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how to load python numpy table with C++

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Guide

python

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import numpy as np
import cv2
from array import array

table = np.float32(np.zeros((4, 5, 2)))

array_0 = np.array([-1.5,-1.5,-1.5,-1.5, -1.5])
table[:,:,0] = np.array([array_0,array_0,array_0,array_0])

array_1 = np.array([1.1,1.1,1.1,1.1,1.1])
table[:,:,1] = np.array([array_1,array_1,array_1,array_1])

table
array([[[-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002]],

       [[-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002]],

       [[-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002]],

       [[-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002]]], dtype=float32)

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# imwrite/imread can only handle 8/16/24/32bit integral data, not floats
# opencv image: hwc, bgr, value range =  x: [-5,5], y= [10,45], z = 0; 255  float

def save_table(table, row, col, filename):

    table_list = []
    for i in range(row):
            for j in range(col):
                x = table[i][j][0] 
                y = table[i][j][1] 
                table_list.append(x)
                table_list.append(y)
    #print("table_list = ",table_list)

    output_file = open(filename, 'wb')
    float_array = array('f', table_list) # f,d
    float_array.tofile(output_file)
    output_file.close()

def load_table(filename, row, col):

    input_file = open(filename, 'rb')
    float_array = array('f')  # f,d
    float_array.fromstring(input_file.read())

    channel = 2 # default 2
    table = np.float32(np.zeros((row, col, 2)))
    for i in range(row):
            for j in range(col):
                table[i][j][0] = float_array[2*(i*col+j) +0]
                table[i][j][1] = float_array[2*(i*col+j) +1]

    return table
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row = 4  # height
col = 5  # width
# table size
# double: （4*5*2）×8 = 320 bytes
# float:  （4*5*2）×4 = 160 bytes
save_table(table, row, col, "table_f.bin")
table2 = load_table("table_f.bin", row, col)
table2 
array([[[-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002]],

       [[-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002]],

       [[-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002]],

       [[-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002],
        [-1.5       ,  1.10000002]]], dtype=float32)

Tips, use d or f to store number as double or float

table size

  • double: （452）×8 = 320 bytes
  • float: （452）×4 = 160 bytes

c++

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std::string table_filepath = "./table_f.bin";
std::ifstream ifs(table_filepath, std::ifstream::binary);
int rows = 4;
int cols = 5; // xy

for(int y=0; y<rows; ++y)
{
	for(int x=0;x<cols;++x)
	{
		float distance_x;
		ifs.read(reinterpret_cast<char*>(&distance_x), sizeof(float)); 
		std::cout<< distance_x << std::endl;

		float distance_y;
		ifs.read(reinterpret_cast<char*>(&distance_y), sizeof(float)); 
		std::cout<< distance_y << std::endl;
	}
}
ifs.close();

Tips: read double

Reference

History

  • 20190228: created.