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如何計算數據集均值和方差

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import os
from PIL import Image  
import matplotlib.pyplot as plt
import numpy as np
from scipy.misc import imread 
 
filepath = /home/JPEGImages # 數據集目錄
pathDir = os.listdir(filepath)
 
R_channel = 0
G_channel = 0
B_channel = 0
for idx in xrange(len(pathDir)):
    filename = pathDir[idx]
    img = imread(os.path.join(filepath, filename))
    R_channel 
= R_channel + np.sum(img[:,:,0]) G_channel = G_channel + np.sum(img[:,:,1]) B_channel = B_channel + np.sum(img[:,:,2]) num = len(pathDir) * 384 * 512 # 這裏(384,512)是每幅圖片的大小,所有圖片尺寸都一樣 R_mean = R_channel / num G_mean = G_channel / num B_mean = B_channel / num
R_channel = 0
G_channel = 0
B_channel = 0
for idx in xrange(len(pathDir)):
    filename = pathDir[idx]
    img = imread(os.path.join(filepath, filename))
    R_channel = R_channel + np.sum((img[:,:,0] - R_mean)**2)
    G_channel = G_channel + np.sum((img[:,:,1] - G_mean)**2)
    B_channel = B_channel + np.sum((img[:,:,2] - B_mean)**2)
    
R_var = R_channel / num
G_var 
= G_channel / num B_var = B_channel / num print("R_mean is %f, G_mean is %f, B_mean is %f" % (R_mean, G_mean, B_mean)) print("R_var is %f, G_var is %f, B_var is %f" % (R_var, G_var, B_var))

如何計算數據集均值和方差