CMSI 284: Systems Programming: Homework #2 Answers

CMSI 284

Homework #2
Partial Answers

/*
 * This application writes a table of piano notes and their frequencies
 * to standard output.
 */

#include <stdio.h>
#include <math.h>

char *names[] = {"A","A#","B","C","C#","D","D#","E","F","F#","G","G#"};
int STEPS = sizeof names / sizeof (char*);
int INITIAL_FREQUENCY = 27.5;
int NUM_KEYS = 88;

int main() {
    int i;
    for (i = 0; i < NUM_KEYS; i++) {
        printf("%s\t%10.4f\n", names[i%STEPS],
                INITIAL_FREQUENCY * pow(2.0, (double)i / STEPS));
    }
    return 0;
}

/*
 * pianoscales.c
 *
 * Takes a single command line argument which is the name of a note
 * and prints, to standard output, the major (Ionian) and minor (Aeolian)
 * scales starting at that note.  The program is very week; note names
 * are constrained to be in {A, A#, B, C, C#, D, D#, E, F, F#, G, G#}.
 */

#include <stdio.h>
#include <string.h>

char *keys[] = {"A","A#","B","C","C#","D","D#","E","F","F#","G","G#"};
int NUMBER_OF_NOTES = sizeof keys / sizeof (char*);
int majorSteps[] = {2,2,1,2,2,2,1};
int minorSteps[] = {2,1,2,2,1,2,2};

/*
 * Writes a progression of notes whose step sizes come from the steps array.
 */
void printScale(int noteIndex, int steps[], int numberOfSteps) {
    int i;
    int j;
    for (i = 0, j = 0; i <= numberOfSteps; j += steps[i++]) {
        printf("%3s", keys[(noteIndex + j) % NUMBER_OF_NOTES]);
    }
}

/*
 * Display the major and minor scales for a specified piano key input.
 */
int main(int argc, char** argv) {
    int j;

    if (argc != 2) {
        printf("This program requires exactly one command line argument\n");
        return -1;
    }

    /* Find the note index for a given note name */
    for (j = 0; j < NUMBER_OF_NOTES && strcmp(argv[1], keys[j]) != 0; j++)
        ;

    /* Not found? */
    if (j == NUMBER_OF_NOTES) {
        printf("No such key: %s\n", argv[1]);
        return -2;
    }

    /* Write major and minor */
    printf("%s major:", argv[1]);
    printScale(j, majorSteps, 7);
    printf("\n%s minor:", argv[1]);
    printScale(j, minorSteps, 7);
    printf("\n");

    return 0;
}

#include <stdlib.h>

#define max(x,y) ((x)<(y)?(y):(x))

/*
 * max_string(s, t) returns the string u with the same length as
 * the longes of s and t, containing, in each position i, the
 * larger of s[i] and t[i].  If a position is part of only one
 * of s or t, then u[i] contains the character in the longer
 * string.
 */
char* max_string(char* s, char* t) {
    int length_s = strlen(s);
    int length_t = strlen(t);

    /* Allocate space for the result */
    int n = max(length_s, length_t);
    char* result = malloc(n * sizeof(char) + 1);

    /* Put in the result characters */
    int i;
    for (i = 0; i < n; i++) {
        if (i >= length_s) result[i] = t[i];
        else if (i >= length_t) result[i] = s[i];
        else result[i] = max(s[i], t[i]);
    }

    /* Insert the string terminator and return. */
    result[i] = '\0';
    return result;
}

/*
 * max_string_test_case.c
 *
 * Test case for the funky max_string function.
 */

#include <assert.h>
#include <string.h>
#include <stdio.h>

/* Usually it's best to include from .h files, but this is ok for a test */
#include "max_string.c"

int main() {

    assert(strcmp(max_string("doghouse", "rats"), "rotsouse") == 0);
    assert(strcmp(max_string("rats", "doghouse"), "rotsouse") == 0);
    assert(strcmp(max_string("", ""), "") == 0);
    assert(strcmp(max_string("", "Abc"), "Abc") == 0);
    assert(strcmp(max_string("Abc", ""), "Abc") == 0);
    assert(strcmp(max_string("123", "abc"), "abc") == 0);
    assert(strcmp(max_string("abc", "123"), "abc") == 0);

    printf("All tests passed\n");

    return 0;
}

#include <string.h>
#include <stdlib.h>

/*
 * Returns the k-fold left-rotation of s, as a newly allocated
 * string. (That means the caller is responsible for the memory.)
 * If k is not in the range 0..strlen(s), returns NULL.
 *
 * Examples:
 *    rotate_left("doghouse", 0)  ===>  "doghouse"
 *    rotate_left("doghouse", 1)  ===>  "oghoused"
 *    rotate_left("doghouse", 2)  ===>  "ghousedo"
 *    rotate_left("doghouse", 3)  ===>  "housedog"
 *    rotate_left("doghouse", 4)  ===>  "ousedogh"
 *    rotate_left("doghouse", 5)  ===>  "usedogho"
 *
 * Rotating a negative number of times returns a copy of the
 * input, as it should, since rotating -4 times is rotating
 * not at all.
 */
char* rotate_left(char* s, int k) {
    int i = 0;
    int j;
    int n = strlen(s);

    /* Handle non-positive k as well as the empty string. */
    if (k <= 0 || n == 0) return strdup(s);

    /* Ensure 0 <= k < n.  This is one reason we checked for n==0 above. */
    if (k >= n) k %= n;

    /* Allocate space for the result */
    char* result = malloc(n * sizeof(char) + 1);

    /* Put in the result characters */
    for (j = k; j < n; j++) result[i++] = s[j];
    for (j = 0; j < k; j++) result[i++] = s[j];

    /* Insert the string terminator and return. */
    result[i] = '\0';
    return result;
}

#include <assert.h>
#include <string.h>
#include <stdio.h>

/* Usually it's best to include from .h files, but this is ok for a test */
#include "rotate.c"

int main() {

    assert(strcmp(rotate_left("doghouse", -5), "doghouse") == 0);
    assert(strcmp(rotate_left("doghouse", -50), "doghouse") == 0);
    assert(strcmp(rotate_left("doghouse", 0), "doghouse") == 0);
    assert(strcmp(rotate_left("doghouse", 8), "doghouse") == 0);
    assert(strcmp(rotate_left("doghouse", 16), "doghouse") == 0);
    assert(strcmp(rotate_left("doghouse", 24), "doghouse") == 0);
    assert(strcmp(rotate_left("doghouse", 32), "doghouse") == 0);
    assert(strcmp(rotate_left("doghouse", 800000), "doghouse") == 0);
    assert(strcmp(rotate_left("doghouse", 1), "oghoused") == 0);
    assert(strcmp(rotate_left("doghouse", 2), "ghousedo") == 0);
    assert(strcmp(rotate_left("doghouse", 3), "housedog") == 0);
    assert(strcmp(rotate_left("doghouse", 4), "ousedogh") == 0);
    assert(strcmp(rotate_left("doghouse", 5), "usedogho") == 0);
    assert(strcmp(rotate_left("doghouse", 6), "sedoghou") == 0);
    assert(strcmp(rotate_left("doghouse", 7), "edoghous") == 0);
    assert(strcmp(rotate_left("doghouse", 11), "housedog") == 0);
    assert(strcmp(rotate_left("doghouse", 19), "housedog") == 0);
    assert(strcmp(rotate_left("", 0), "") == 0);
    assert(strcmp(rotate_left("", 1), "") == 0);
    assert(strcmp(rotate_left("", 60000), "") == 0);
    assert(strcmp(rotate_left("", -20), "") == 0);
    assert(strcmp(rotate_left("x", -20), "x") == 0);
    assert(strcmp(rotate_left("x", 0), "x") == 0);
    assert(strcmp(rotate_left("x", 20), "x") == 0);
    assert(strcmp(rotate_left("x", 5000), "x") == 0);

    printf("All tests passed\n");

    return 0;
}

Common problems with many of the submissions:

For rotate, no checking for negative or huge counts. You must think: I BETTER CHECK EVERY ARGUMENT SOMEONE PASSES IN OR ELSE SOMETHING COULD GO WRONG. (For example you could be indexing an array outside of its allocated range.)
Unit tests without troublesome arguments. You gotta think: AM I TESTING ALL POSSIBLE **KINDS** OF INPUTS? (zeros, negatives, positives, "boundary cases", empty, almost empty, full, almost full, same, different, smaller, larger, etc.)
Inconsistent (or non-existent) spacing and indentation. This should not be an issue with second year programmers!
Magic Numbers. Ack! Almost anything other than 0 or 1 should not be hardcoded.
Evil copy/paste and other violations of the DRY principle. DON'T REPEAT YOURSELF! If a change to one part of the code requires changes to another, then you have to factor out that dependency.
Saying "if blah then return true else return false" instead of just "return blah". Some people never learn.
Accumulating multiplications in the keys problem.
An "if" condition with a bunch of ors instead of a lookup.
Not leaving space for the zero byte at the end of a string.
Hardcoding data that could be generated algorithmically.
Multiple calls to strlen().
Forgetting a destroy() function for the queue.
Missing unit tests.
Not unit testing properly. PRINTF IS FOR DEBUGGING, NOT UNIT TESTING!
An inexplicable lack of comments. There should be at least a comment at the top of a file describing what the file contains (in general terms).
Things like "if (blah == 1)" instead of "if (blah)".
Not checking for NULL before dereferencing a pointer. AAAAAAAAHHHHH! If you ever use "->" then ask yourself whether there is any way the pointer can be null.
A couple people did the most evil thing of all: strcpy of a passed in string into a fixed-size local string variable. NEVER ASSUME YOUR BUFFER IS "BIG ENOUGH".