python如何读取二进制mnist实例详解

python读取二进制mnist实例详解

training data 数据结构：

<br>[offset] [type] [value] [description]
0000 32 bit integer 0x00000803(2051) magic number
0004 32 bit integer 60000 number of images
0008 32 bit integer 28 number of rows
0012 32 bit integer 28 number of columns
0016 unsigned byte ?? pixel
0017 unsigned byte ?? pixel
........
xxxx unsigned byte ?? pixel

将整个文件读入：

filename = 'train-images.idx3-ubyte'
binfile = open(filename , 'rb')
buf = binfile.read()

读取头四个32bit的interger:

index = 0
magic, numImages , numRows , numColumns = struct.unpack_from('>IIII' , buf , index)
index += struct.calcsize('>IIII')

读取一个图片，784=28*28 ：

im = struct.unpack_from('>784B' ,buf, index)
index += struct.calcsize('>784B')
 
im = np.array(im)
im = im.reshape(28,28)
 
fig = plt.figure()
plotwindow = fig.add_subplot(111)
plt.imshow(im , cmap='gray')
plt.show()