Lab #4: Simple Exercises in the C Programming Language
Assigned: Thu 21 Feb 02
Due: Thu 28 Feb 02
About this lab
This week's lab will be the first of a couple of studies of individual programming languages we will make over the course of the term. The character of these labs will be quite different from the earlier, implementation-oriented labs: rather than solving a significant problem about languages using a known vehicle (Java), you will be solving smaller problems of more general character using a new language. The point, of course, is to familiarize you with some of the features of these new languages, although you will not be able to get any significant experience from a few small problems. Rather, you should try to concentrate on the novel features of these languages and how they affect your programming tasks.Unfortunately, our textbook does not provide any consolidated description of the C language, so we will need to depend on some other reference books and on-line materials for background on C: I have placed several links to good references on the course homepage, as shown in lecture.
In addition to these reference materials, you will need to settle on a C compiler to use for your work and find out how to call it. Although there are several compilers available on the Gemini/Hudson systems, I recommend the gcc demonstrated in class. You may also want to use a C compiler on Windows, which may come with a more graphical interface. Your particular choice of compiler is not too important, since we will not be making use of any too-sophisticated features.
Basics of C programming
As described in lecture, there are a number of tricky points of C syntax and semantics which you will have to overcome in order to get through even the most basic programs. Syntactically, C is rather like Java (of course, historically C came first), but here's a quick list of difference which are likely to be difficult at first:
- lack of objects; use of structures
C has no built-in object-oriented notions, so the overall structure of programs is instead a collection of function definitions contained in a file at the top level (data declarations may also be made at the top level). Function definitions are similar to methods, although of course they are "free-standing", not belonging to any class or instance. Every C program must have a (single) main function which will be used as the point of entry when the program is run. Different versions of C compilers may require that the main be declared as returning either a void type or an int type (the latter is used for returning error codes). Check your implementation of C for details.
- strings as pointers/arrays
As discussed in lecture, C uses pointers or arrays of characters to represent string values. Recall that a string in C must be terminated with a null character (value 0 or '\0') in order to work as expected with various library routines. In most versions of C, you may declare parameters and other abstract arrays using an empty set of brackets (e.g., "char[] foo;") in order to indicate an array of unknown length. Any actual concrete array (i.e., not a parameter, etc.) will of course need to have some specific length declared (e.g., "char[100] mystring;").
- use of pointers for simulating references
In Java, all objects (though not primitive data) are held as references, i.e., essentially as pointers to the actual object values. In C, we must explicitly declare our variables as being pointers if that is our intent. So, for example, a variable might be declared as "datastruc* mystruc;" in order to indicate that the variable mystruc is a pointer to a datastruc value.
Two operators are available for manipulating pointers, the '*' operator for de-referencing (i.e., "following the pointer to see what's at the end") and the address operator '&' for returning the address of a variable (in the general sense, i.e., including possibly an element of an array). These are in some sense inverses of each other, in that "*(&x)" is the same value as just "x" and "&(*p)" is the same pointer value as p.
- lack of objects; use of structures
C has no built-in object-oriented notions, so the overall structure of programs is instead a collection of function definitions contained in a file at the top level (data declarations may also be made at the top level).
In order to simulate objects after a fashion, we can use C's structures: these are essentially collections of named fields which may hold values of any data type, including other structures. See the reference books for example of how to declare structures in C.
A common use of structures involves pointing at them (e.g., in a dynamic data structure such as a linked list or tree) and then extracting the field values (or members) of the structure at the end of the pointer. The dot construct (structure.member) is used to select members from a structure. Thus we can follow a pointer and select a member using the notation "(*p).name", for example. This combination is used so often that a special syntax exists, using an arrow("->") to select member field, as in "p->name" (this also gets around the unfortunate need for parentheses in the other syntax).
Programming problems
Write solutions to each of the following problems. Remember that the point of these exercises is to help you get familiar with the language, not to be the best algorithm or the most useful program, or to have the slickest interface. When you demo and hand in your program, be sure to document which platform and compiler you used. You should also provide a short (small paragraph) summary of your reaction to C and your prior exposure, if any. You can include this information in a comment at the top of one of the programs if you like. solution
- Write a function called squareVar which will take a variable in as a parameter and square it "in place". In other words, in terms of pre-condition/post-condition, we should have:
{ x has value n }The ellipses are here to indicate that you'll need to add a little bit of code around the variable before you make the call in order for this to work.
squareVar( ... x ... );
{ x has value n * n }Show that your function works by squaring a "regular" variable (such as x above) and by using it to square every element of an array of 10 integers.
- Write a function replace which will take a string (i.e., character pointer) as parameter, and which will replace every occurrence of one word with another. The other two words will come as the second and third parameters. You should check that the two words are of the same length before you start the replacement, returning the string unchanged if they are not.
The following call exemplifies the intended behavior of replace:
char* message = "Hello, Occupant! You and the whole Occupant family ...";
char msgcopy[100];
strcpy(msgcopy, message);
replace(msgcopy,"Occupant", "Miss Foo");
{ msgcopy has value "Hello, Miss Foo! You and the whole Miss Foo family ..." }
- Write a program which will read in student records and print them out in sorted order. Each record should be represented by a C structure having member/fields for first name, last name and student number (an integer). You should use a stable sorting algorithm and sort first by first name, then by last name (this way they will be fully sorted by name when you finish). You can read the information in from the console or from a file, and should print out the sorted list to the console. You can assume some reasonable upper limit on number of records (say, 25) if you want to use an array.
Challenge: use the same sorting routine to perform both sorts on the same data, but using different fields (hint: use pointers to functions and pass two different comparison functions).
