Notes and Errata for Prelim 2 Review

Problem 1

Your method swapPairs should swap two consecutive elements repeatedly, not just the first two.  In the sample solution, the head of the while loop should read

while (i < letters.length - 1) {

instead of while (i < letters.length - 2).

Problem 3

There's a small flaw in the example: Your function should return 3 because 1, 4, and 7 appear in both arrays.

Problem 4

The expression (int) Math.random() * N returns a random element of { 0, 1, 2, ..., N-1 }.  It never returns N.  In the sample solution, the while loop for initializing faceShown is missing a statement. The loop should read

while (i < 6) {
    faceShown[i] = false;
    i = i + 1;
}

Problem 5

The sample solution will not print the length of the longest, but the length of the last nondecreasing sequence.  The correct solution should look like

public static void main(String[] args) {
    int lastNum = -1; // number previously entered
    int length = 0;  // length of current nondecr. subsequence
    int maxLength = 0;  // length of longest nondecr. subsequence so far
    int num; // current number

    while (lastNum != 0) {
        num = in.ReadInt();
        if (num != 0) {
            if (num >= lastNum)   // if new elem is at least as big as old one
                length = length + 1;  // count it
            else
                length = 1; // one element is always nondecreasing
    if (length > maxLength)
        maxLength = length;  // update info on longest sequence
            lastNum = num; // save number for next round
}
    }
    System.out.println("Length of longest nondecr. subseq. is " + length);
}

Problem 7

Two lines are missing in the answer to problem 7.  Function compress should read like

void compress() {
    int currentPixel = 0;
    int i = 0;
    int j = 0;
    int length = 0;

    while (i < image.length) {
        if (image[i] == currentPixel)
            length = length + 1;
        else {
            reducedImage[j] = length;
            j = j + 1;
            currentPixel = image[i];
            length = 1;
        }
        i = i + 1;
    }
    reducedImage[j] = length;  // store last count
}