Python: Cut and Paste Data on Text File.

Question

I have a text file that contains numbers.

I need to move some of the numbers from the beginning of the file to the end in the correct order.

For example, the Original TEXT file has the following content: 0123456789 .

I need to move the first 4 numbers to the end in the same order so it'll look like this:

4567890123.

Unfortunately i have no idea how to do this with Python, I don't know even where to start.

Any pointers to solving this problem would be highly appreciated.

Answer 1

See the Python tutorial ( section "Strings" ; search for "slice notation"):

>>> a = "0123456789"
>>> b = a[4:] + a[:4]
>>> b
'4567890123'

Or what is it you're really trying to do?

Answer 2

The individual characters of the string a = '0123456789' can be accessed through a[i] , where for i=0 you get the character at the first position (indexes are numbered from 0), so '0' . You can also extract several characters at once in the form a[i:j] , where i is the position of the first character and j is the position of the character after the last character. If you omit one of i or j , it will take all characters from the beginning or until the end of the string.

So:

a[0] = a[0:1] = a[:1] = '0'
a[1] = a[1:2] = '1'
a[4] = a[4:5] = '4'
a[0:3] = a[:3] = '012'
a[3:5] = '34'
a[4:] = '456789'

So the first 4 characters are a[:4] and the rest is a[4:] . Now you concatenate them together:

a[4:] + a[:4]

and it will return

'4567890123'

In order to read the file, you will have to open it in the read mode and use the first line, stripping any whitespace/newlines:

with open('filename.txt', 'r') as f:
    line = f.readline().strip()
    print(line[4:] + line[:4])