Coding Practice — How to Think Like a Computer Scientist

An image is just a matrix of pixels. Write the struct definition for Image, which should store information about its height and width and contain a matrix of Pixels. Select the Parsonsprob tab for hints for the construction of the code.

An image is just a matrix of pixels. Write the struct definition for Image, which should store information about its height and width and contain a matrix of Pixels. Use the lines to construct the code, then go back to complete the Activecode tab.

        struct Image {
---
    int height;
---
    int width;
---
    vector> matrix;
---
    vector matrix; #distractor
---
    vector matrix; #distractor
---
};

Now let’s print an Image. Unfortunately we can’t print out the actual image to the terminal, but we can print out the Pixels in the Image matrix. Write the Image member function printImage. Separate pixels in the same row with a space and add a new line at the end of each row. Use the printPixel function we created previously. Select the Parsonsprob tab for hints for the construction of the code.

Now let’s print an Image. Unfortunately we can’t print out the actual image to the terminal, but we can print out the Pixels in the Image matrix. Write the Image member function printImage. Separate pixels in the same row with a space and add a new line at the end of each row. Use the printPixel function we created previously. Use the lines to construct the code, then go back to complete the Activecode tab.

        void Image::printImage() {
---
    for (int r = 0; r < height; ++r) {
---
    for (int c = 0; c < width; ++ c) {
---
        matrix[r][c].printPixel();
---
        cout << " ";
---
    }
---
    cout << endl;
---
    }
---
}

Let’s write a swapPixel member function for Image. swapPixel takes two pairs of row indices and column indices from a matrix and swaps the two Pixels at those locations. Note that these indices are 0-indexed, unlike the previous cropIndex parameters. Select the Parsonsprob tab for hints for the construction of the code.

#include <iostream>
#include <vector>
using namespace std;

struct Pixel {
    int r;
    int g;
    int b;
    void printPixel();
};

struct Image {
    int height;
    int width;
    vector<vector<Pixel> > matrix;
    void printImage();
    void cropImage(int startRow, int stopRow, int startCol, int stopCol);
    void swapPixel(int row1, int col1, int row2, int col2);
};

// Write your implementation of swapPixel here.

int main() {
    vector<vector<Pixel> > matrix = { { { 0, 140, 255 }, { 0, 0, 0 }, { 15, 20, 255 } } };
    Image image = { 1, 3, matrix };
    image.printImage();
    cout << endl;
    image.swapPixel(0, 0, 0, 2);
    image.printImage();
}
====
void Pixel::printPixel() {
    cout << "("<< r << ", " << g << ", " << b << ")";
}

void Image::printImage() {
    for (int r = 0; r < height; ++r) {
    for (int c = 0; c < width; ++ c) {
        matrix[r][c].printPixel();
        cout << " ";
    }
    cout << endl;
    }
}

void Image::cropImage(int startRow, int stopRow, int startCol, int stopCol) {
    vector<vector<Pixel> > newMatrix(stopRow - startRow + 1);
    for (int r = startRow - 1; r < stopRow; ++r) {
        for (int c = startCol - 1; c < stopCol; ++c) {
            newMatrix[r - (startRow - 1)].push_back(matrix[r][c]);
        }
    }
    height = stopRow - startRow + 1;
    width = stopCol - startCol + 1;
    matrix = newMatrix;
}

Let’s write a swapPixel member function for Image. swapPixel takes two pairs of row indices and column indices from a matrix and swaps the two Pixels at those locations. Note that these indices are 0-indexed, unlike the previous cropIndex parameters. Use the lines to construct the code, then go back to complete the Activecode tab.

        void Image::swapPixel(int row1, int col1, int row2, int col2) {
---
    Pixel temp = { matrix[row1][col1].r, matrix[row1][col1].g,  matrix[row1][col1].b };
---
    matrix[row1][col1] = matrix[row2][col2];
---
    matrix[row2][col2] = temp;
---
}

Oops! Somehow our image came out upside down. Let’s write the Image member function flipVertical, which reverts an image to be right side up. Select the Parsonsprob tab for hints for the construction of the code.

#include <iostream>
#include <vector>
using namespace std;

struct Pixel {
    int r;
    int g;
    int b;
    void printPixel();
};

struct Image {
    int height;
    int width;
    vector<vector<Pixel> > matrix;
    void printImage();
    void cropImage(int startRow, int stopRow, int startCol, int stopCol);
    void swapPixel(int row1, int col1, int row2, int col2);
    void flipHorizontal();
    void flipVertical();
};

// Write your implementation of flipVertical here.

int main() {
    vector<vector<Pixel> > matrix = { { { 255, 255, 255 }, { 255, 255, 255 }, { 255, 255, 255 } },
                                    { { 50, 50, 50 }, { 10, 10, 10 }, { 50, 50, 50 } },
                                    { { 30, 30, 30 }, { 70, 70, 70 }, { 30, 30, 30 } },
                                    { { 0, 0, 0 }, { 0, 0, 0 }, { 0, 0, 0 } } };
    Image image = { 4, 3, matrix };
    image.printImage();
    cout << endl;
    image.flipVertical();
    image.printImage();
}
====
void Pixel::printPixel() {
    cout << "("<< r << ", " << g << ", " << b << ")";
}

void Image::printImage() {
    for (int r = 0; r < height; ++r) {
    for (int c = 0; c < width; ++ c) {
        matrix[r][c].printPixel();
        cout << " ";
    }
    cout << endl;
    }
}

void Image::cropImage(int startRow, int stopRow, int startCol, int stopCol) {
    vector<vector<Pixel> > newMatrix(stopRow - startRow + 1);
    for (int r = startRow - 1; r < stopRow; ++r) {
        for (int c = startCol - 1; c < stopCol; ++c) {
            newMatrix[r - (startRow - 1)].push_back(matrix[r][c]);
        }
    }
    height = stopRow - startRow + 1;
    width = stopCol - startCol + 1;
    matrix = newMatrix;
}

void Image::swapPixel(int row1, int col1, int row2, int col2) {
    Pixel temp = { matrix[row1][col1].r, matrix[row1][col1].g,  matrix[row1][col1].b };
    matrix[row1][col1] = matrix[row2][col2];
    matrix[row2][col2] = temp;
}

void Image::flipHorizontal() {
    for (int r = 0; r < height; ++r) {
        int start = 0;
        int end = width - 1;
        while (start < end) {
            swapPixel(r, start, r, end);
            ++start;
            --end;
        }
    }
}

Oops! Somehow our image came out upside down. Let’s write the Image member function flipVertical, which reverts an image to be right side up. Use the lines to construct the code, then go back to complete the Activecode tab.

        void Image::flipVertical() {
    for (int c = 0; c < width; ++c) {
---
        int start = 0;
---
        int end = height - 1;
---
        while (start < end) {
---
            swapPixel(start, c, end, c);
---
            ++start;
---
            --end;
---
        }
---
    }
---
}

Let’s return our image to the state of a clean slate. Write the function clearImage, which sets the color of every Pixel to white. Select the Parsonsprob tab for hints for the construction of the code.

#include <iostream>
#include <vector>
using namespace std;

struct Pixel {
    int r;
    int g;
    int b;
    void printPixel();
};

struct Image {
    int height;
    int width;
    vector<vector<Pixel> > matrix;
    void printImage();
    void cropImage(int startRow, int stopRow, int startCol, int stopCol);
    void swapPixel(int row1, int col1, int row2, int col2);
    void flipHorizontal();
    void flipVertical();
    void createBorder(Pixel p);
    void clearImage();
};

// Write your implementation of clearImage here.

int main() {
    vector<vector<Pixel> > matrix = { { { 0, 0, 0 }, { 10, 10, 10 }, { 65, 70, 255 } },
                                    { { 26, 48, 205 }, { 43, 12, 15 }, { 45, 30, 70 } },
                                    { { 89, 36, 65 }, { 75, 43, 26 }, { 40, 75, 70 } } };
    Image image = { 3, 3, matrix };
    image.printImage();
    cout << endl;
    image.clearImage();
    image.printImage();
}
====
void Pixel::printPixel() {
    cout << "("<< r << ", " << g << ", " << b << ")";
}

void Image::printImage() {
    for (int r = 0; r < height; ++r) {
    for (int c = 0; c < width; ++ c) {
        matrix[r][c].printPixel();
        cout << " ";
    }
    cout << endl;
    }
}

void Image::cropImage(int startRow, int stopRow, int startCol, int stopCol) {
    vector<vector<Pixel> > newMatrix(stopRow - startRow + 1);
    for (int r = startRow - 1; r < stopRow; ++r) {
        for (int c = startCol - 1; c < stopCol; ++c) {
            newMatrix[r - (startRow - 1)].push_back(matrix[r][c]);
        }
    }
    height = stopRow - startRow + 1;
    width = stopCol - startCol + 1;
    matrix = newMatrix;
}

void Image::swapPixel(int row1, int col1, int row2, int col2) {
    Pixel temp = { matrix[row1][col1].r, matrix[row1][col1].g,  matrix[row1][col1].b };
    matrix[row1][col1] = matrix[row2][col2];
    matrix[row2][col2] = temp;
}

void Image::flipHorizontal() {
    for (int r = 0; r < height; ++r) {
        int start = 0;
        int end = width - 1;
        while (start < end) {
            swapPixel(r, start, r, end);
            ++start;
            --end;
        }
    }
}

void Image::flipVertical() {
    for (int c = 0; c < width; ++c) {
        int start = 0;
        int end = height - 1;
        while (start < end) {
            swapPixel(start, c, end, c);
            ++start;
            --end;
        }
    }
}

void Image::createBorder(Pixel p) {
    for (int r = 0; r < height; ++r) {
        matrix[r][0] = p;
        matrix[r][width - 1] = p;
    }
    for (int c = 0; c < width; ++c) {
        matrix[0][c] = p;
        matrix[height - 1][c] = p;
    }
}

Let’s return our image to the state of a clean slate. Write the function clearImage, which sets the color of every Pixel to white. Use the lines to construct the code, then go back to complete the Activecode tab.

        void Image::clearImage () {
---
    for (int r = 0; r < height; r++) {
---
        for (int c = 0; c < width; c++) {
---
            matrix[r][c].r = 255;
---
            matrix[r][c].g = 255;
---
            matrix[r][c].b = 255;
---
        }
---
    }
---
}

Coding Practice¶