Python use for loop to read specific multiply lines from txt files

Question

I want use python to read specific multiply lines from txt files. For example ,read line 7 to 10, 17 to 20, 27 to 30 etc.

Here is the code I write, but it will only print out the first 3 lines numbers. Why? I am very new to use Python.

with open('OpenDR Data.txt', 'r') as f:  
    for poseNum in range(0, 4):               
        Data = f.readlines()[7+10*poseNum:10+10*poseNum] 
        for line in Data: 
            matAll = line.split()    
            MatList = map(float, matAll) 
            MatArray1D = np.array(MatList)
            print MatArray1D

Possible duplicate of [Reading specific lines only (Python)](http://stackoverflow.com/questions/2081836/reading-specific-lines-only-python) — Two-Bit Alchemist, Nov 12 '15 at 21:40
You're trying to read the file over and over again. Besides being inefficient, it won't work unless you rewind the file each time. Read the file before the loop. — saulspatz, Nov 12 '15 at 21:44
@saulspatz Hi, thanks. I use 7+10*poseNum:10+10*poseNum to control reading different rows in every loop, why this is not working? — SimaGuanxing, Nov 12 '15 at 21:46
`readlines()` reads the whole file at once. Each time you hit the line `Data = f.readlines()[7+10*poseNum:10+10*poseNum] ` you try to read the file again. Reading the file and retrieving and retrieving specific rows from the list are two distinct operations. — saulspatz, Nov 12 '15 at 21:50

score 2 · Answer 1 · answered Nov 12 '15 at 21:48

2

with open('OpenDR Data.txt') as f:
    lines = f.readlines()

for poseNum in range(0, 4):               
    Data = lines[7+10*poseNum:10+10*poseNum]

answered Nov 12 '15 at 21:48

Hasan Ramezani

5,004
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vk1011 · Accepted Answer · 2015-11-12T22:03:39.467

2

This simplifies the math a little to choose the relevant lines. You don't need to use readlines().

with open('OpenDR Data.txt', 'r') as fp:
    for idx, line in enumerate(fp, 1):
        if idx % 10 in [7,8,9,0]:
            matAll = line.split()    
            MatList = map(float, matAll) 
            MatArray1D = np.array(MatList)
            print MatArray1D

edited Nov 12 '15 at 22:03

answered Nov 12 '15 at 21:50

vk1011

7,011
6
26
42

Thank you! Beautiful solution! – SimaGuanxing Nov 12 '15 at 22:01

wvdz · Answer 3 · 2015-11-12T21:59:26.573

1

You should only call readlines() once, so you should do it outside the loop:

with open('OpenDR Data.txt', 'r') as f:  
    lines = f.readlines()
    for poseNum in range(0, 4):               
        Data = lines[7+10*poseNum:10+10*poseNum] 
        for line in Data: 
            matAll = line.split()    
            MatList = map(float, matAll) 
            MatArray1D = np.array(MatList)
            print MatArray1D

edited Nov 12 '15 at 21:59

answered Nov 12 '15 at 21:45

wvdz

16,251
4
53
90

`poseNum` needs to be defined prior to being used on line 2. – vk1011 Nov 12 '15 at 21:51

score 0 · Answer 4 · answered Nov 12 '15 at 21:50

You can use a combination list slicing and comprehension.

start = 7
end = 10
interval = 10
groups = 3

with open('data.txt') as f:
    lines = f.readlines()
    mult_lines = [lines[start-1 + interval*i:end + interval*i] for i in range(groups)]

This will return a list of lists containing each group of lines (i.e. 7 thru 10, 17 thru 20).

Python use for loop to read specific multiply lines from txt files

4 Answers4