从.txt文件中读取第N行（Python）

Question

I am making a program that reads time values and readings from a .txt file and plots them on a web-page with matplotlib. In my case there are lots of values, and the plot can look very confusing, and with thousands of data points it can also be sluggish.

I am currently reading the .txt lines with this code to put the values into plottable lists:

with open('C:/Email/file.txt') as f:   


    lines = f.readlines()



    dates = [str(line.split(';')[0]) for line in lines]
    y = [float(line.split(';')[1]) for line in lines]
    z = [float(line.split()[2]) for line in lines]

The .txt file content looks like this:

30.05.18_12:27:17;  13.0;  -0.0
30.05.18_12:27:18;  14.0;  -0.0
30.05.18_12:27:19;  15.0;  -0.0
30.05.18_12:27:20;  16.0;  -0.0

So, is there any way to read and plot every Nth (for example every 30. would be ideal) timestamp and value?

I've been researching different ways but they seem confusing.

One solution that I was thinking about is to make a counter that counts the amount of read lines, then when the counter is 30, skip 30 the next lines and read and plot the data in line 31. I have no idea how to do this, since I am relatively new to Python.

Help would be appreciated.

Just in case, the whole code here:

from flask import Flask
import numpy as np
import matplotlib.pyplot as plt, mpld3
from datetime import datetime

app = Flask(__name__)



@app.route("/")
def hello():

    with open('C:/Email/file.txt') as f:   


        lines = f.readlines()



        dates = [str(line.split(';')[0]) for line in lines]
        y = [float(line.split(';')[1]) for line in lines]
        z = [float(line.split()[2]) for line in lines]


        date = [datetime.strptime(x,'%d.%m.%y_%H:%M:%S') for x in dates]
        plt.figure(figsize=(10,5))
        plt.title('Temperature', fontsize=15)
        plt.ylabel('Temperature' + u'\u2103', fontsize=15)

        plt.plot_date(date, y, 'r-', label='quadratic')
        plt.ylim([10,35])

        # Print as HTML
        return mpld3.show()

if __name__ == "__main__":
    app.run()

EDIT: HUGE THANKS TO MR. F-ROCHE

Got it working with the following code:

@app.route("/")
def hello():

    with open('C:/Email/file.txt') as f:   

        # Counts lines in text (Use later to delete lines every X readings)
        # lines = f.readlines()
        cpt = 0
        all_lines = []
        for line in f:
            cpt += 1
            if cpt == 30:
                all_lines.append(line)
                cpt = 0
        dates = [str(line.split(';')[0]) for line in all_lines]
        date = [datetime.strptime(x,'%d.%m.%y_%H:%M:%S') for x in dates]
        y = [float(line.split(';')[1]) for line in all_lines]
        z = [float(line.split()[2]) for line in all_lines]

        plt.figure(figsize=(10,5))
        plt.title('Temperature', fontsize=15)
        plt.ylabel('Temperature' + u'\u2103', fontsize=15)


        plt.plot_date(date, y, 'r-', label='quadratic')
        plt.ylim([10,35])

        # Print as HTML
        return mpld3.show()

Answer 1

Something like that will only display every 30 lines:

with open('C:/Email/file.txt') as f:
    cpt = 0
    for line in f:
        cpt += 1
        if cpt == 30:
            print(line)
            cpt = 0

you can put line variable in a list like this:

all_lines = []
...
all_lines.append(line)

EDIT: or even better with one line of code:

with open('C:/Email/file.txt') as f:
    all_lines = [v for i, v in enumerate(f, start=1) if i % 30 == 0]

Then you can apply that to dates , y and z instead.