I have this C++
int n = 0;
file >> n;
for (int i = 0; i < n; i++) file >> array[i];
etc.
How do i write it succinctly in Python?
This is Python:
string = string.lstrip()
n = 0
for c in string:
if not c.isdigit():
break
n = n * 10 + int(c)
for i in range(n):
string = string.lstrip()
for c in string:
if not c.isdigit():
break
array[i] = array[i] * 10 + int(c)
I thought Python was supposed to be more "expressive" than C++.
The io class tells us what methods a file object has (the thing returned by open()). I do not see anything like "read word" or "read int".
I was going to say it's simple to write some small functions that do that, but it turned into a 4-hour expedition and became rather large to be just an example in a Stack Overflow answer. I've posted it on Github instead: https://github.com/lgommans/OpenSesame
The file you need is just open_sesame.py. Basic usage is:
from open_sesame import OpenSesame
myfile = OpenSesame("data.txt")
records = myfile.int()
for i in range(0, records):
array.append(myfile.number())
I tried commenting the code and making it quite clear, so it should be understandable if you want to make modifications or see what it does.
Don't parse integers from strings manually. Python is perfectly capable of doing that for you:
>>> s = "12345"
>>> i = int(s)
>>> print(i)
12345
That already cleans up a substantial part of your code:
string = string.lstrip()
n = int(string)
for i in range(n):
string = string.lstrip()
array[i] = int(string)
I don't see any logic that moves around the string, so I assume you have left those pieces out. You aren't explicit about what exactly it is that separates these integers either (your code says "anything that isn't a digit"), so I'll assume instead it's whitespace-delimited.
Python can split such strings for you through one of the methods in str: split.
>>> s = "1 2\n3\t4 5" # Notice: all kinds of whitespace here.
>>> arr = s.split() # By default, split will split on whitespace.
>>> print(arr)
['1', '2', '3', '4', '5']
Notice that the split leaves the values as strings. That means we aren't done yet, we also have to convert each individual element into an integer as I demonstrated before.
Here, I'll use a Python feature called list comprehensions:
>>> s = "1 2\n3\t4 5"
>>> arr = [int(n) for n in s.split()]
>>> print(arr)
[1, 2, 3, 4, 5]
This is what people are talking about when they mention the "expressiveness" of Python :) This turns all of the code you've written into a one-liner. However, this assumes your data is in a string already. It seems you're reading from a file, so there's a bit more work required to get it working properly...
arr = [] # Empty list.
with open("path/to/file.txt") as f:
for line in f: # Will read all lines.
arr += [int(x) for x in line.split()]
# Use arr...
...which assumes you have multiple ints in a line. If instead you have a single int on every line, your code becomes much simpler:
with open("path/to/file.txt") as f:
arr = [int(line) for line in f] # Will read all lines.
# Use arr...
However, this still isn't a complete solution to your original problem... But I hope it's educational regardless. FWIW, this is how I would solve your particular problem:
with open("path/to/file.txt") as f:
ints_of_f = (int(line) for line in f) # A *GENERATOR*, not a *LIST*.
n = next(ints_of_f)
arr = [next(ints_of_f) for _ in range(n)] # _ is a throwaway variable.
Finally, here's a great talk on how to write "beautiful, expressive" Python code.
