3-1)*4 = 108

Most of this is compile-time!

2-d arrays (column major)

(1,2)

(1,1)

(2,2)

(2,1)

(3,2)

(3,1)

Loc (a[I,J]) = base address (a) (I-lb1)*size element + (J-lb2)*size of column

size of column=number rows allocated * size element

100

104

108

112

116

120

Loc (a[1,2]) = 100 + (1-1)*4 + (2-1)*3*4 = 100 + 0 + 12 = 112

Passing 2-d arrays as parameters

• The receiving procedure needs to have DIMENSION information

• Some languages are tightly bound and force that .. Pascal by requiring it to be a declared type

• Others have strange rules– Fortran (column major)

Caller:INTEGER A(10,20)CALL PROCESS(A,10)

Called:SUBROUTINE PROCESS(A,N)INTEGER A(N,1)

Associative arrays

• Not common… in perl

• Uses a hash function

• Stores Key and Value

cedric

perry

mary

gary 4785057000

5575075000

%salaries{“gary”} -> 47850

%salaries

hash“gary” 47850

In math class: hash(key) = value or hash(“gary”)=47850

Arrays as pointers in c

• Use of array name in c is the same as a pointer to the beginning element

• Incrementing the associated pointer increments by the true memory size– integers are 4 bytes– int * j;– j++; // increments j by 4.. assuming byte addressable

Example code in c

int c[10], *j;

for (j=c; j<&c[10]; j++)

{ *j = 0; }

Assign j to be the address of c[0]

As long as the addressof j is within the bounds of c

Increment j by size of integer

Set the element to 0

for (int j=0; j<10; j++) { c[j] = 0; }

Records

• Record operations– assignment– comparison– block operations without respect to fields

• Strange syntax in c

• Unions

Record pointers in c

Struct person{ int weight; int age; char name[20]; }; // not exact format

person teacher;

In declaring routine:

teacher.age=35;

When passing to functionand inside function:

teacher->age=35;

Unions

• Free unions– two names for the same place– it’s up to you to keep them straight– no support for checking

• Discriminated unions– a value in the record indicates how to interpret

the associated data.– Not always easy to check.. Sometimes not done

Ada example (p.231)

filled

color

Discriminant(form)

triangle:leftside, rightside, angle

circle:diameter

rectangle:side1,side2

Sets

• Bit fields implemented as binary values (below)

• fast implementation

• set operations are easy binary operations– try set union

• limit to size of set related to binary ops

Type colors = (red,blue,green,yellow,orange,white,black); colorset = set of colors;var set1 : colorset;set1 := [red,orange,blue];

implemented as ( 1 1 0 0 1 0 0 )

Pointers

• Lots of flexibility

• Data from heap

• Difficult to manage what you are pointing at

• Many languages strongly manage the types to which the pointers point– c doesn’t care– c++ does

• Real problems are programmer management

Pointer problems

Dangling reference:

int *p1, *p2;p1 = new (int);p2=p1;delete(p1);

p1

p2

Lost heap-dynamic:

int *p1, *p2;p1 = new (int);p1 = p2;

p1

p2

(lost)

Handling Pointer Problems

• Tombstones– always stays even after memory deallocated– never have a variable pointing at deallocated data

cell

Before

null cell

After

tombstone

Handling Pointer Problems

REFERENCE COUNTERS

cell 3

3 pointers at same cell

cell 2

2 pointers at same cell

Delete cell when reference count is 0

Other than efficiency, trick is with circular lists

Handling Pointer Problems

GARBAGE COLLECTION

Initial scenario Mark all w/0 Mark all pointed at w/1

0

0

0

0

0

0

1

1

1

1