How to execute Program structure Variables name, keywords, binding, scope, lifetime Data types – type system – primitives, strings, arrays, hashes – pointers/references – type conversions and equality Expressions – Operators, overloading, booleans, short-circuiting, conditional expression – Referential transparency – Statements vs Expressions Control flow – conditionals – loops Functions Classes Exception handling Other features Threads Reflection Libraries Functional Language – other aspects, covered later

 A binding is an association, such as: bind type of variable bind operation to symbol (e.g., meaning of *) bind function to its definition  Binding time is the time at which a binding takes place.  Bindings may be static or dynamic explicit or implicit

 Language design time -- bind operator symbols to operations : sum = sum + count msg = “Hello” + name  Language implementation time-- bind type to a representation : int => number of bits, etc.  Compile time -- bind a variable to a type: int count;  Link time – bind library subprogram to code: cout << x;  Load time -- bind a FORTRAN 77 variable to a memory cell (or a C static variable)  Runtime -- bind a nonstatic local variable to a memory cell

 A binding is static if: it first occurs before run time and it remains unchanged throughout program execution.  A binding is dynamic if: it first occurs during execution or it can change during execution of the program  NOTE: doesn't consider paging etc. which is at the hardware level

This distinction may apply to:  variable typing  variable lifetime  variable scope  polymorphism overloaded operators vs late binding Static is also used to identify class vs instance variables – not really a static vs dynamic example

 Type not specified by declaration, not determined by name (JavaScript, PHP, Ruby)  Specified through an assignment statement list = [2, 4.33, 6, 8]; list = 17.3; Advantage: flexibility (generic program units) Disadvantages:  High cost (dynamic type checking requires run-time descriptors, normally interpreted… upcoming discussion)  Type error detection by the compiler is difficult How are generic program units done in C++? Java?

 Read this:  With a partner: Look at how List is defined Write down how you think Set would be defined (just write 1-2 methods, not the entire interface)  Read this:  Discuss the Stack example Turn in for class participation

 How would you implement dynamic types?  What data structure(s) would you use? list = 3.6 list = [3.4, 5.6] list = [1,2,3]  How does this impact your code – consider efficiency, reliability.  Now think about challenges with + total = message = “hello” + “ world” something = “count “ other = 3 + “count”

i = x; // desired, x is scalar i = y; // typed accidentally, y is array

 Definitions not precise  In general, a strongly typed language will generate a compiler error if the value used (e.g., passed to a function, assigned to a variable) does not match the expected type  May also be considered strongly typed if type errors are prevented at runtime due to dynamic typing (so type safety is more important for reliability)  A language may be considered weakly typed if it includes features that allow types to be used interchangeably

 Implicit type conversions  Pointers*  Untagged unions* * covered later

 Widening Conversions: can include at least approximations to all of the values of the original type. Examples (Java/C++) byte to short, int, long, float or double short to int, long, float or double char to int, long, float or double int to long, float or double long to float or double float to double  Narrowing conversions: cannot include all of the values of the original type short to byte or char char to byte or short int to byte, short, or char long to byte, short, char or int float to byte, short, char, int or long double to byte, short, char, int, long or float

 Even widening conversions may lose accuracy.  Example: integers stored in 32 bits, 9 digits of precision. Floating point values also stored in 32 bits, only about 7 digits of precision (because of space used for exponent). Conversions should be used with care! Warnings should not just be ignored… Strongly typed language minimizes type conversions

 A language with more implicit conversions is considered less strongly typed  C supports more implicit conversions than Java

 Based on:  Implicit conversions. Write a line of code that would illustrate one of the scenarios.  Array to pointer conversion. Draw a picture and 1-2 lines of code that illustrate.  Did you know: C++ will do an implicit conversion if there is a single-argument ctor that will do the needed conversion? Add to page for class participation

 Type safety is the extent to which a programming language discourages or prevents type errors  A type error is erroneous or undesirable program behavior caused by a discrepancy between differing data types (e.g., trying to perform an operation that is not valid for that type)  Type enforcement can be static (compile time) or dynamic (run-time)

 Explicit – stated by programmer  Implicit – determined by language  Can be applied to: type declaration variable lifetime These are NOT the same as static/dynamic, but are often confused

 An explicit declaration is a program statement used for declaring the types of variables: int count;  An implicit declaration is a default mechanism for specifying types of variables (the first appearance of the variable in the program)  Both create static bindings to types (i.e., type doesn’t change during execution of program)  FORTRAN, PL/I, BASIC, and Perl provide implicit declarations Advantage: writability Disadvantage: reliability  is array, % is hash, $ is scalar  Fortran: I-N integer, others single precision, can override

 many words have special meaning (e.g. if, true, def, etc.)  Keyword: has special meaning in particular context, but can be used as variable name Algol, PL/I, Fortran  Reserved: can’t be used as variable COBOL has ~400, Java has ~50 Ruby has reserved words (evolving, no fixed number) Advantage: may avoid confusion Disadvantage: may need to be aware of language parts you aren’t even using

 In Fortran, this is potentially valid: if if then then else else  In Java, goto is a reserved word (you can’t use) but not a keyword (language doesn’t use)  Functions in libraries are not keywords OR reserved words

function bark() {}; bark.prototype.alert = "woof"; bark.prototype.alertOwner = function(){return this.alert}; var myDog = new bark(); alert(myDog.alert); // displays dialog saying woof

Unconditional Branching  Transfers execution control to a specified place in the program  Represented one of the most heated debates in 1960’s and 1970’s  Well-known mechanism: goto statement  Major concern: Readability  Some languages do not support goto statement (e.g., Module-2, Java, Python, Ruby)  C# offers goto statement (can be used in switch statements)  Loop exit statements are restricted and somewhat camouflaged goto ’s

 Read this:   Why are goto statements harmful?? Add brief explanation to class participation