如何在一定范围内找到最大值的x值？

Question

如何确定特定范围内数据集最大值的x值？

对于上下文，这些记录数据集如下所示：

value['Wavelength']:

PixelNo
0       1167.467506
1       1167.219570
2       1166.971628
3       1166.723680
4       1166.475725
           ...     
3635     233.904486
3636     233.641110
3637     233.377731
3638     233.114350
3639     232.850967
Name: Wavelength, Length: 3640, dtype: float64

value['Calib_1']:

PixelNo
0       1.000000e+00
1       5.500000e+01
2       3.500000e+01
3       1.000000e-07
4       4.600000e+01
            ...     
3635    4.000000e+01
3636    7.400000e+01
3637    1.000000e-07
3638    3.000000e+00
3639    1.600000e+01
Name: Calib_1, Length: 3640, dtype: float64

这是所有数据集对应图的示例。

Answer 1

您可以使用argmax方法，其中returns最大值的索引。
假设您有这样的曲线：

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt


value = pd.DataFrame({'Wavelength': np.linspace(250, 600, 10000)})
value['Calib_1'] = (np.sin(2*np.pi/20*value['Wavelength'])*np.exp(-(value['Wavelength']-455)**2/20/(600-250)))**2

fig, ax = plt.subplots()

ax.plot(value['Wavelength'], value['Calib_1'], color = 'blue')

ax.set_xlabel('Wavelength')
ax.set_ylabel('Calib_1')

plt.show()

上面的曲线被设计成在455.
有最大值您可以找到最大值的索引：

value['Calib_1'].argmax()
# 5857

并用它通过loc方法得到对应的x轴值：

value.loc[value['Calib_1'].argmax(), 'Wavelength']
# 455.015501550155

如您所见，它 returns 的值大约接近 455 的精确值。错误将通过 numpy.linspace.
解决到 x 轴离散化您可以在上面的图上绘制最大值：

id_max = value['Calib_1'].argmax()
x_max = value.loc[id_max, 'Wavelength']
y_max = value.loc[id_max, 'Calib_1']


fig, ax = plt.subplots()

ax.plot(value['Wavelength'], value['Calib_1'], color = 'blue')
ax.plot(x_max, y_max, marker = 'o', color = 'red')

ax.set_xlabel('Wavelength')
ax.set_ylabel('Calib_1')

plt.show()

如果你只需要一个特定范围的最大值，比如说[500, 550]，那么首先你必须过滤数据帧，然后计算最大值的索引：

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt


value = pd.DataFrame({'Wavelength': np.linspace(250, 600, 10000)})
value['Calib_1'] = (np.sin(2*np.pi/20*value['Wavelength'])*np.exp(-(value['Wavelength']-455)**2/20/(600-250)))**2

value_filt = value[(value['Wavelength'] >= 500) & (value['Wavelength'] <= 550)].reset_index()
id_max = value_filt['Calib_1'].argmax()
x_max = value_filt.loc[id_max, 'Wavelength']
y_max = value_filt.loc[id_max, 'Calib_1']


fig, ax = plt.subplots()

ax.plot(value['Wavelength'], value['Calib_1'], color = 'blue')
ax.plot(x_max, y_max, marker = 'o', color = 'red')

ax.axvline(x = 500, color = 'black', linestyle = '--')
ax.axvline(x = 550, color = 'black', linestyle = '--')

ax.set_xlabel('Wavelength')
ax.set_ylabel('Calib_1')

plt.show()

如何在一定范围内找到最大值的x值？

How to find x-value of maximum in a certain range?

python

data-visualization

matplotlib

dataframe

pandas