How to read MNIST data in C++?

Question

I'm having trouble reading the MNIST database of handwritten digits in C++.

It's in a binary format, which I know how to read, but I don't know the exact format of MNIST.

Therefore, I want to ask people who have read the MNIST data about the format of MNIST data and do you have any suggestions for how to read this data in C++?

I think that you can found all you need about MNIST and C++ in [Neural Network for Recognition of Handwritten Digits by Mike O'Neill](http://www.codeproject.com/KB/library/NeuralNetRecognition.aspx) — dani herrera, Nov 27 '11 at 15:46
You can find an already decoded version of the MNIST dataset here: http://mnist-decoded.000webhostapp.com/ — SomethingSomething, Mar 31 '17 at 14:19

score 28 · Answer 1 · answered May 02 '12 at 07:22

int reverseInt (int i) 
{
    unsigned char c1, c2, c3, c4;

    c1 = i & 255;
    c2 = (i >> 8) & 255;
    c3 = (i >> 16) & 255;
    c4 = (i >> 24) & 255;

    return ((int)c1 << 24) + ((int)c2 << 16) + ((int)c3 << 8) + c4;
}
void read_mnist(/*string full_path*/)
{
    ifstream file (/*full_path*/"t10k-images-idx3-ubyte.gz");
    if (file.is_open())
    {
        int magic_number=0;
        int number_of_images=0;
        int n_rows=0;
        int n_cols=0;
        file.read((char*)&magic_number,sizeof(magic_number)); 
        magic_number= reverseInt(magic_number);
        file.read((char*)&number_of_images,sizeof(number_of_images));
        number_of_images= reverseInt(number_of_images);
        file.read((char*)&n_rows,sizeof(n_rows));
        n_rows= reverseInt(n_rows);
        file.read((char*)&n_cols,sizeof(n_cols));
        n_cols= reverseInt(n_cols);
        for(int i=0;i<number_of_images;++i)
        {
            for(int r=0;r<n_rows;++r)
            {
                for(int c=0;c<n_cols;++c)
                {
                    unsigned char temp=0;
                    file.read((char*)&temp,sizeof(temp));

                }
            }
        }
    }
}

Note to save others from duplicating my silly mistake: although the filenames in this answer have a ".gz" extension, it is, in fact, necessary that the files be uncompressed before they are used. (There is a comment on the original website that could be interpreted to suggest otherwise.) If the first four bytes of the file are `0x1f8b0808` instead of `0x00000801` or `0x00000803`, that means the file has not been uncompressed. — Charles Staats, Dec 09 '14 at 18:22
u're missing ios::binary in ifstream file() as the second argument. For me it doesn't work otherwise. — Niklas Hantke, Jun 22 '20 at 08:13

score 12 · Accepted Answer · answered Nov 28 '11 at 20:34

I did some work with the MNIST data recently. Here's some code that I wrote in Java that should be pretty easy for you to port over:

import net.vivin.digit.DigitImage;    
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteBuffer;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * Created by IntelliJ IDEA.
 * User: vivin
 * Date: 11/11/11
 * Time: 10:07 AM
 */
public class DigitImageLoadingService {

    private String labelFileName;
    private String imageFileName;

    /** the following constants are defined as per the values described at http://yann.lecun.com/exdb/mnist/ **/

    private static final int MAGIC_OFFSET = 0;
    private static final int OFFSET_SIZE = 4; //in bytes

    private static final int LABEL_MAGIC = 2049;
    private static final int IMAGE_MAGIC = 2051;

    private static final int NUMBER_ITEMS_OFFSET = 4;
    private static final int ITEMS_SIZE = 4;

    private static final int NUMBER_OF_ROWS_OFFSET = 8;
    private static final int ROWS_SIZE = 4;
    public static final int ROWS = 28;

    private static final int NUMBER_OF_COLUMNS_OFFSET = 12;
    private static final int COLUMNS_SIZE = 4;
    public static final int COLUMNS = 28;

    private static final int IMAGE_OFFSET = 16;
    private static final int IMAGE_SIZE = ROWS * COLUMNS;


    public DigitImageLoadingService(String labelFileName, String imageFileName) {
        this.labelFileName = labelFileName;
        this.imageFileName = imageFileName;
    }

    public List<DigitImage> loadDigitImages() throws IOException {
        List<DigitImage> images = new ArrayList<DigitImage>();

        ByteArrayOutputStream labelBuffer = new ByteArrayOutputStream();
        ByteArrayOutputStream imageBuffer = new ByteArrayOutputStream();

        InputStream labelInputStream = this.getClass().getResourceAsStream(labelFileName);
        InputStream imageInputStream = this.getClass().getResourceAsStream(imageFileName);

        int read;
        byte[] buffer = new byte[16384];

        while((read = labelInputStream.read(buffer, 0, buffer.length)) != -1) {
           labelBuffer.write(buffer, 0, read);
        }

        labelBuffer.flush();

        while((read = imageInputStream.read(buffer, 0, buffer.length)) != -1) {
            imageBuffer.write(buffer, 0, read);
        }

        imageBuffer.flush();

        byte[] labelBytes = labelBuffer.toByteArray();
        byte[] imageBytes = imageBuffer.toByteArray();

        byte[] labelMagic = Arrays.copyOfRange(labelBytes, 0, OFFSET_SIZE);
        byte[] imageMagic = Arrays.copyOfRange(imageBytes, 0, OFFSET_SIZE);

        if(ByteBuffer.wrap(labelMagic).getInt() != LABEL_MAGIC)  {
            throw new IOException("Bad magic number in label file!");
        }

        if(ByteBuffer.wrap(imageMagic).getInt() != IMAGE_MAGIC) {
            throw new IOException("Bad magic number in image file!");
        }

        int numberOfLabels = ByteBuffer.wrap(Arrays.copyOfRange(labelBytes, NUMBER_ITEMS_OFFSET, NUMBER_ITEMS_OFFSET + ITEMS_SIZE)).getInt();
        int numberOfImages = ByteBuffer.wrap(Arrays.copyOfRange(imageBytes, NUMBER_ITEMS_OFFSET, NUMBER_ITEMS_OFFSET + ITEMS_SIZE)).getInt();

        if(numberOfImages != numberOfLabels) {
            throw new IOException("The number of labels and images do not match!");
        }

        int numRows = ByteBuffer.wrap(Arrays.copyOfRange(imageBytes, NUMBER_OF_ROWS_OFFSET, NUMBER_OF_ROWS_OFFSET + ROWS_SIZE)).getInt();
        int numCols = ByteBuffer.wrap(Arrays.copyOfRange(imageBytes, NUMBER_OF_COLUMNS_OFFSET, NUMBER_OF_COLUMNS_OFFSET + COLUMNS_SIZE)).getInt();

        if(numRows != ROWS && numRows != COLUMNS) {
            throw new IOException("Bad image. Rows and columns do not equal " + ROWS + "x" + COLUMNS);
        }

        for(int i = 0; i < numberOfLabels; i++) {
            int label = labelBytes[OFFSET_SIZE + ITEMS_SIZE + i];
            byte[] imageData = Arrays.copyOfRange(imageBytes, (i * IMAGE_SIZE) + IMAGE_OFFSET, (i * IMAGE_SIZE) + IMAGE_OFFSET + IMAGE_SIZE);

            images.add(new DigitImage(label, imageData));
        }

        return images;
    }
}

Where is `DigitImage` ? And `numRows != COLUMNS` doesn't seem right... — Yassin Hajaj, Oct 20 '19 at 17:25

dariush · Answer 3 · 2016-09-22T10:09:53.737

For what it's worth, I have tuned the @mrgloom's code:

For reading images dataset:

uchar** read_mnist_images(string full_path, int& number_of_images, int& image_size) {
    auto reverseInt = [](int i) {
        unsigned char c1, c2, c3, c4;
        c1 = i & 255, c2 = (i >> 8) & 255, c3 = (i >> 16) & 255, c4 = (i >> 24) & 255;
        return ((int)c1 << 24) + ((int)c2 << 16) + ((int)c3 << 8) + c4;
    };

    typedef unsigned char uchar;

    ifstream file(full_path, ios::binary);

    if(file.is_open()) {
        int magic_number = 0, n_rows = 0, n_cols = 0;

        file.read((char *)&magic_number, sizeof(magic_number));
        magic_number = reverseInt(magic_number);

        if(magic_number != 2051) throw runtime_error("Invalid MNIST image file!");

        file.read((char *)&number_of_images, sizeof(number_of_images)), number_of_images = reverseInt(number_of_images);
        file.read((char *)&n_rows, sizeof(n_rows)), n_rows = reverseInt(n_rows);
        file.read((char *)&n_cols, sizeof(n_cols)), n_cols = reverseInt(n_cols);

        image_size = n_rows * n_cols;

        uchar** _dataset = new uchar*[number_of_images];
        for(int i = 0; i < number_of_images; i++) {
            _dataset[i] = new uchar[image_size];
            file.read((char *)_dataset[i], image_size);
        }
        return _dataset;
    } else {
        throw runtime_error("Cannot open file `" + full_path + "`!");
    }
}

For reading labels dataset:

uchar* read_mnist_labels(string full_path, int& number_of_labels) {
    auto reverseInt = [](int i) {
        unsigned char c1, c2, c3, c4;
        c1 = i & 255, c2 = (i >> 8) & 255, c3 = (i >> 16) & 255, c4 = (i >> 24) & 255;
        return ((int)c1 << 24) + ((int)c2 << 16) + ((int)c3 << 8) + c4;
    };

    typedef unsigned char uchar;

    ifstream file(full_path, ios::binary);

    if(file.is_open()) {
        int magic_number = 0;
        file.read((char *)&magic_number, sizeof(magic_number));
        magic_number = reverseInt(magic_number);

        if(magic_number != 2049) throw runtime_error("Invalid MNIST label file!");

        file.read((char *)&number_of_labels, sizeof(number_of_labels)), number_of_labels = reverseInt(number_of_labels);

        uchar* _dataset = new uchar[number_of_labels];
        for(int i = 0; i < number_of_labels; i++) {
            file.read((char*)&_dataset[i], 1);
        }
        return _dataset;
    } else {
        throw runtime_error("Unable to open file `" + full_path + "`!");
    }
}

EDIT: Thanks to @Jürgen Brauer for reminding me to correct my answer, although I have fixed it in my code back in time, but forgot to update the answer.

I printed the values of a single 28x28 digit and they were all nearly 0. I found out, that it is import to write ifstream file(full_path, ios::binary); and not ifstream file(full_path); in the code above. Hope that helps to prevent others from loosing time. — Jürgen Brauer, Sep 20 '16 at 12:25
In reverseInt, is the purpose of the bitwise & to limit the number of bits with data filled? — Victor Lockwood, Aug 09 '23 at 22:34

score 1 · Answer 4 · answered Sep 19 '18 at 13:15

The below code is from caffe,and I have do some change and convert it to cv::Mat:

uint32_t swap_endian(uint32_t val) {
    val = ((val << 8) & 0xFF00FF00) | ((val >> 8) & 0xFF00FF);
    return (val << 16) | (val >> 16);
}

void read_mnist_cv(const char* image_filename, const char* label_filename){
    // Open files
    std::ifstream image_file(image_filename, std::ios::in | std::ios::binary);
    std::ifstream label_file(label_filename, std::ios::in | std::ios::binary);

    // Read the magic and the meta data
    uint32_t magic;
    uint32_t num_items;
    uint32_t num_labels;
    uint32_t rows;
    uint32_t cols;

    image_file.read(reinterpret_cast<char*>(&magic), 4);
    magic = swap_endian(magic);
    if(magic != 2051){
        cout<<"Incorrect image file magic: "<<magic<<endl;
        return;
    }

    label_file.read(reinterpret_cast<char*>(&magic), 4);
    magic = swap_endian(magic);
    if(magic != 2049){
        cout<<"Incorrect image file magic: "<<magic<<endl;
        return;
    }

    image_file.read(reinterpret_cast<char*>(&num_items), 4);
    num_items = swap_endian(num_items);
    label_file.read(reinterpret_cast<char*>(&num_labels), 4);
    num_labels = swap_endian(num_labels);
    if(num_items != num_labels){
        cout<<"image file nums should equal to label num"<<endl;
        return;
    }

    image_file.read(reinterpret_cast<char*>(&rows), 4);
    rows = swap_endian(rows);
    image_file.read(reinterpret_cast<char*>(&cols), 4);
    cols = swap_endian(cols);

    cout<<"image and label num is: "<<num_items<<endl;
    cout<<"image rows: "<<rows<<", cols: "<<cols<<endl;

    char label;
    char* pixels = new char[rows * cols];

    for (int item_id = 0; item_id < num_items; ++item_id) {
        // read image pixel
        image_file.read(pixels, rows * cols);
        // read label
        label_file.read(&label, 1);

        string sLabel = std::to_string(int(label));
        cout<<"lable is: "<<sLabel<<endl;
        // convert it to cv Mat, and show it
        cv::Mat image_tmp(rows,cols,CV_8UC1,pixels);
        // resize bigger for showing
        cv::resize(image_tmp, image_tmp, cv::Size(100, 100));
        cv::imshow(sLabel, image_tmp);
        cv::waitKey(0);
    }

    delete[] pixels;
}

Usage(I have simplify the code, ommited headers and namespace):

string base_dir = "/home/xy/caffe-master/data/mnist/";
string img_path = base_dir + "train-images-idx3-ubyte";
string label_path = base_dir + "train-labels-idx1-ubyte";

read_mnist_cv(img_path.c_str(), label_path.c_str());

Output as below:

张云铭 · Answer 5 · 2019-03-04T07:36:54.463

By using in() , you can read any size data you want.

const int MAXN = 6e4 + 7;
unsigned int image[MAXN][30][30];
unsigned int num, magic, rows, cols;
unsigned int label[MAXN];
unsigned int in(ifstream& icin, unsigned int size) {
    unsigned int ans = 0;
    for (int i = 0; i < size; i++) {
        unsigned char x;
        icin.read((char*)&x, 1);
        unsigned int temp = x;
        ans <<= 8;
        ans += temp;
    }
    return ans;
}
void input() {
    ifstream icin;
    icin.open("train-images.idx3-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4), rows = in(icin, 4), cols = in(icin, 4);
    for (int i = 0; i < num; i++) {
        for (int x = 0; x < rows; x++) {
            for (int y = 0; y < cols; y++) {
                image[i][x][y] = in(icin, 1);
            }
        }
    }
    icin.close();
    icin.open("train-labels.idx1-ubyte", ios::binary);
    magic = in(icin, 4), num = in(icin, 4);
    for (int i = 0; i < num; i++) {
        label[i] = in(icin, 1);
    }
}

How to read MNIST data in C++?

5 Answers5

For reading images dataset:

For reading labels dataset:

Linked