Python: numpy and minimize: set the minimum and/or maximum array

Question

if I have a picture like Result of the autocorrelation function: and I need to find my first maximum as well as my first minimum

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 6, 1000)

sig = np.sin(2 * np.pi * x)

ACF = np.correlate(sig, sig, mode='full')
res_ACF = ACF[len(sig)-1:]
res_ACF /= np.max(res_ACF)


plt.subplot(211)
plt.plot(x, sig, lw=0.5, label='signal originel')
plt.legend()
plt.subplot(212)
plt.plot(x, res_ACF, lw=3, label='ACF_with_np_correlate_in_full_mode')
plt.legend()
plt.show()

ndarray.argmax and ndarray.argmin

Mechanic Pig
– Mechanic Pig

2022-06-03 10:16:50 +00:00
Commented Jun 3, 2022 at 10:16 — Mechanic Pig
– Mechanic Pig, Commented Jun 3, 2022 at 10:16

Nawal · Accepted Answer · 2022-06-03 10:14:54Z

1

2 simple for loops can do that

for i in range(1,len(res_ACF)-1):
    if res_ACF[i-1] > res_ACF[i] < res_ACF[i+1]:
        print('first minima: ', i, res_ACF[i])
        break

for i in range(1,len(res_ACF)-1):
    if res_ACF[i-1] < res_ACF[i] > res_ACF[i+1]:
        print('first maxima: ', i, res_ACF[i])
        break

yes, you can optimize it to use single for-loop.

answered Jun 3, 2022 at 10:14

Nawal

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8 Comments

Mechanic Pig Over a year ago

In numpy, vectorization is much faster than a for loop, so you should try to avoid using for loop.

yassine dhifaoui Over a year ago

thank you for your answer, but you can see with this picture that if I use the loop for it gives me false results.

Nawal Over a year ago

@yassinedhifaoui can't see the picture :|

yassine dhifaoui Over a year ago

Sorry, but I don’t know how to put the picture here please you can launch this program and see the result to chat. plt.axvline(res_max,color='red') plt.axvline(res_min,color='red')

yassine dhifaoui Over a year ago

i.sstatic.net/EZsWj.png You can see here this picture

|

Mechanic Pig · Accepted Answer · 2022-06-03 12:55:01Z

0

argmin/argmax return the index of the min/max value:

plt.axvline(x[res_ACF.argmax()], c='r')
plt.axvline(x[res_ACF.argmin()], c='r')

answered Jun 3, 2022 at 12:55

Mechanic Pig

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1 Comment

yassine dhifaoui Over a year ago

I have the data,I work for this program to watch this first minimal and the first maximum.But with double dip autocorrelation it does not work.But in simple dip he give me this result.Can you do something for me? Can't see the picture: i.stack.imgur.com/FVC3J.png AND THIS FOR SIMPLE DIP : i.stack.imgur.com/NrCWA.png

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