incrementing list values Python 3

Question

infile = open('numbers.txt','r')

c = 0
for i in infile:
    if int(i) > c:
        c = int(i)
hist = [0]*c

for i in infile:   #checking
    ii = int (i)-1
    hist[ii] += 1  # problem area seemingly

for i in range(c):
    print(str(i+1),':',end=' ')
    print(str(hist[i]))

The purpose of the code above is to open number.txt, which has a 100 numbers, all within range(1,101), all with '\n' characters at their ends, and to count the number of times each number has been encountered.

First the greatest number in the file is found, then a list with the number of element equals the greatest number in the file, is made with all elements initially set to zero. Then the numbers are checked. All numbers in file are integers.

If the greatest number contained in the file is 100, then the hist[] list has initially 100 elements which are all 0.

During checking, say if a number 78 is encountered, the element with index [77] ie hist[77] is updated from value 0 to 1. If 78 is encountered again, hist[77] is changed from 1 to 2.

This way whatever number occurs in the file, each occurring number has a counter (also numbers that don't appear but are less that the greatest occurring no. have counters but thats not a problem).

Having checked that the correct file is being opened, the hist list is initially being set up properly, the problem is that the values of the hist[] list are not getting increment when the corresponding number is being encountered. When i print the list at the end, all values are still zero.

I am using Python 3.4.3. Both the script and 'numbers.txt' are on my desktop. Any help appreciated.

Martijn Pieters · Accepted Answer · 2015-05-04 11:49:56Z

1

You are looping over the file twice, but did not rewind the file reading pointer to the start. Add a file.seek() call before the second loop:

infile.seek(0)
for i in infile:   #checking

Without the seek iteration over infile will not yield any further lines; you reached the end of the file already.

You'd be better off using a collections.Counter() object here though:

from collections import Counter

with open('numbers.txt','r') as infile:
    hist = Counter(int(line) for line in infile)

You can then either get the highest counted number using max(hist) or instead retrieve the numbers in most common to least order with the Counter.most_common() method:

# In numerical order, setting missing numbers to 0
for i in range(1, max(hist) + 1):
    print('{}: {}'.format(i, hist.get(i, 0)))

# In most common to least order
# In numerical order
for number, count in hist.most_common():
    print('{}: {}'.format(number, count))

edited May 4, 2015 at 11:49

answered May 4, 2015 at 11:44

Martijn Pieters

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1 Comment

Keyboard Penman Over a year ago

Thank you very much. I had the misconception that only methods like read(), readline() advanced the file pointer and not syntax based statements like for i in infile: I'm a python newbie, so.. thanks again anyway.

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