How to get length of a list of lists in python

Question

So, if I have a list called myList I use len(myList) to find the number of elements in that list. Fine. But how do I find the number of lists in a list?

text = open("filetest.txt", "r")
myLines = text.readlines()
numLines=len(myLines)
print numLines

The above text file used has 3 lines of 4 elements separated by commas. The variable numLines prints out as '4' not '3'. So, len(myLines) is returning the number of elements in each list not the length of the list of lists.

When I print myLines[0] I get the first list, myLines[1] the second list, etc. But len(myLines) does not show me the number of lists, which should be the same as 'number of lines'.

I need to determine how many lines are being read from the file.

Romaromagna

In your code, you have a list of lines (strings). not a list of lists. And python will happily count empty lines... Did you check to make sure you don't have any empty lines? – Fusee

len(myLines) will give you the number of objects inside that list no matter if they are lists, dicts or strings. – Delrosario

Your code works absolutely fine. Give us the file as is. – Vikkivikky

Just print myLines and the reason will be clear. – Tapia

Python will not automatically split text read from a file on commas. Use the csv module if you want that to happen. – Teador

If the text file used has 3 lines, len(myLines) will be 3 regardless of the content of each. Perhaps you have a blank line at the end of the file. Another thing to try is using open("filetest.txt", "rt") because some platforms have multiple characters at the end of each line of a text file which could confuse readlines(). – Trafficator

Answer 1

This saves the data in a list of lists.

text = open("filetest.txt", "r")
data = [ ]
for line in text:
    data.append( line.strip().split() )

print "number of lines ", len(data)
print "number of columns ", len(data[0])

print "element in first row column two ", data[0][1]

Kasper

Answer 2

You can do it with functools.reduce:

from functools import reduce 

a = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [], [1, 2]]
print(reduce(lambda count, l: count + len(l), a, 0))
# result is 11

Encyclopedist

Answer 3

"The above text file used has 3 lines of 4 elements separated by commas. The variable numLines prints out as '4' not '3'. So, len(myLines) is returning the number of elements in each list not the length of the list of lists."

It sounds like you're reading in a .csv with 3 rows and 4 columns. If this is the case, you can find the number of rows and lines by using the .split() method:

text = open("filetest.txt", "r").read()
myRows = text.split("\n")      #this method tells Python to split your filetest object each time it encounters a line break 
print len(myRows)              #will tell you how many rows you have
for row in myRows:
  myColumns = row.split(",")   #this method will consider each of your rows one at a time. For each of those rows, it will split that row each time it encounters a comma.  
  print len(myColumns)         #will tell you, for each of your rows, how many columns that row contains

Mcclary

Answer 4

if the name of your list is listlen then just type len(listlen). This will return the size of your list in the python.

Crispin

Answer 5

The method len() returns the number of elements in the list.

 list1, list2 = [123, 'xyz', 'zara'], [456, 'abc']
    print "First list length : ", len(list1)
    print "Second list length : ", len(list2)

When we run above program, it produces the following result −

First list length : 3 Second list length : 2

Crispin