plot from csv data in python

Question

I'm new to python and I'm practicing csv data operation. Now I have a situation where I have 100 * 4 datas where the i-th row corresponds to i-th example (x_i, y_i) Columns 1,2, and 3 are for each input variable (x_1, x_2, 1) abd 4-th column is output y. Then, determine appropriate constants (a,b) (Let's say (6, 5) here).

Now I want to draw graphs, the x-axis being ax_1+bx_2 and y-axis being y.

here are the parts of the datas.

"x1","x2","1","y"
-0.626453810742332,-0.620366677224124,1,0.28239638205273
0.183643324222082,0.0421158731442352,1,1.73290072129656
-0.835628612410047,-0.910921648552446,1,-0.293950695808836

and here is the code im trying to complete

import numpy as np
import matplotlib.pyplot as plt

if __name__ == '__main__':

    x1 = []
    x2 = []
    y = []
    a = 6
    b = 5

    with open('data.csv', 'r') as f:
        lines = f.readlines()
        tmp_x1 = np.array(lines)[:,0]
        tmp_x2 = np.array(lines)[:,1]
        tmp_y = np.array(lines)[:,3]
        x1.append(tmp_x1)
        x2.append(tmp_x2)
        y.append(tmp_y)

    ax1 = list()
    bx2 = list()

    for i in range(100):
        ax1.append(a * x1)
        bx2.append(b * x2)

    plt.figure()
    plt.xlabel('a*x1 + b*x2')
    plt.ylabel('y')
    plt.plot(ax1 + bx2, y)

and this code gives me error

IndexError: too many indices for array

Can someone help me to solve this?

[edit][graph]1

code:

import pandas as pd
import matplotlib.pyplot as plt

df = pd.read_csv('data.csv')

a,b = 6,5

df['x'] = df['x1']*a + df['x2']*b

plt.figure()
plt.plot(df['x'], df['y'])
plt.show()

Yi Hang Tay · Accepted Answer · 2020-05-19 03:36:26Z

1

f.readlines() will read everything in the file and put it into the list. The list is only a one dimensional array with length number of numbers in the file. You can use the numpy function genfromtxt to load the data directly into a numpy array

lines = np.genfromtxt('data.csv', delimiter=',' ,skip_header=1)

answered May 19, 2020 at 3:36

Yi Hang Tay

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Comments

Quang Hoang · Accepted Answer · 2020-05-19 03:46:02Z

0

I recommend using pandas to handle tabular data:

import pandas as pd

df = pd.read_csv('data.csv')

a,b = 6,5

df['x'] = df['x1']*a + df['x2']*b

df.plot(x='x',y='y')

Output:

Or in the case you want to plot one line for each input identified by field 1:

# continue the code above
# replace the last line with
fig, ax = plt.subplots()
for k,d in df.groupby('1'):
    d.plot(x='x',y='y', ax=ax, label=k)

Output:

answered May 19, 2020 at 3:46

Quang Hoang

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1 Comment

J.Maisel Over a year ago

thank you for the answer. I ran this code but come up with no graphs, so i rewrite the code using matplotlib, then the plot was like this (added to the post). what am i missing?

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