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Introduction to Programming: 

Introduction to Programming Seif Haridi KTH Peter Van Roy UCL


Introduction An introduction to programming concepts Simple calculator Declarative variables Functions Structured data (example: lists) Functions over lists Correctness and complexity Lazy functions Concurrency and dataflow State, objects, and classes Nondeterminism and atomicity

A calculator: 

A calculator Use the system as a calculator andgt; Oz {Browse 9999*9999}


Variables Variables are short-cuts for values, they cannot be assigned more than once declare V = 9999*9999 {Browse V*V} Variable identifiers: is what you type Store variable: is part of the memory system The declare statement creates a store variable and assigns its memory address to the identifier ’V’ in the environment


Functions Compute the factorial function: Start with the mathematical definition declare fun {Fact N} if N==0 then 1 else N*{Fact N-1} end end Fact is declared in the environment Try large factorial {Browse {Fact 100}}

Composing functions: 

Composing functions Combinations of r items taken from n. The number of subsets of size r taken from a set of size n declare fun {Comb N R} {Fact N} div ({Fact R}*{Fact N-R}) end Example of functional abstraction Comb Fact Fact Fact

Structured data (lists): 

Structured data (lists) Calculate Pascal triangle Write a function that calculates the nth row as one structured value A list is a sequence of elements: [1 4 6 4 1] The empty list is written nil Lists are created by means of '|' (cons) declare H=1 T = [2 3 4 5] {Browse H|T} % This will show [1 2 3 4 5]

Lists (2): 

Lists (2) Taking lists apart (selecting components) A cons has two components a head, and tail declare L = [5 6 7 8] L.1 gives 5 L.2 give [6 7 8] ‘|’ ‘|’ ‘|’ 6 7 8 nil

Pattern matching: 

Pattern matching Another way to take a list apart is by use of pattern matching with a case instruction case L of H|T then {Browse H} {Browse T} end

Functions over lists: 

Functions over lists Compute the function {Pascal N} Takes an integer N, and returns the Nth row of a Pascal triangle as a list For row 1, the result is [1] For row N, shift to left row N-1 and shift to the right row N-1 Align and add the shifted rows element-wise to get row N 1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 (0) (0) [0 1 3 3 1] [1 3 3 1 0] Shift right Shift left

Functions over lists (2): 

Functions over lists (2) declare fun {Pascal N} if N==1 then [1] else {AddList {ShiftLeft {Pascal N-1}} {ShiftRight {Pascal N-1}}} end end AddList ShiftLeft ShiftRight Pascal N-1 Pascal N-1 Pascal N

Functions over lists (3): 

Functions over lists (3) fun {ShiftLeft L} case L of H|T then H|{ShiftLeft T} else [0] end end fun {ShiftRight L} 0|L end fun {AddList L1 L2} case L1 of H1|T1 then case L2 of H2|T2 then H1+H2|{AddList T1 T2} end else nil end end

Top-down program development: 

Top-down program development Understand how to solve the problem by hand Try to solve the task by decomposing it to simpler tasks Devise the main function (main task) in terms of suitable auxiliary functions (subtasks) that simplifies the solution (ShiftLeft, ShiftRight and AddList) Complete the solution by writing the auxiliary functions

Is your program correct?: 

Is your program correct? 'A program is correct when it does what we would like it to do' In general we need to reason about the program: Semantics for the language: a precise model of the operations of the programming language Program specification: a definition of the output in terms of the input (usually a mathematical function or relation) Use mathematical techniques to reason about the program, using programming language semantics

Mathematical induction: 

Mathematical induction Select one or more input to the function Show the program is correct for the simple cases (base case) Show that if the program is correct for a given case, it is then correct for the next case. For integers base case is either 0 or 1, and for any integer n the next case is n+1 For lists the base case is nil, or a list with one or few elements, and for any list T the next case H|T

Correctness of factorial: 

Correctness of factorial fun {Fact N} if N==0 then 1 else N*{Fact N-1} end end Base Case: {Fact 0} returns 1 (Nandgt;1), N*{Fact N-1} assume {Fact N-1} is correct, from the spec we see the {Fact N} is N*{Fact N-1} More techniques to come !


Complexity Pascal runs very slow, try {Pascal 24} {Pascal 20} calls: {Pascal 19} twice, {Pascal 18} four times, {Pascal 17} eight times, ..., {Pascal 1} 219 times Execution time of a program up to a constant factor is called program’s time complexity. Time complexity of {Pascal N} is proportional to 2N (exponential) Programs with exponential time complexity are impractical declare fun {Pascal N} if N==1 then [1] else {AddList {ShiftLeft {Pascal N-1}} {ShiftRight {Pascal N-1}}} end end

Faster Pascal: 

fun {FastPascal N} if N==1 then [1] else local L in L={FastPascal N-1} {AddList {ShiftLeft L} {ShiftRight L}} end end end Faster Pascal Introduce a local variable L Compute {FastPascal N-1} only once Try with 30 rows. FastPascal is called N times, each time a list on the average of size N/2 is processed The time complexity is proportional to N2 (polynomial) Low order polynomial programs are practical.

Lazy evaluation: 

Lazy evaluation The functions written so far are evaluated eagerly (as soon as they are called) Another way is lazy evaluation where a computation is done only when the results is needed declare fun lazy {Ints N} N|{Ints N+1} end Calculates the infinite list: 0 | 1 | 2 | 3 | ...

Lazy evaluation (2): 

Lazy evaluation (2) Write a function that computes as many rows of Pascal’s triangle as needed We do not know how many beforehand A function is lazy if it is evaluated only when its result is needed The function PascalList is evaluated when needed fun lazy {PascalList Row} Row | {PascalList {AddList {ShiftLeft Row} {ShiftRight Row}}} end

Lazy evaluation (3): 

Lazy evaluation (3) Lazy evaluation will avoid redoing work if you decide first you need the 10th row and later the 11th row The function continues where it left off declare L = {PascalList [1]} {Browse L} {Browse L.1} {Browse L.2.1} Landlt;Futureandgt; [1] [1 1]

Higher-order programming: 

Higher-order programming Assume we want to write another Pascal function which instead of adding numbers performs exclusive-or on them It calculates for each number whether it is odd or even (parity) Either write a new function each time we need a new operation, or write one generic function that takes an operation (another function) as argument The ability to pass functions as argument, or return a function as result is called higher-order programming Higher-order programming is an aid to build generic abstractions

Variations of Pascal: 

Variations of Pascal Compute the parity Pascal triangle 1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 1 1 1 0 1 1 1 1 1 1 0 0 0 1 fun {Xor X Y} if X==Y then 0 else 1 end end

Higher-order programming: 

Higher-order programming fun {GenericPascal Op N} if N==1 then [1] else L in L = {GenericPascal Op N-1} {OpList Op {ShiftLeft L} {ShiftRight L}} end end fun {OpList Op L1 L2} case L1 of H1|T1 then case L2 of H2|T2 then {Op H1 H2}|{OpList Op T1 T2} end end else nil end end fun {Add N1 N2} N1+N2 end fun {Xor N1 N2} if N1==N2 then 0 else 1 end end fun {Pascal N} {GenericPascal Add N} end fun {ParityPascal N} {GenericPascal Xor N} end


Concurrency How to do several things at once Concurrency: running several activities each running at its own pace A thread is an executing sequential program A program can have multiple threads by using the thread instruction {Browse 99*99} can immediately respond while Pascal is computing thread P in P = {Pascal 21} {Browse P} end {Browse 99*99}


Dataflow What happens when multiple threads try to communicate? A simple way is to make communicating threads synchronize on the availability of data (data-driven execution) If an operation tries to use a variable that is not yet bound it will wait The variable is called a dataflow variable + * * X Y Z U

Dataflow (II): 

Dataflow (II) Two important properties of dataflow Calculations work correctly independent of how they are partitioned between threads (concurrent activities) Calculations are patient, they do not signal error; they wait for data availability The dataflow property of variables makes sense when programs are composed of multiple threads declare X thread {Delay 5000} X=99 end {Browse ‘Start’} {Browse X*X} declare X thread {Browse ‘Start’} {Browse X*X} end {Delay 5000} X=99


State How to make a function learn from its past? We would like to add memory to a function to remember past results Adding memory as well as concurrency is an essential aspect of modeling the real world Consider {FastPascal N}: we would like it to remember the previous rows it calculated in order to avoid recalculating them We need a concept (memory cell) to store, change and retrieve a value The simplest concept is a (memory) cell which is a container of a value One can create a cell, assign a value to a cell, and access the current value of the cell Cells are not variables declare C = {NewCell 0} {Assign C {Access C}+1} {Browse {Access C}}


Example Add memory to Pascal to remember how many times it is called The memory (state) is global here Memory that is local to a function is called encapsulated state declare C = {NewCell 0} fun {FastPascal N} {Assign C {Access C}+1} {GenericPascal Add N} end


Objects Functions with internal memory are called objects The cell is invisible outside of the definition declare local C in C = {NewCell 0} fun {Bump} {Assign C {Access C}+1} {Access C} end end declare fun {FastPascal N} {Browse {Bump}} {GenericPascal Add N} end


Classes A class is a ’factory’ of objects where each object has its own internal state Let us create many independent counter objects with the same behavior fun {NewCounter} local C Bump in C = {NewCell 0} fun {Bump} {Assign C {Access C}+1} {Access C} end Bump end end

Classes (2): 

Classes (2) Here is a class with two operations: Bump and Read fun {NewCounter} local C Bump in C = {NewCell 0} fun {Bump} {Assign C {Access C}+1} {Access C} end fun {Read} {Access C} end [Bump Read] end end

Object-oriented programming: 

Object-oriented programming In object-oriented programming the idea of objects and classes is pushed farther Classes keep the basic properties of: State encapsulation Object factories Classes are extended with more sophisticated properties: They have multiple operations (called methods) They can be defined by taking another class and extending it slightly (inheritance)


Nondeterminism What happens if a program has both concurrency and state together? This is very tricky The same program can give different results from one execution to the next This variability is called nondeterminism Internal nondeterminism is not a problem if it is not observable from outside

Nondeterminism (2): 

Nondeterminism (2) declare C = {NewCell 0} thread {Assign C 1} end thread {Assign C 2} end time C = {NewCell 0} cell C contains 0 {Assign C 1} cell C contains 1 {Assign C 2} cell C contains 2 (final value) t0 t1 t2

Nondeterminism (3): 

Nondeterminism (3) declare C = {NewCell 0} thread {Assign C 1} end thread {Assign C 2} end time C = {NewCell 0} cell C contains 0 {Assign C 2} cell C contains 2 {Assign C 1} cell C contains 1 (final value) t0 t1 t2

Nondeterminism (4): 

Nondeterminism (4) declare C = {NewCell 0} thread I in I = {Access C} {Assign C I+1} end thread J in J = {Access C} {Assign C J+1} end What are the possible results? Both threads increment the cell C by 1 Expected final result of C is 2 Is that all?

Nondeterminism (5): 

Nondeterminism (5) Another possible final result is the cell C containing the value 1 declare C = {NewCell 0} thread I in I = {Access C} {Assign C I+1} end thread J in J = {Access C} {Assign C J+1} end time C = {NewCell 0} I = {Access C} I equal 0 t0 t1 t2 J = {Access C} J equal 0 {Assign C J+1} C contains 1 {Assign C I+1} C contains 1 t3 t4

Lessons learned: 

Lessons learned Combining concurrency and state is tricky Complex programs have many possible interleavings Programming is a question of mastering the interleavings Famous bugs in the history of computer technology are due to designers overlooking an interleaving (e.g., the Therac-25 radiation therapy machine giving doses 1000’s of times too high, resulting in death or injury) If possible try to avoid concurrency and state together Encapsulate state and communicate between threads using dataflow Try to master interleavings by using atomic operations


Atomicity How can we master the interleavings? One idea is to reduce the number of interleavings by programming with coarse-grained atomic operations An operation is atomic if it is performed as a whole or nothing No intermediate (partial) results can be observed by any other concurrent activity In simple cases we can use a lock to ensure atomicity of a sequence of operations For this we need a new entity (a lock)

Atomicity (2): 

Atomicity (2) declare L = {NewLock} lock L then sequence of ops 1 end Thread 1 lock L then sequence of ops 2 end Thread 2

The program: 

The program declare C = {NewCell 0} L = {NewLock} thread lock L then I in I = {Access C} {Assign C I+1} end end thread lock L then J in J = {Access C} {Assign C J+1} end end The final result of C is always 2

Additional exercises: 

Additional exercises Write the memorizing Pascal function using the store abstraction (available at Reason about the correctness of AddList and ShiftLeft using induction

Memoizing FastPascal: 

Memoizing FastPascal {FastPascal N} New Version Make a store S available to FastPascal Let K be the number of the rows stored in S (i.e. max row is the Kth row) if N is less or equal K retrieve the Nth row from S Otherwise, compute the rows numbered K+1 to N, and store them in S Return the Nth row from S Viewed from outside (as a black box), this version behaves like the earlier one but faster declare S = {NewStore} {Put S 2 [1 1]} {Browse {Get S 2}} {Browse {Size S}}

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