Questions and Answers of Programming Language Pragmatics

Suppose we wish, as described at the end of Example 16.36, to accurately attribute sampled time to the various contexts in which a subroutine is called. Perhaps the most straightforward approach
Using your local implementations of Java and C#, compile the code of Figures 16.2 and C 16.7 all the way to machine language. Disassemble and compare the results. Can all the differences be
Rewrite the list insertion method of Example C 16.40 in F# instead of C#. Compile to CIL and compare to the right side of Figure C 16.7. Discuss any differences you find.Figure 16.7 . method private
Building on the previous exercise, rewrite your list insertion routine (both C# and F# versions) to be generic in the type of the list elements. Compare the generic and non generic versions of the
Extend your F# code from Exercise C 16.16 to include list removal and search routines. After finding and reading appropriate documentation, package these routines in a library that can be called in a
Consider the following pseudocode:procedure P(A, B : real) X : realprocedure Q(B, C : real) Y : real . . .procedure
Rewrite the attribute grammar of Figure 4.14 in the form of an ad hoc tree traversal consisting of mutually recursive subroutines in your favorite programming language. Keep the symbol table in a
Write the equivalent of Figure 6.5 in C# or Ruby. Write a second version that performs an in-order enumeration, rather than preorder.Figure 6.5: class BinTree: def -_init_ (self): # constructor
Building on the previous exercise, implement higher-level union, intersection, and difference functions that operate on ordered sets. Note that these should not be members of the ordered_set class,
Continuing Example C 7.59, the callcsNames.consider(null);will generate a run-time exception, because String. compare To is not designed to take null arguments.(a) Modify Figure C 7.6 to guard
(a) Modify your solution to Exercise 7.14 so that the comparison routine is an explicit generic parameter, reminiscent of the chooser.(b) Give an alternative solution in which the comparison routine
Consider the C++ program. Explain why the final call to first_n generates a compile-time error, but the call to last_n does not. Show how to modify the final call to first_n so that the compiler will
Consider the following code in C++:Here *Lp will be a “deep copy” of L, containing a copy of each foo object. Try to write equivalent code in Java. What goes wrong? How might you get around the
Suppose we are compiling for a machine with 1-byte characters, 2-byte shorts, 4-byte integers, and 8-byte reals, and with alignment rules that require the address of every primitive data element to
In Example 8.10 we suggested the possibility of sorting record fields by their alignment requirement, to minimize holes. In the example, we sorted smallest-alignment-first. What would happen if we
Give Ada code to map from lowercase to uppercase letters, using(a) An array(b) A functionNote the similarity of syntax: in both cases upper(‚a‚) is ‚A‚.
In Section 8.2.2 we noted that in a language with dynamic arrays and a value model of variables, records could have fields whose size is not known at compile time. To accommodate these, we suggested
Explain how to extend Figure 8.7 to accommodate subroutine arguments that are passed by value, but whose shape is not known until the subroutine is called at run time. sp- Ada: -- procedure foo (size
Explain how to obtain the effect of Fortran 90’s allocate statement for one-dimensional arrays using pointers in C. You will probably find that your solution does not generalize to multidimensional
Example 8.24, which considered the layout of a two-dimensional array of characters, counted only the space devoted to characters and pointers. This is appropriate if the space is allocated
Consider the array indexing calculation of Example 8.25. Suppose that i, j, and k are already loaded into registers, and that A’s elements are integers, allocated contiguously in memory on a 32-bit
Continuing the previous exercise, suppose that A has row-pointer layout, and that i, j, and k are again available in registers. Show pseudo assembler code to load A[i, j, k] into a register. Assuming
Repeat the preceding two exercises, modifying your code to include runtime checking of array subscript bounds.
In Section 8.2.3 we discussed how to differentiate between the constant and variable portions of an array reference, in order to efficiently access the subparts of array and record objects. An
Consider the following C declaration, compiled on a 64-bit x86 machine:struct { int n; char c;} A[10][10];If the address of A[0][0] is 1000
Suppose we are generating code for an imperative language on a machine with 8-byte floating-point numbers, 4-byte integers, 1-byte characters, and 4-byte alignment for both integers and
Suppose A is a 10×10 array of (4-byte) integers, indexed from [0][0] through [9][9]. Suppose further that the address of A is currently in register r1, the value of integer i is currently in
Pointers and recursive type definitions complicate the algorithm for determining structural equivalence of types. Consider, for example, the following definitions:type A = record x :
Explain the meaning of the following C declarations:double *a[n];double (*b)[n];double (*c[n])();double (*d())[n];
In Ada 83, pointers (access variables) can point only to objects in the heap. Ada 95 allows a new kind of pointer, the access all type, to point to other objects as well, provided that those objects
As noted in Section 8.5.2, Ada 95 forbids an access all pointer from referring to any object whose lifetime is briefer than that of the pointer’s type. Can this rule be enforced completely at
In much of the discussion of pointers in Section 8.5, we assumed implicitly that every pointer into the heap points to the beginning of a dynamically allocated block of storage. In some languages,
(a) Occasionally one encounters the suggestion that a garbage-collected language should provide a delete operation as an optimization: by explicitly delete-ing objects that will never be used again,
In Example 8.52 we noted that functional languages can safely use reference counts since the lack of an assignment statement prevents them from introducing circularity. This isn’t strictly true;
Here is a skeleton for the standard quicksort algorithm in Haskell:quicksort [] = []quicksort (a : l) = quicksort [...] ++ [a] ++ quicksort [...]The ++ operator denotes list concatenation (similar to
In Example 6.70 we described a programming idiom in which an iterator takes a “loop body” function as argument, and applies it to every element of a given ontainer or set. Show how to use this
Show how unions can be used in C to interpret the bits of a value of one type as if they represented a value of some other type. Explain why the same technique does not work in Ada. After consulting
Are variant records a form of polymorphism? Why or why not?
Learn the details of variant records in Pascal.(a) You may have noticed that the language does not allow you to pass the tag field of a variant record to a subroutine by reference. Why not?(b)
We noted in Section C 8.1.3 that Ada requires the variant portions of a record to occur at the end, to save space when a particular record is constrained to have a comparatively small variant part.
In Example 8.52 we noted that reference counts can be used to reclaim tombstones, failing only when the programmer neglects to manually delete the object to which a tombstone refers. Explain how to
In Section 8.5.3 we introduced the notion of smart pointers in C++. Learn how these are implemented, and write an explanation. Discuss the relationship to tombstones.
Rewrite Example C 8.80 using fgets, strtol, strtod, etc. (read the man pages), so that it is guaranteed not to result in buffer overflow.
The output routines of several languages (e.g., println in Swift) give special treatment to ends of lines. By contrast, C’s printf does not; it treats newlines and carriage returns the same as any
Describe as many ways as you can in which functions in imperative programming languages differ from functions in mathematics.
Consider the following code in C++:Suppose that string_map::operator[] contains the only call to complex_ lookup anywhere in the program. Explain why it would be unwise for the programmer to expand
Using your favorite language and compiler, write a program that can tell the order in which certain subroutine parameters are evaluated.
Consider the following (erroneous) program in C:Local variable i in subroutine foo is never initialized. On many systems, however, the program will display repeatable behavior, printing 0 1 2 3 4 5 6
The standard calling sequence for the c. 1980 Digital VAX instruction set employed not only a stack pointer (sp) and frame pointer (fp), but a separate arguments pointer (ap) as well. Under what
Write (in the language of your choice) a procedure or function that will have four different effects, depending on whether arguments are passed by value, by reference, by value/result, or by name.
Consider an expression like a + b that is passed to a subroutine in Fortran. Is there any semantically meaningful difference between passing this expression as a reference to an unnamed temporary (as
Consider the following subroutine in Fortran 77:subroutine shift(a, b, c)integer a, b, ca = bb = cendSuppose we want to call shift(x, y, 0) but we don’t want to change the value of y. Knowing that
In some implementations of Fortran IV, the following code would print a 3. Can you suggest an explanation? How do you suppose more recent Fortran implementations get around the problem?c
Suppose you are writing a program in which all parameters must be passed by name. Can you write a subroutine that will swap the values of its actual parameters? Explain.
Can you write a swap routine in Java, or in any other language with only call-by-sharing parameters? What exactly should swap do in such a language?
As noted in Section 9.3.1, out parameters in Ada 83 can be written by the callee but not read. In Ada 95 they can be both read and written, but they begin their life uninitialized. Why do you think
Taking a cue from Ada, Swift provides an in out parameter mode. The language manual does not specify whether in out parameters are to be passed by reference or value-result. Write a program that
Fields of packed records (Example 8.8) cannot be passed by reference in Pascal. Likewise, when passing a subrange variable by reference, Pascal requires that all possible values of the corresponding
Consider the following declaration in C:double(*foo(double (*)(double, double[]), double)) (double, ...);Describe in English the type of foo.
Why do you suppose that variable-length argument lists are so seldom supported by high-level programming languages?
Building on Exercise 6.35, show how to implement exceptions using call with- current-continuation in Scheme. Model your syntax after the handler-case of Common Lisp. As in Exercise 6.35, you will
Given what you have learned about the implementation of structured exceptions, describe how you might implement the nonlocal gotos of Pascal or the label parameters of Algol 60 (Section 6.2). Do you
Describe a plausible implementation of C++ destructors or Java try. . . finally blocks. What code must the compiler generate, at what points in the program, to ensure that cleanup always occurs when
Use threads to build support for true iterators in Java. Try to hide as much of the implementation as possible behind a reasonable interface. In particular, hide any uses of new thread, thread.start,
In Common Lisp, multilevel returns use catch and throw; exception handling in the style of most other modern languages uses handler-case and error. Show that the distinction between these is mainly a
Compile and run the program in Figure 9.6. Explain its behavior. Create a new version that behaves more predictably. #include #include #include char* days [7] = {"Sunday", "Monday", "Tuesday",
In C#, Java, or some other language with thread-based event handling, build a simple program around the “pause button” of Examples 9.51–9.54. Your program should open a small window containing
Extend your answer to the previous problem by adding a “clone” button. Pushing this button should create an additional window containing another counter. This will, of course, require additional
Suppose you wish to minimize the size of closures in a language implementation that uses a display to access nonlocal objects. Assuming a language like Pascal or Ada, in which subroutines have
Elaborate on the reasons why even parameters passed in registers may sometimes need to have locations in the stack. Consider all the cases in which it may not suffice to keep a parameter in a
Most versions of the C library include a function, alloca, that dynamically allocates space within the current stack frame.2 It has two advantages over the usual malloc, which allocates space in the
Explain how to extend the conventions of Figure C 9.9 and Section C 9.2.2 to accommodate arrays whose bounds are not known until elaboration time (as discussed in Section 8.2.2). What ramifications
In all three of our case studies, stack-based arguments were placed into the argument build area in “reverse” order, with the lowest-numbered argument at the top. Explain why this is important.
How would you implement nested subroutines as parameters on a machine that doesn’t let you execute code in the stack? Can you pass a simple code address, or do you require that closures be
If you have read the rest of Chapter 9, you may have noticed that the term “trampoline” is also used in conjunction with the implementation of signal handlers (Section 9.6.1). What is the
Explain how you might implement setjmp and longjmp on a SPARC.
Build a preorder iterator for binary trees in Java, C#, or Python. Do not use a true iterator or an explicit stack of tree nodes. Rather, create nested iterator objects on demand, linking them
One source of inaccuracy in the traffic simulation of Section C 9.5.4 has to do with the timing at traffic signals. If a signal is currently green in the EW direction, but the queue of waiting cars
Some language designers argue that object orientation eliminates the need for nested subroutines. Do you agree? Why or why not?
Design a class hierarchy to represent syntax trees for the CFG of Figure 4.5. Provide a method in each class to return the value of a node. Provide constructors that play the role of the make_leaf,
Repeat the previous exercise, but using a variant record (union) type to represent syntax tree nodes. Repeat again using type extensions. Compare the three solutions in terms of clarity, abstraction,
Using the C# indexer mechanism, create a hash table class that can be indexed like an array. (In effect, create a simple version of the System.Collections.Hashtable container class.) Alternatively,
In the spirit of Example 10.8, write a double-ended queue (deque) abstraction (pronounced “deck”), derived from a doubly linked list base class.
Use templates (generics) to abstract your solutions to the previous two questions over the type of data in the container.
Repeat Exercise 10.5 in Python or Ruby. Write a simple program to demonstrate that generics are not needed to abstract over types. What happens if you mix objects of different types in the same
When using the list class of Example 10.17, the typical C++ programmer will use a pointer type for generic parameter V, so that list_nodes point to the elements of the list. An alternative
Can you emulate the inner class of Example 10.22 in C# or C++?
Write a package body for the list abstraction of Figure 10.2.Figure 10.2 generic type item is private; default_value : item; Ada supports both type -- and value generic parameters -- package g_list
Rewrite the list and queue abstractions in Eiffel, Java, and/or C#.
Using C++, Java, or C#, implement a Complex class in the spirit of Example 10.25. Discuss the time and space tradeoffs between maintaining all four values (x, y, ρ, and θ) in the state of the
Repeat the previous two exercises for Python and/or Ruby.
Compare Java final methods with C++ nonvirtual methods. How are they the same? How are they different?
In several object-oriented languages, including C++ and Eiffel, a derived class can hide members of the base class. In C++, for example, we can declare a base class to be public, protected, or
What happens to the implementation of a class if we redefine a data member? For example, suppose we haveDoes the representation of a bar object contain one b field or two? If two, are both
Discuss the relative merits of classes and type extensions. Which do you prefer? Why?
Building on the outline of Example 10.28, write a program that illustrates the difference between copy constructors and operator= in C++. Your code should include examples of each situation in which
What do you think of the decision, in C++, C#, and Ada 95, to use static method binding, rather than dynamic, by default? Is the gain in implementation speed worth the loss in abstraction and
If foo is an abstract class in a C++ program, why is it acceptable to declare variables of type foo*, but not of type foo?
Consider the Java program shown in Figure 10.8. Assume that this is to be compiled to native code on a machine with 4-byte addresses.Figure 10.8(a) Draw a picture of the layout in memory of the
In Ruby, as in Java 8 or Scala, an interface (mix-in) can provide method code as well as signatures. (It can’t provide data members; that would be multiple inheritance.) Explain why dynamic typing

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