Python：读取文件并计算总和和平均值

Question

The question is to read the file line by line and calculate and display the sum and average of all of the valid numbers in the file.

The text file is

contains text
79.3
56.15
67
6
text again 
57.86
6
37.863
text again 
456.675

That's all I have so far.

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

line = file_contents.readline()

numbers.close()

try:
    sum = line + line
    line = file_contents.readline()
    print "The sum of the numbers is", sum


except ValueError:
    print line

Answer 1

Using with notation can make dealing with files a lot more intuitive.

For instance, changing the opening and closing to this:

summation = 0

# Within the with block you now have access to the source variable
with open('fileofnumbers.txt', 'r') as source:
    for line in source: #iterate through all the lines of the file 
        try:
            # Since files are read in as strings, you have to cast each line to a float
            summation += float(line)
        except ValueError:
            pass

Might get you started

If you want to be a little more clever, there's a convenient python function called isdigit , which checks if a string is all integer values, which can let you do very clever things like this:

is_number = lambda number: all(number.split('.').isdigit())
answer = [float(line) for line in open('fileofnumbers.txt') if is_number(line)]

Which then makes sum and average trivial:

print sum(answer) # Sum
print sum(answer)/len(answer) #Average

Answer 2

Let's try list comprehension with try-except . This might be an overkill but surely a good tool to keep in your pocket, first you write a function that will silence the errors as such in http://code.activestate.com/recipes/576872-exception-handling-in-a-single-line/ .

Then you can use list comprehension by passing in argv like you do in Unix:

intxt = """contains text
29.3423
23.1544913425
4
36.5
text again 
79.5074638
3
76.451
text again 
84.52"""

with open('in.txt','w') as fout:
    fout.write(intxt)

def safecall(f, default=None, exception=Exception):
    '''Returns modified f. When the modified f is called and throws an
    exception, the default value is returned'''
    def _safecall(*args,**argv):
        try:
            return f(*args,**argv)
        except exception:
            return default
    return _safecall

with open('in.txt','r') as fin:
    numbers = [safecall(float, 0, exception=ValueError)(i) for i in fin] 
    print "sum:", sum(numbers)
    print "avg:", sum(numbers)/float(len(numbers))

[out]:

sum: 336.475255142
avg: 30.5886595584