Python Plot- Multiple the data in plot figure

Question

I am reading data from a text file but when I do so, I need to multiple this values like 3*sqrt(col1)= x1.append(3*math.sqrt(float(p[1]))) in plot function. How can I multiple column number data before plotting? For example, I will multiple col3 data by 3*sqrt(col3) and after plot that data.

#-------input.dat---------
#   x        y     z
# col 1    col 2  col 3
# 3          5      5
# 5          6      4
# 7          7      3
import matplotlib.pyplot as plt
import numpy as np
import pylab as pl
import math

data = open('input.dat')
lines = data.readlines()
data.close()
x1=[]
y1=[]
z1=[]
plt.plot(1)
for line in lines[2:]:
p= line.split()
x1.append(3*math.sqrt(float(p[1])))
y1.append(3*math.sqrt(float(p[2])))
z1.append(3*math.sqrt(float(p[3])))
x=np.array(x1)
y=np.array(y1)
z=np.array(z1)
plt.subplot(311)
plt.plot(x,'b',label=" X figure ")
plt.subplot(312)
plt.plot(y,'r',label=" Y figure ")
plt.subplot(313)
plt.plot(x,z,'g',label=" X,Z figure ")
plt.show()

This probably means you have negative numbers in your table. When you try to do sqrt on a negative number you get this error. — joaquin, Jun 15 '18 at 10:47

tmdavison · Accepted Answer · 2018-06-15T10:16:43.047

Again, this is easier if you just use numpy arrays from the start.

By reading the data in as I showed you in your last question, your data will already be in numpy arrays. Then you can use the numpy.sqrt function to perform the square-root operation element-wise on the array.

#-------input.dat---------
#   x        y     z
# col 1    col 2  col 3
# 3          5      5
# 5          6      4
# 7          7      3
import matplotlib.pyplot as plt
import numpy as np

data = np.genfromtxt('input.dat', skip_header=2)

x = 3. * np.sqrt(data[:, 0])
y = 3. * np.sqrt(data[:, 1])
z = 3. * np.sqrt(data[:, 2])

plt.subplot(311)
plt.plot(x, 'b', label=" X figure ")
plt.subplot(312)
plt.plot(y, 'r', label=" Y figure ")
plt.subplot(313)
plt.plot(x, z, 'g', label=" X,Z figure ")
plt.show()

However, if you really want to stick with your old code, it can be fixed by

fixing the indentation,
changing the indexing to p[0], p[1] and p[2] (instead of p[1], p[2] and p[3])

This code produces the same plot as above:

import matplotlib.pyplot as plt
import numpy as np
import pylab as pl
import math

data = open('input.dat')
lines = data.readlines()
data.close()
x1=[]
y1=[]
z1=[]
plt.plot(1)
for line in lines[2:]:
    p= line.split()
    x1.append(3*math.sqrt(float(p[0])))
    y1.append(3*math.sqrt(float(p[1])))
    z1.append(3*math.sqrt(float(p[2])))
x=np.array(x1)
y=np.array(y1)
z=np.array(z1)
plt.subplot(311)
plt.plot(x,'b',label=" X figure ")
plt.subplot(312)
plt.plot(y,'r',label=" Y figure ")
plt.subplot(313)
plt.plot(x,z,'g',label=" X,Z figure ")
plt.show()

thanks Tom i will change the code regarding your solution. Also I have one question, For example, I want to define xlabel with column 1. plt.xlim(0,col1). Because I wan to see in the x range, col1 number — Nobody, Jun 15 '18 at 10:36

Python Plot- Multiple the data in plot figure

1 Answers1