CSE 142 : 2 December 1999

Strings and File I/O

A string in C is only an array of characters with the "null" character '\0' (backslash zero, not to be confused with the null pointer). What does the following output?

    #include <string.h>

    char hello[]   = "Hello, world\n"; /* null is implicit */
    char world[14] = {'H', 'e', 'l', 'l', 'o', ',',  ' ',
                      'w', 'o', 'r', 'l', 'd', '\n', '\0'};
    char mychars[20]; /* NOT a string yet, just an array of char */

    printf("%s", hello);
    printf("%s", world);

    /* strcmp: return value is
         < 0 : if first < second
           0 : if first == second
         > 0 : if first > second. */
    printf("%d\n", strcmp(hello, world));
    hello[1] = 'u';
    printf("%d\n", strcmp(hello, world));
    hello[1] = 'a';
    printf("%d\n", strcmp(hello, world));

    /* strcpy: note that the SECOND argument is the source.
       Think, "I strcpy TO mystr FROM world." */
    strcpy(mychars, world);
    printf("%s", mychars);

    /* strlen: note that the length DOES NOT include the null
       character or anything after it. */
    printf("%d\n", strlen(world));
    printf("%d\n", strlen(mychars));
    

Working with files

Files are represented by a struct that typically contains some hairy operating system information, like a pointer to a list of disk blocks, a pointer to buffer memory, etc.

Fortunately, we don't have to worry about it, because all file functions operate on pointers to FILE structs. This is an example of a "handle" or "magic cookie", which is frequently used to provide modular programming interfaces to C libraries.

    #include <stdio.h>

    FILE *infile, *outfile;
    char c;

    infile  = fopen("in.txt", "r");               /* Open filehandles */
    outfile = fopen("out.txt", "w");

    if (infile != NULL && outfile != NULL) {      /* If open successful,   */
        while ( fscanf(infile, "%c", &c) != EOF ) /* read until EOF,       */
            fprintf(outfile, "%c", c);            /* writing the character */
    }

    fclose(infile);
    fclose(outfile);
    

The NAW server: serving files

Recall the NAW server example we did, so long ago. Let's say we wanted to make the GET print a file's contents, like a real web server, instead of hardcoded strings. How would we do it?

    void HandleGet(void) {
        /* declarations */



        /* Get page name from user; need string input. */
        printf("Enter page name: ");




        /* Print selected page: need to open a file, and print its
           contents. */






    }
    

Sorting

So far, you have learned mostly the mechanics of programming: control structures and primitive data types. However, the purpose of programming is to solve actual problems with these building blocks.

To solve actual problems, we use algorithms. Sorting is one problem frequently taught in introductory courses because it is a simple proble, with (mostly) simple algorithms.

You did SelectSort in lecture. Here are some other sorting algorithms, in order of increasing speed:

• Bogo-sort: Check if the array is sorted. If not, randomly shuffle all the values. Repeat. Pathologically bad: O(nⁿ⁺¹) in the average case.
• BubbleSort: For each element from 1 to n, move the current element left until it either "bumps into" a smaller value or reaches the left end of the array. Somewhat worse than SelectSort. O(n²).
• QuickSort: Choose the first element n in the array. Put all the values less than this element in one pile, and put all the values greater than or equal to this element in another pile. Put element n in between the piles, and repeat for each pile. O(n * log(n)) on average.

What lies in store for you?

If we liken programming to literature, right now you're at the stage where you can read "See Spot run. Good Spot!" This is fun for a while but gets boring eventually. Fortunately, things only get much more interesting from here on out.

• Object-oriented programming: Describing the world with objects, rather than with procedures.
• Algorithms: Sorting isn't a very cool problem, so the algorithms are boring. There are many nicer ones. For example: What's the shortest path between the cities on a road map? How do I compress a text file to a smaller size?
• Graphics: If you enjoyed playing with GP142, you can persevere until the graphics for majors course and learn how to program much more pleasing visual effects.
• Networking: How would I write a real web server or a multiplayer online game? What makes the Internet work?
• Architecture, compilers, languages, artificial intelligence, theory of computation...