1. 159.331Prog Lang & Alg1 Programming Languages Imperative Programming Languages - Part 1 - Principles & Data

2. 159.331Prog Lang & Alg2 Principles of Imperative Languages Procedural or Imperative Languages involve: Data & Data declarations State changing commands (or statements) Stepping the flow of control We will also look at: Program Composition Examples of Imperative Programming

3. 159.331Prog Lang & Alg3 Data All data in a computer has some representation and number of properties - known as its type Most data items have names Relationships between data items, names, types and representations specified by the programmer in data declarations

4. 159.331Prog Lang & Alg4 Data Declarations Data item in memory has number of bits Need a structure to know how to interpret the bits appropriately. Usually needs a name so we can refer to it too A data declaration binds a name to the (particular) data item and its structure Data in the machine can be changed - hence we call it a variable - with name, type and value Note that a data item exists in the machine Its structure and name exist only in the program In C: int i, j;

5. 159.331Prog Lang & Alg5 Data Initialization Some languages allow us to combine data declaration with initialization In C: int i=5, j=5; C has facility to initialize different variables in single declaration to different values Remember the declaration is doing two things: –Creating or Allocating memory for the data item –Binding a name to it

6. 159.331Prog Lang & Alg6 Constant Declarations Some languages have constant declarations so that the variable’s bit pattern cannot be changed once it is initialized. –In C: const int i = 42;

7. 159.331Prog Lang & Alg7 Uninitialised Variables Non-initializing declaration creates uninitialized variable Different languages provide different safety treatments: 1.Problem ignored, it’s the programmer’s responsibility - inelegant 2.Uninitialized variable declared erroneous and it is compiler writer’s responsibility to enforce the check - expensive 3.All uninitialized variables silently initialized to some default value eg zero bit pattern, pointers set to null - creates false sense of security for the programmer 4.Disallow non initializing declarations - forces programmer to deal with the issue 5.For each type have a special representation meaning “not initialized - often called the omega value - this propagates and allows dynamic checking but changes meaning of “uninitialized” Most languages fully or partially ignore the problem

8. 159.331Prog Lang & Alg8 Renaming & Aliasing A second sort of declaration - a renaming or aliasing one In Ada: K: Integer renames I; No extra storage allocated - just another name bound to same variable Potentially dangerous - can have surprise side effects Some parameter transfer mechanisms need this feature No such facility in C - not the same as a pointer…

9. 159.331Prog Lang & Alg9 Overloading The opposite or counterpart of aliasing Aliasing - multiple names bound to one name Overloading - binds more than one object code to the same name eg “+” works for ints and floats Which actual object code used will depend upon context - enough contextual info must be present to resolve the ambiguity C uses overloading for built-in operators only Ada has elaborate set of resolution rules

10. 159.331Prog Lang & Alg10 Types & Type Constructors Hardware: type is a way of interpreting data bits Programming: type specifies set of allowed values and set of operations defined on those values Task of language compiler writer to find most efficient hardware representation for a given language Basic types or primitive types in many languages –Characters - eg char –Integers eg short, int, long, long long –Floating point numbers - eg float, double

11. 159.331Prog Lang & Alg11

12. 159.331Prog Lang & Alg12 Literals Explicit values of the basic types are written down “literally” in a program - hence the term literal –eg -123 or 3.14156984 or ‘K’ or “Ken” Sometimes we need some extra syntax –eg 0x19AF (Hex) or 0123 (Octal) –or 1234L (long) or 3.14F (float) Void is technically a type - often used to specify set of no values or a related idea

13. 159.331Prog Lang & Alg13 Type Constructors The primitive types usually built into a language and set up automatically The user/programmer can specify their own types - use type construction In C: typedef int int_10_array[10]; int_10_array ia; In Ada: type Int10-Array is array (Integer range 1.. 10) of Integer; IA: Int10_Array; Data types composed of other types are called compound types

14. 159.331Prog Lang & Alg14 Enumeration Types Simplest type constructor in enumeration No new type just a binding of names to set of values in an existing type In C: typedef enum { red, amber, green } traffic_light_colour; traffic_light_colour n, e, s, w; typedef enum{ false, true } boolean; In Ada: Type Traffic_Light_Colour is (Red, Amber, Green); N, E, S, W: Traffic_Light_Colour; Type Boolean is (False, True); Actually Boolean is a built-in type in Ada.

15. 159.331Prog Lang & Alg15 What can we do with enum types? Might expect just to be able to do copying and comparison for equality Most languages implement them as integers and let you do comparisons (greater/smaller) or incrementing them (successor operation) This makes sense for some enumerated types eg “days of week” or “months of year” or character set with lexical ordering eg ASCII or Unicode

16. 159.331Prog Lang & Alg16 Arrays Simplest type constructor that builds upon another type Array is a series of a known number of items - all of the same type An item is called an element Accessed by an ordinal known as its index Array represents a mapping from some type (typically integers) to values of the element type Language might supply operators for lower bound, upper bound and size of an array

17. 159.331Prog Lang & Alg17 Array Syntax In C: int ia[10] First element is ia[0], last element is ia[9] sizeof(ia) gives size in bytes, so use: (sizeof(ia ) / sizeof( ia[0] ) ) to get number of elements Initialisation requires a compound value (called an aggregate in Ada) eg int ia[3] = {3, 5, 8}; Note all our bounds here are constants - ie they are known at compile time (known as static bounds) Some languages have dynamic bounded arrays (Ada does, Java does, but not C)

18. 159.331Prog Lang & Alg18 More Array Syntax In Ada: IA: array (Integer range 1..10) of Integer; IA(1) is first element, IA(10) is last element IA’First is lower bound IA’Last is upper bound IA’Length is the length or size of the array Initialising: IA: array (Integer range 1..3) of Integer := (3, 5, 8); Dynamic bounding: IA: array (Integer range M..N-1) of Integer;

19. 159.331Prog Lang & Alg19 Flexible Arrays flexible arrays can be resized dynamically Neither Ada nor C has this feature although some languages like Algol, Orca and Icon do In C we can simulate this feature using library functions malloc() and realloc() Note this may involve copying data around to new locations in memory to ensure array is contiguous in its new memory location. This can be slow - sometime use special fast copy function memcpy() to do this.

20. 159.331Prog Lang & Alg20 Some languages have multi-dimensional arrays and some also arrays of arrays. Common Syntax is iaa[3,5] for multi-dimensional arrays Ada array-of-arrays must have same bounds (sizes) so less useful. ANSI C uses [][] notation for indexing multi-dimensional arrays

21. 159.331Prog Lang & Alg21 Row and Column Major Some algorithms step through multi dimensional data in nested repetition - it is important to known how data is stored in memory to get best data caching performance C is “row major” (So are C++ and Java) So in an array myArray[i][j], think of j is the one that moves fastest in memory, i moves more slowly. Fortran is “column-major” - the opposite - comes from Fortran’s roots in maths and matrix terminology - Matrix element Aij row i, column j

22. 159.331Prog Lang & Alg22 Slicing array slicing is similar to indexing, but accesses a subarray rather than a single element In Ada the elements IA(3) through IA(6) can be accessed (in one go) as the slice IA(3..6) C has no slicing. Modern data-parallel Fortrans use slicing to good effect to specify chunks of data that can be distributed across processors in parallel computer system. Some languages like Fortran90 allow you to specify a stride in the slice specification A(4:10:2) means the slice containing A(4), A(6), A(8) and A(10)

23. 159.331Prog Lang & Alg23 Associative Arrays Array mapping need not be just with integer index values As long as we can resolve ambiguities, we can use any mapping (in principle - although may be inefficient) Associative arrays use potentially arbitrary types for the index (the language ABC supports this) They associate an index value of one type with elements in the array of a second type Supported in many scripting languages eg Perl and Python to index into an array using a string as the “index” C and Ada allow indexing with enumerated types and characters ( they map to integers anyway) Other languages support this with library functions - eg C++ Standard Template Library, or java.util package

24. 159.331Prog Lang & Alg24 Sequences or Lists sequences or lists are constructed types that build a new type using only one base type. Unlike arrays they hold an unknown number of elements - all of the same type First element can be accessed using a head operator; the rest using a tail operator No simple way to determine the size or length of the list (or indeed the last element) If there were, these would just be “arrays in disguise” Arrays and sequences are often confused by programmers Strings are generally used as sequences of characters, although often implemented (modelled) as arrays with messy function calls in a library

25. 159.331Prog Lang & Alg25 Sets sets also build a new type using only one type A set over type T with set of values V is a type whose values are sets of the elements of V Usual operators are set-union, set-difference and set-intersection Neither C nor Ada provides sets. Modula-2 does. C++ and Java provide set libraries. number of elements in a set is its cardinality A powerset operator yields the set of all sets of its argument set. The SETL language has this.

26. 159.331Prog Lang & Alg26 Bags Bags are variants of sets - they can contain a value more than once They record how many times a value has been inserted Primary associated operations are insert-value and remove-value Sometimes known as multi-sets The language ABS has bags. C and Ada do not, C++ and Java have multi-set libraries.

27. 159.331Prog Lang & Alg27 Imperative - Part 1 -Summary Data & Data Declarations –Constant Declarations –Uninitialized variables –Renaming & Aliasing –Overloading –Types and Type Constructors –Enumeration Types –Arrays –Sequences –Sets & Bags Next - Records, Pointers, and Unions Bal & Grune Chapter 2 - Sections 2.1 and 2.2 Sebesta Chapters 5 & 6