Read and count

[Jussi Piitulainen]

Val Krem writes:

> Hi all,
>
> I am a new learner about python (moving from R to python) and trying
> read and count the number of observation by year for each city.
>
>
> The data set look like
> city year  x 
>
> XC1 2001  10
> XC1   2001  20
> XC1   2002   20
> XC1   2002   10
> XC1 2002   10
>
> Yv2 2001   10
> Yv2 2002   20
> Yv2 2002   20
> Yv2 2002   10
> Yv2 2002   10
>
> out put will be
>
> city
> xc1  2001  2
> xc1   2002  3
> yv1  2001  1
> yv2  2002  3
>
>
> Below is my starting code
> count=0
> fo=open("dat", "r+")
> str = fo.read();
> print "Read String is : ", str
>
> fo.close()

Below's some of the basics that you want to study. Also look up the csv
module in Python's standard library. You will want to learn these things
even if you end up using some sort of third-party data-frame library (I
don't know those but they exist).

from collections import Counter

# collections.Counter is a special dictionary type for just this
counts = Counter()

# with statement ensures closing the file
with open("dat") as fo:
    # file object provides lines
    next(fo) # skip header line
    for line in fo:
        # test requires non-empty string, but lines
        # contain at least newline character so ok
        if line.isspace(): continue
        # .split() at whitespace, omits empty fields
        city, year, x = line.split()
        # collections.Counter has default 0,
        # key is a tuple (city, year), parentheses omitted here
        counts[city, year] += 1

print("city")
for city, year in sorted(counts): # iterate over keys
    print(city.lower(), year, counts[city, year], sep = "\t")

# Alternatively:
# for cy, n in sorted(counts.items()):
#   city, year = cy
#   print(city.lower(), year, n, sep = "\t")