Be sure you have read the assigned readings for classes through 4/01.
Do the following programming problems. You will end up with at least one code file per problem. Submit your program source (and any other needed files) by sending mail to bmassing@cs.trinity.edu, with each file as an attachment. Please use a subject line that mentions the course number and the assignment (e.g., ``csci 1120 homework 5''). You can develop your programs on any system that provides the needed functionality, but I will test them on one of the department's Linux machines, so you should probably make sure they work in that environment before turning them in.
To do this, I think you will need to read the whole file into memory. There are various ways to do this and perform the sort; the one I want you to use is somewhat involved but intended to give you more practice working with pointers. To get full credit you must use the approach described here.
First, read the whole file into memory. To do this you will need to know its size, and interestingly enough there does not appear to be any truly portable and reliable way to find that out. My suggestion is therefore to just open the file, read it a character at a time, counting the number of characters but not trying to save them, and close it again. Reading the file twice (once only to find its size) is of course inefficient but will give the desired result (unless some other application is changing the file at the same time) using only standard and portable C functions, and coming up with a nicer way to accomplish this task is beyond the scope of this assignment.
Once you have the (best estimate for) file size,
you can allocate a single array for the file using malloc, something
like this:
char * data = malloc(size_in_bytes);
You can now operate on data as if it had been declared as an array of char. (Check first that malloc succeeded.)
Read in the contents of the file; a character at a time is probably simplest. Notice that as you do this you will get the newline characters at the ends of lines. (You might write this much of the program and check that it works before going on.)
Once you have the whole file in memory, the objective is to sort it with qsort. The sample program sort-improved.c has an example of using qsort. It needs four parameters: an array to sort (of elements of fixed size), a count of elements, a size for each element, and a comparison function. Your first thought may be to wonder how this can work, since text strings aren't of fixed size. But we can play a trick ...
The idea will be to build an array of pointers pointing to starts of lines, sort the pointers so the first one points to the first line to print, etc., and use them to print the lines in order. (If you think you know at this point how to proceed, you could try doing so, and then come back and read the rest of this description.)
So the next step is to
build the array of pointers to lines. How many do
you need? Well, you could figure that out as you're reading the
file into memory. Say you have that in a variable called N.
Then you can allocate space for an array of pointers like this:
char ** lines = malloc(sizeof(lines[0]) * N);
The first one should point to data[0], which you can accomplish like this:
lines[0] = &data[0];
Then the idea is to go through the rest of the characters, and make lines[1] point to the character after the first newline, lines[2] point to the character after the second newline, etc.
Once you get this array built, you can check it by printing the
file contents out again using the array of pointers;
for example, to print the first line you can write
printf("%s\n", lines[0]);
(You'll need this code anyway, so might as well write it now and check that it works. You may get a surprise when you first run it, as a result of which you may decide you need to do more processing of your data array.)
Now the missing piece of the puzzle is to use qsort to actually sort the lines before printing them. Its parameters were described earlier; the only one you don't yet have is the comparison function. It can look a lot like the one in the sample program; all that's different is that you're sorting pointers-to-strings rather than integers. You will probably want to use the C library function strcmp for the actual comparison. Read its man page to find out what it does and what parameters it takes.
You can check your program's output by using the sort command to sort the input file and comparing its result (captured with I/O redirection!) with your result (also captured with I/O redirection).