本次作业右两个问题: 1)用C实现一个排序算法, 2) 用shifting alphabet的方式给文本加密。

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 and the assignment (e.g., ``csci 1120 hw 5’’ or ``LL hw 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.

  1. (10 points) In CSCI 1320 you probably learned about sorting algorithms and implemented one or more of them. A simple way to test such an algorithm is to generate a sequence of ``random’’ numbers, sort them, and check that the result is in ascending order. Sample program sorter-start.c shows how this might be done in C (leaving out the actual sorting). For this problem you will turn in two revisions of this program:

    • First, fill in code for the sort function so that it actually sorts. It’s completely up to you which sorting algorithm to implement, though I’m inclined to recommend that you just do one of the simple-but-slow ones (e.g., bubble sort or selection sort). If you feel ambitious, you could try quicksort or mergesort, though mergesort is apt to be more trouble since it requires a work array.
    • Next, revise the program so that rather than generating random data it reads the values to sort from a file and writes the sorted values to another file. The completed program should take two command-line arguments giving the names of the input and output files. (It should not prompt the user for anything.) The program should print appropriate error messages if not enough arguments are supplied, if it cannot open the input and output files, or if the input file contains anything but a sequence of integers. Since we have not yet talked about how to make arrays larger at runtime, just write the program with a fixed-size array for holding input, and have the program print an error message if the number of input values exceeds the size of the array. It’s up to you whether you keep the part of the existing program that checks whether the sort succeeds (I say ``might as well’’); if you do, just have it print to standard output as before.

    To repeat: You will turn in two programs, one that just fills in the sort function but sorts the randomly-generated data, and one that gets input from a file and writes to another file.


    • Sample program whilesum-fromfile.c illustrates reading a sequence of integers from an input file. Notice that the while loop to read integers stops when fscanf detects either an error or the end of the file. The if after the loop uses feof to find out which of these two things happened – feof returns a nonzero value (``true’’) when the previous attempt to read something detected end of file, zero (``false’’) otherwise (i.e., an error). Be advised that ferror is useful only for detecting I/O errors and is not set if fscanf can read input from the stream but can’t convert it to the requested format.
  2. (10 points) A very simple way to encrypt text is to rotate each alphabetic character N positions. For example, if N is 1, ``abc XYZ 1234’’ becomes ``bcd YZA 1234’’. (This is obviously not industrial-strength encryption but is good enough to somewhat obscure the plaintext.) Write a C program that implements this scheme. The program should take three command-line arguments: the number of positions to rotate (which for simplicity should be a positive integer), the name of the input file, and the name of the output file. It should print error messages as appropriate (not enough command-line arguments, non-numeric N, input or output file cannot be opened). For valid arguments, it should encrypt the input file and write the result to the output file. To get full credit, your program must encrypt characters as discussed in the hint below.


    • You don’t need to try to read input a line at a time; you can just read and process it a character at a time using fgetc, fputc, and a function that encrypts a single character. Sample program copy-file.c illustrates how to process a file one character at a time.
    • You can use library function strtol to convert a command-line argument string into an integer. (You could also use atoi, which is simpler, but it doesn’t provide a nice way to check for errors.) There’s an example of using strtol in sample program echo-args.c.
    • There are probably several ways you could approach encoding each character. The one I want you to use here – partly for practice working with strings, but also because it doesn’t rely on characters being encoded in ASCII (which on most systems these days they are, but C doesn’t require it) – begins by looking up the character in a string representing the alphabet. Starter code for such a scheme, to encode int variable inchar, is available here, and to get full credit for this problem you must use this approach (rather than other ways you may have seen for doing this sort of thing).


… apply).1
Credit where credit is due: I based the wording of this list on a posting to a SIGCSE mailing list. SIGCSE is the ACM’s Special Interest Group on CS Education.

Berna Massingill

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