error in python: list assignment index out of range

Question

I searched everywhere and even though there were a couple of questions and answers regarding this error I couldn't find a solution to fix my problem

I'm reading in from a file that contains letters and numbers and I'm populating my matrix depending on the values in that file. ex: file description of letters and numbers ... table:

Here's the code

matrix = [[0 for j in range(4)] for i in range(4)]
i = 0
j = 0

for line in file:
   for a in line:
      if is_number(a):
         matrix[i][j] = int(a)
         j+= 1
      if matrix.count(0) < 2: #since matrix already populated with zeroes. it shouldn't have 
                               #many per program specifications, that's why I use this 
                               #conditional to increment i and reset j back to 0
         i += 1
         j = 0

file.close()

I don't understand why I keep getting that error.

@AshwiniChaudhary my output is basically supposed to be a 2D array that holds the integer values in the above table — evthim
– evthim, Commented Nov 13, 2012 at 22:14

Wilduck · Accepted Answer · 2012-11-14 16:16:47Z

I see two possible ways you could end up with an IndexError in your code.

The first problem occurs because of the way you are iterating through the file that you're reading. Your code:

for line in file:
    for a in line:
        if is_number(a):
            # do stuff

Reads a line in the file into the variable line. Then, each character is stored in the variable a and you check if it is a number. If any of the integers you are reading in are greater than 9 you will see an IndexError since it will count each digit as a separate number, causing you to eventually run out of room in your pre-allocated array.

A possible fix would be to change the line:

for a in line:

to

for a in line.split()

which will split the line into a list of words (that is, a new entry for everything separated by whitespace). So, "6 12 4 5" will become [6,12,4,5], making it so that you don't count the 1 and 2 in 12 separately.

The second issue I see with your code is in the line:

if matrix.count(0) < 2:

If your input file ever contains a zero, it will cause this line to stay true for one iteration of the loop longer than you would like. A possible fix would be to change the line to:

if j == len(matrix[0]) - 1:

Ashwini Chaudhary · Accepted Answer · 2012-11-13 21:42:24Z

1

try something like this:

with open("data1.txt") as f:
    next(f)                  #skip the first line
    lis=[map(int,x.split()[1:]) for x in f]  #use x.split()[1:] to remove the alphabet
    print lis

output:

[[1, 2, 5, 6], [5, 6, 3, 4], [3, 2, 1, 4], [2, 4, 6, 8]]

answered Nov 13, 2012 at 21:42

Ashwini Chaudhary

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2 Comments

evthim Over a year ago

I got this as output when I used your method: <map object at 0x1007a7590>, <map object at 0x1007a75d0>, <map object at 0x1007a7650>, <map object at 0x1007a76d0>, <map object at 0x1007a7750>, <map object at 0x1007a77d0>]

Ashwini Chaudhary Over a year ago

you're on python 3.x, use list(map(int,x.split()[1:])) in python 3.x. or simply use a list-comprehension lis=[[int(y) for y in x.split()[1:]] for x in f]

Julien Vivenot · Accepted Answer · 2012-11-13 22:06:42Z

1

If you know the input file already has the right matrix (line by line) layout you could use the following :

matrix = filter(lambda x: len(x)>0, [[int(a) for a in l.split() if is_number(a)] for l in file])

If you cannot expect anything from the input layout, you could try:

data = open("test").read()
l = filter(lambda x: is_number(x), data.replace("\n"," ").split())
width = int(math.sqrt(len(l)))
print [[int(l[i+width*j]) for i in range(width)] for j in range(width)]

answered Nov 13, 2012 at 22:06

Julien Vivenot

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2 Comments

evthim Over a year ago

do you mean, trix = filter(lambda x: len(x)>0, [[int(a) for a in l.split() if is_number(a)] for l])? Where would the last bracket and parentheses be?

Julien Vivenot Over a year ago

I am sorry but I do not understand your comment. There are already enough brackets and parentheses as is in my answer.

Adam Rosenfield · Accepted Answer · 2012-11-13 21:37:54Z

0

You're constructing a 4x4 matrix in the first line of code, but your data is a 6x6 matrix. When you try to store an element at index 4 in row 0, you get an IndexError.

answered Nov 13, 2012 at 21:37

Adam Rosenfield

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1 Comment

evthim Over a year ago

my bad, I was just giving an example of a possible file, but the matrix is supposed to be 4x4

Chris Down · Accepted Answer · 2012-11-13 21:38:24Z

0

The problem is here:

matrix = [[0 for j in range(4)] for i in range(4)]

Your matrix is 6x6, but your code only compensates for a 4x4 matrix.

answered Nov 13, 2012 at 21:38

Chris Down

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1 Comment

evthim Over a year ago

I fixed the example file. I'm actually supposed to be reading in a 4x4 matrix

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