Yongtao Hu's blog

fwrite/fread for different data types

Posted on Edited on Disqus:

We use fwrite to write formated data to disk and use fread to later read them back from disk. Different data types are very different in using fwrite/fread. The following shows how to do for vector (simple), Mat, vector<Mat>, struct (simple), struct (complex).

for vector (simple):

  1. fwrite:

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    void write_vector(string file, vector<int> a)
    {
        // write data to file
        FILE *fo;
        fo = fopen(file.c_str(), "wb");
        //size_element = result[0].elemSize()*result[0].total();
        fwrite(&a[0], a.size(), sizeof(int), fo);
        fclose(fo);
    }

  2. fread:

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    vector<int> load_vector(string file, int size)
    {
        vector<int> result;
        result.resize(size);

        // read data from file
        FILE *fi;
        fi = fopen(file.c_str(), "rb");

        fread( &result[0], sizeof(int), result.size(), fi );
        fclose(fi);

        return result;
    }

  3. Usage:

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    vector<int> a;
    a.push_back(0);
    a.push_back(1);
    a.push_back(2);
    write_vector("vector.dat", a);
    vector<int> b = load_vector("vector.dat", 3);

for Mat:

  1. fwrite:

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    void write_mat(string file, Mat a)
    {
        // write data to file
        FILE *fo;
        fo = fopen(file.c_str(), "wb");
        //size_element = result[0].elemSize()*result[0].total();
        fwrite(a.ptr(), 1, (size_t)(a.elemSize()*a.total()), fo);
        fclose(fo);
    }

  2. fread:

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    Mat load_mat(string file, int rows, int cols, int type)
    {
        Mat result(rows, cols, type);

        // read data from file
        FILE *fi;
        fi = fopen(file.c_str(), "rb");
        fread(result.ptr(), (size_t)(result.elemSize()*result.total()), 1, fi);
        fclose(fi);

        return result;
    }

  3. Usage:

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    Mat a = Mat::eye(3, 3, CV_32FC1);
    write_mat("mat.dat", a);
    Mat b = load_mat("mat.dat", 3, 3, CV_32FC1);

for vector<Mat> (every Mat have similar data):

  1. fwrite:

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    void write_all_images_content_to_disk(vector<Mat> result, string file, size_t & size_mat)
    {
        // write data to file
        FILE *fo;
        fo = fopen(file.c_str(), "wb");
        for (int i=0; i<result.size(); i++)
        {
            Mat stub = result[i];

            size_mat = (size_t)(stub.elemSize()*stub.total());
            fwrite(stub.ptr(), size_mat, 1, fo);
        }
        fclose(fo);
    }

  2. fread:

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    vector<Mat> load_images_content_from_disk(string file, size_t size_mat, int size)
    {
        vector<Mat> result;
        result.resize(size);

        // read data from file
        FILE *fi;
        fi = fopen(file.c_str(), "rb");
        for (int i=0; i<size; i++)
        {
            Mat stub(WIDTH_IMAGE, HEIGHT_IMAGE, CV_8UC3);

            fread(stub.ptr(), size_mat, 1, fi);

            result.push_back(stub);
        }
        fclose(fi);

        return result;
    }

  3. Usage:

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    // for vector<Mat> IMAGES_CONTENT;
    size_t size_mat;
    write_all_images_content_to_disk(IMAGES_CONTENT, "images.dat", size_mat);
    vector<Mat> stub = load_images_content_from_disk("images.dat", size_mat, IMAGES_CONTENT.size());

for struct (simple):

Take the following struct as an example:

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struct PERSON
{
    char name[30];
    int age;
};

  1. fwrite:

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    void write_struct(string file, PERSON a)
    {
        // write data to file
        FILE *fo;
        fo = fopen(file.c_str(), "wb");
        fwrite(&a, sizeof(struct PERSON), 1, fo);
        fclose(fo);
    }

  2. fread:

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    void load_struct(string file, PERSON & result)
    {
        // read data from file
        FILE *fi;
        fi = fopen(file.c_str(), "rb");
        fread(&result, sizeof(struct PERSON), 1, fi);
        fclose(fi);
    }

  3. Usage:

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    PERSON a;
    strcpy(a.name, "Tommy");
    a.age = 25;

    write_struct("person.dat", a);
    PERSON b;
    load_struct("person.dat", b);

for struct (contains vector and Mat):

Take the following struct as an example:

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struct FEATURE_RECT
{
    Rect rect;

    vector<float> color_hist;
    vector<float> hog;
    Mat region_mat;
};

  1. fwrite:

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    void write_struct(string file, FEATURE_RECT a)
    {
        // write data to file
        FILE *fo;
        fo = fopen(file.c_str(), "wb");
        fwrite(&a, sizeof(struct FEATURE_RECT), 1, fo);
        fwrite(&a.color_hist[0], sizeof(float), a.color_hist.size(), fo);
        fwrite(&a.hog[0], sizeof(float), a.hog.size(), fo);
        fclose(fo);
    }

  2. fread:

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    void load_struct(string file, FEATURE_RECT & result)
    {

        // read data from file
        FILE *fi;
        fi = fopen(file.c_str(), "rb");
        fread(&result, sizeof(struct FEATURE_RECT ), 1, fi);
        fread(&result.color_hist[0], sizeof(float), result.color_hist.size(), fi);
        fread(&result.hog[0], sizeof(float), result.hog.size(), fi);
        fclose(fi);
    }

  3. Usage:

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    FEATURE_RECT test;
    test.rect = Rect(1,2,100,200);
    test.color_hist.push_back(0);
    test.hog.push_back(1);
    test.region_mat = Mat::eye(3,3,CV_32FC1);

    write_struct("struct.dat", test);
    FEATURE_RECT test2;
    load_struct("struct.dat", test2);