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How to create a 2d nested list from a text file using python?

I'm a beginner programmer, and I'm trying to figure out how to create a 2d nested list (grid) from a particular text file. For example, the text file would look like this:
3
3
150
109
80
892
123
982
0
98
23
The first two lines in the text file would be used to create the grid, meaning that it is 3x3. The next 9 lines would be used to populate the grid, with the first 3 making up the first row, the next 3 making up the middle row, and the final 3 making up the last row. So the nested list would look like this:
[[150, 109, 80] [892, 123, 982] [0, 98, 23]]
How do I go about doing this? I was able to make a list of all of the contents, but I can't figure out how to use the first 2 lines to define the size of the inner lists within the outer list:
lineContent = []
innerList = ?
for lines in open('document.txt','r'):
value = int(lines)
lineContent.append(value)
From here, where do I go to turn it into a nested list using the given values on the first 2 lines?
Thanks in advance.

You can make this quite neat using list comprehension.
def txt_grid(your_txt):
with open(your_txt, 'r') as f:
# Find columns and rows
columns = int(f.readline())
rows = int(f.readline())
your_list = [[f.readline().strip() for i in range(rows)] for j in range(columns)]
return your_list
print(txt_grid('document.txt'))
strip() just clears the newline characters (\n) from each line before storing them in the list.
Edit: A modified version with logic for if your txt file didn't have enough rows for the defined dimensions.
def txt_grid(your_txt):
with open(your_txt, 'r') as f:
# Find columns and rows
columns = int(f.readline())
rows = int(f.readline())
dimensions = columns * rows
# Test to see if there are enough rows, creating grid if there are
nonempty_lines = len([line.strip("\n") for line in f]) # This ignores the first two lines as they have already been written
if nonempty_lines < dimensions:
# Either raise an error
# raise ValueError("Insufficient non-empty rows in text file for given dimensions")
# Or return something that's not a list
your_list = None
else:
# Creating grid
your_list = [[f.readline().strip() for i in range(rows)] for j in range(columns)]
return your_list
print(txt_grid('document.txt'))

def parse_txt(filepath):
lineContent = []
with open(filepath, 'r') as txt: # The with statement closes the txt file after its been used
nrows = int(txt.readline())
ncols = int(txt.readline())
for i in range(nrows): # For each row
row = []
for j in range(ncols): # Grab each value in the row
row.append(int(txt.readline()))
lineContent.append(row)
return lineContent
grid_2d = parse_txt('document.txt')

lineContent = []
innerList = []
for lines in open('testQuestion.txt', 'r'):
value = int(lines)
lineContent.append(value)
rowSz = lineContent[0] # row size
colSz = lineContent[1] # column size
del lineContent[0], lineContent[0] # makes line contents just the values in the matrix, could also just start currentLine at 2, notice 0 index is repeated because 1st element was deleted
assert rowSz * colSz == len(lineContent), 'not enough values for array' # to ensure there are enough entries to complete array of rowSz * colSz elements
arr = []
currentLine = 0
for x in range(rowSz):
arr.append([])
for y in range(colSz):
arr[x].append(lineContent[currentLine])
currentLine += 1
print(arr)

How to separate different input formats from the same text file with Python

I'm new to programming and python and I'm looking for a way to distinguish between two input formats in the same input file text file. For example, let's say I have an input file like so where values are comma-separated:
5
Washington,A,10
New York,B,20
Seattle,C,30
Boston,B,20
Atlanta,D,50
2
New York,5
Boston,10
Where the format is N followed by N lines of Data1, and M followed by M lines of Data2. I tried opening the file, reading it line by line and storing it into one single list, but I'm not sure how to go about to produce 2 lists for Data1 and Data2, such that I would get:
Data1 = ["Washington,A,10", "New York,B,20", "Seattle,C,30", "Boston,B,20", "Atlanta,D,50"]
Data2 = ["New York,5", "Boston,10"]
My initial idea was to iterate through the list until I found an integer i, remove the integer from the list and continue for the next i iterations all while storing the subsequent values in a separate list, until I found the next integer and then repeat. However, this would destroy my initial list. Is there a better way to separate the two data formats in different lists?

You could use itertools.islice and a list comprehension:
from itertools import islice
string = """
5
Washington,A,10
New York,B,20
Seattle,C,30
Boston,B,20
Atlanta,D,50
2
New York,5
Boston,10
"""
result = [[x for x in islice(parts, idx + 1, idx + 1 + int(line))]
for parts in [string.split("\n")]
for idx, line in enumerate(parts)
if line.isdigit()]
print(result)
This yields
[['Washington,A,10', 'New York,B,20', 'Seattle,C,30', 'Boston,B,20', 'Atlanta,D,50'], ['New York,5', 'Boston,10']]
For a file, you need to change it to:
with open("testfile.txt", "r") as f:
result = [[x for x in islice(parts, idx + 1, idx + 1 + int(line))]
for parts in [f.read().split("\n")]
for idx, line in enumerate(parts)
if line.isdigit()]
print(result)

You're definitely on the right track.
If you want to preserve the original list here, you don't actually have to remove integer i; you can just go on to the next item.
Code:
originalData = []
formattedData = []
with open("data.txt", "r") as f :
f = list(f)
originalData = f
i = 0
while i < len(f): # Iterate through every line
try:
n = int(f[i]) # See if line can be cast to an integer
originalData[i] = n # Change string to int in original
formattedData.append([])
for j in range(n):
i += 1
item = f[i].replace('\n', '')
originalData[i] = item # Remove newline char in original
formattedData[-1].append(item)
except ValueError:
print("File has incorrect format")
i += 1
print(originalData)
print(formattedData)

The following code will produce a list results which is equal to [Data1, Data2].
The code assumes that the number of entries specified is exactly the amount that there is. That means that for a file like this, it will not work.
2
New York,5
Boston,10
Seattle,30
The code:
# get the data from the text file
with open('filename.txt', 'r') as file:
lines = file.read().splitlines()
results = []
index = 0
while index < len(lines):
# Find the start and end values.
start = index + 1
end = start + int(lines[index])
# Everything from the start up to and excluding the end index gets added
results.append(lines[start:end])
# Update the index
index = end

Python: filling a List of objects from a .txt file

For starters I've programmed in C++ for the past year and a half, and this is the first time I'm using Python.
The objects have two int attributes, say i_ and j_.
The text file is as follows:
1,0
2,0
3,1
4,0
...
What I want to do is have the list filled with objects with correct attributes. For example,
print(myList[2].i_, myList[2].j_, end = ' ')
would return
3 1
Here's my attempt after reading a little online.
class myClass:
def __init__(self, i, j):
self.i_ = i
self.j_ = j
with open("myFile.txt") as f:
myList = [list(map(int, line.strip().split(','))) for line in f]
for line in f:
i = 0
while (i < 28):
myList.append(myClass(line.split(","), line.split(",")))
i +=1
But it doesn't work obviously.
Thanks in advance!

Since you're working with a CSV file you might want to use the csv module. First you would pass the file object to the csv.reader function and it will return an iterable of rows from the file. From there you can cast it to a list and slice it to the 29 rows you are required to have. Finally, you can iterate over the rows (e.g. [1,0]) and simply unpack them in the class constructor.
class MyClass:
def __init__(self, i, j):
self.i = int(i)
self.j = int(j)
def __repr__(self):
return f"MyClass(i={self.i}, j={self.j})"
with open('test.txt') as f:
rows = [r.strip().split(',') for r in f.readlines()[:29]]
my_list = [MyClass(*row) for row in rows]
for obj in my_list:
print(obj.i, obj.j)
print(len(my_list))

I not sure you really what to stick with this format
print(myList[2].i_, myList[2].j_, end = ' ')
My solution is quite manual coded and i am using dictionary to store i and j
result = {'i':[],
'j':[]}
and below is my code
result = {'i':[],
'j':[]}
with open('a.txt', 'r') as myfile:
data=myfile.read().replace('\n', ',')
print(data)
a = data.split(",")
print (a)
b = [x for x in a if x]
print(b)
for i in range( 0, len(b)):
if i % 2 == 0:
result['i'].append(b[i])
else:
result['j'].append(b[i])
print(result['i'])
print(result['j'])
print(str(result['i'][2])+","+ str(result['j'][2]))
The result: 3,1

I'm not sure what you're trying to do with myList = [list(map(int, line.strip().split(','))) for line in f]. This will give you a list of lists with those pairs converted to ints. But you really want objects from those numbers. So let's do that directly as we iterate through the lines in the file and do away with the next while loop:
my_list = []
with open("myFile.txt") as f:
for line in f:
nums = [int(i) for i in line.strip().split(',') if i]
if len(nums) >= 2:
my_list.append(myClass(nums[0], nums[1]))

Printing to a .csv file from a Random List

When I create a random List of numbers like so:
columns = 10
rows = 10
for x in range(rows):
a_list = []
for i in range(columns):
a_list.append(str(random.randint(1000000,99999999)))
values = ",".join(str(i) for i in a_list)
print values
then all is well.
But when I attempt to send the output to a file, like so:
sys.stdout = open('random_num.csv', 'w')
for i in a_list:
print ", ".join(map(str, a_list))
it is only the last row that is output 10 times. How do I write the entire list to a .csv file ?

In your first example, you're creating a new list for every row. (By the way, you don't need to convert them to strs twice).
In your second example, you print the last list you had created previously. Move the output into the first loop:
columns = 10
rows = 10
with open("random_num.csv", "w") as outfile:
for x in range(rows):
a_list = [random.randint(1000000,99999999) for i in range(columns)]
values = ",".join(str(i) for i in a_list)
print values
outfile.write(values + "\n")

Tim's answer works well, but I think you are trying to print to terminal and the file in different places.
So with minimal modifications to your code, you can use a new variable all_list
import random
import sys
all_list = []
columns = 10
rows = 10
for x in range(rows):
a_list = []
for i in range(columns):
a_list.append(str(random.randint(1000000,99999999)))
values = ",".join(str(i) for i in a_list)
print values
all_list.append(a_list)
sys.stdout = open('random_num.csv', 'w')
for a_list in all_list:
print ", ".join(map(str, a_list))

The csv module takes care of a bunch the the crap needed for dealing with csv files.
As you can see below, you don't need to worry about conversion to strings or adding line-endings.
import csv
columns = 10
rows = 10
with open("random_num.csv", "wb") as outfile:
writer = csv.writer(outfile)
for x in range(rows):
a_list = [random.randint(1000000,99999999) for i in range(columns)]
writer.writerow(a_list)

nested for loop in python not working

We basically have a large xcel file and what im trying to do is create a list that has the maximum and minimum values of each column. there are 13 columns which is why the while loop should stop once it hits 14. the problem is once the counter is increased it does not seem to iterate through the for loop once. Or more explicitly,the while loop only goes through the for loop once yet it does seem to loop in that it increases the counter by 1 and stops at 14. it should be noted that the rows in the input file are strings of numbers which is why I convert them to tuples and than check to see if the value in the given position is greater than the column_max or smaller than the column_min. if so I reassign either column_max or column_min.Once this is completed the column_max and column_min are appended to a list( l ) andthe counter,(position), is increased to repeat the next column. Any help will be appreciated.
input_file = open('names.csv','r')
l= []
column_max = 0
column_min = 0
counter = 0
while counter<14:
for row in input_file:
row = row.strip()
row = row.split(',')
row = tuple(row)
if (float(row[counter]))>column_max:
column_max = float(row[counter])
elif (float(row[counter]))<column_min:
column_min = float(row[counter])
else:
column_min=column_min
column_max = column_max
l.append((column_max,column_min))
counter = counter + 1

I think you want to switch the order of your for and while loops.
Note that there is a slightly better way to do this:
with open('yourfile') as infile:
#read first row. Set column min and max to values in first row
data = [float(x) for x in infile.readline().split(',')]
column_maxs = data[:]
column_mins = data[:]
#read subsequent rows getting new min/max
for line in infile:
data = [float(x) for x in line.split(',')]
for i,d in enumerate(data):
column_maxs[i] = max(d,column_maxs[i])
column_mins[i] = min(d,column_mins[i])
If you have enough memory to hold the file in memory at once, this becomes even easier:
with open('yourfile') as infile:
data = [map(float,line.split(',')) for line in infile]
data_transpose = zip(*data)
col_mins = [min(x) for x in data_transpose]
col_maxs = [max(x) for x in data_transpose]

Once you have consumed the file, it has been consumed. Thus iterating over it again won't produce anything.
>>> for row in input_file:
... print row
1,2,3,...
4,5,6,...
etc.
>>> for row in input_file:
... print row
>>> # Nothing gets printed, the file is consumed
That is the reason why your code is not working.
You then have three main approaches:
Read the file each time (inefficient in I/O operations);
Load it into a list (inefficient for large files, as it stores the whole file in memory);
Rework the logic to operate line by line (quite feasible and efficient, though not as brief in code as loading it all into a two-dimensional structure and transposing it and using min and max may be).
Here is my technique for the third approach:
maxima = [float('-inf')] * 13
minima = [float('inf')] * 13
with open('names.csv') as input_file:
for row in input_file:
for col, value in row.split(','):
value = float(value)
maxima[col] = max(maxima[col], value)
minima[col] = min(minima[col], value)
# This gets the value you called ``l``
combined_max_and_min = zip(maxima, minima)