How to read values one whitespace separated value at a time?-CodePudding

In C you can read one value at a time like this:

//from console
cin >> x;

//from file:
ifstream fin("file name");
fin >> x;

I would like to emulate this behaviour in Python. It seems, however, that the ordinary ways to get input in Python read either whole lines, the whole file, or a set number of bits.

I would like a function, let's call it one_read(), that reads from a file until it encounters either a white-space or a newline character, then stops. Also, on subsequent calls to one_read() the input should begin where it left off. Examples of how it should work:

# file input.in is:
# 5 4
# 1 2 3 4 5
n = int(one_read())
k = int(one_read())
a = []
for i in range(n):
    a.append(int(one_read()))
# n = 5 , k = 4 , a = [1,2,3,4,5]

How can I do this?

CodePudding user response：

Try creating a class to remember where the operation left off.

The __init__ function takes the filename, you could modify this to take a list or other iterable.

read_one checks if there is anything left to read, and if there is, removes and returns the item at index 0 in the list; that being everything until the first whitespace.

class Reader:
    def __init__(self, filename):
        self.file_contents = open(filename).read().split()

    def read_one(self):
        if self.file_contents != []:
            self.file_contents.pop(0)

Initalise the function as follows and adapt to your liking:

reader = Reader(filepath)
reader.read_one()

CodePudding user response：

More or less anything that operates on files in Python can operate on the standard input and standard output. The sys standard library module defines stdin and stdout which give you access to those streams as file-like objects.

Reading a line at a time is considered idiomatic in Python because the other way is quite error-prone (just one C example question on Stack Overflow). But if you insist: you will have to build it yourself.

As you've found, .read(n) will read at most n text characters (technically, Unicode code points) from a stream opened in text mode. You can't tell where the end of the word is until you read the whitespace, but you can .seek back one spot - though not on the standard input, which isn't seekable.

You should also be aware that the built-in input will ignore any existing data on the standard input before prompting the user:

>>> sys.stdin.read(1) # blocks
foo
'f'
>>> # the `foo` is our input, the `'f'` is the result
>>> sys.stdin.read(1) # data is available; doesn't block
'o'
>>> input()
bar
'bar'
>>> # the second `o` from the first input was lost