See the following video for an introduction to files and while loops.
from IPython.display import YouTubeVideo
YouTubeVideo("9-YH_M6_J-g")
To summarize:
In order to open a file, you need to first set Python's working directory to the directory where the file is (if you're reading the file) or will be (if you're writing to the file).
On different systems, directories are accessed slightly differently. Assuming that your username is guerzhoy and you have created the folder c4mW4 on your desktop, here is how you would do things on different systems:
Mac:
import os
os.chdir("/Users/username/Desktop/c4mW4")
Windows:
import os
os.chdir("c:/Users/username/Desktop/c4mW4")
Linux:
import os
os.chdir("/home/guerzhoy/Desktop/c4mW4")Once you're in the right directory, you can open the filen, and then write to it:
f = open("rain.txt", "w") #"w" means you will be writing to the file. Use "r"
#(or nothing) if you want to read from the file
f.write("There are holes in the sky\n")
f.write("Where the rain gets in\n")
f.write("But they're ever so small\n")
f.write("That's why the rain is thin\n")
f.close() #Close the file once you're done with it
#This is sometimes necessary to avoid problems with
#Writing files
Above, f is a file object. The file object keeps track of where you are in the file. By default, you just keep writing (and reading) where you left off (as opposed to, for example, always writing to the beginning of the file). Note one thing that's different here: unlike with print, we need to explcitly add the newline (\n) characters to strings, or else everything we write to rain.txt will appear on a single line.
To read the entire file into a string, you can use
f = open("rain.txt", "r")
text = f.read()
f.close()
print(text)
You can use for-loops in order to read in the file line-by-line:
f = open("rain.txt")
for line in f:
print(line[:-1])
line is assigned the first line from the file, then the second line from the file, and so on.
Note that we printed line[:-1] rather than line. That's because be default, print adds a newline to the string anyway, and the last character in line is \n. If we just print line, we'll get the following:
f = open("rain.txt")
for line in f:
print(line)
You can also read lines from a file using readline
f = open("rain.txt")
print(f.readline()[:-1])
This got as the first line. This is where the fact that f remembers where we are in the file is useful. If we call f.readline again, we'll get the next line
print(f.readline()[:-1])
This will be a brief diversion into another kind of loop: the while-loop. The structure of the while-loop is as follows:
while <cond>:
<block>
Every time, we check whether the condition cond is true, and if it is, we run <block>. We then repeat the same thing again.
This is useful when interacting with the user: the computer keeps waiting for input from the user, and reading in the input, until some the user types in input that signifies the end of the interaction,
Here's a silly example:
def dishes():
'''Repeatedly ask the user for input,
and returns "Yes" or "No" once the user
inputs "Yes" or "No"'''
answer = ""
while answer != "Yes" and answer != "No":
answer = input("Did you do the dishes? ")
return answer
The function will keep running and getting input from the user until the user inputs either "Yes" or "No".
Let's read in the file january06.txt. Note that the first two lines are the "header":
Year Day Hour Temp RH WndSp WndDir Precip Solar
Julian EST *C % kmh degrees mm W/m2
2005 355 1100 -3.377 78.4 5.128 268 0 113.8
2005 355 1200 -2.559 72.1 6.285 236.8 0 172.7
2005 355 1300 -2.445 66.72 6.721 246.3 0 155.9
2005 355 1400 -1.791 61.91 6.07 258.2 0 184.4
2005 355 1500 -2.795 66.26 6.71 250.8 0 95.7
2005 355 1600 -3.336 69.96 6.74 244.2 0 59.73
2005 355 1700 -3.999 75.6 5.915 250.4 0 11.03
2005 355 1800 -4.782 81.8 6.395 247.7 0 -3.375
2005 355 1900 -4.27 83.3 5.279 240 0 -1.233The fourth column is the temperature. Let's obtain the temperature from the first line automatically:
os.chdir("/home/guerzhoy/Desktop/c4m")
f = open("january06.txt")
header = f.readline()
temp_index = header.split().index('Temp') #temp_index will be 3m subce header.split()[3] is "Temp"
f.readline() #Skip the next line, don't store it anywhere since it's not needed
line = f.readline() #The first line of the data
#Now, get the fourth item in line:
print("Temperature on first line:", float(line.split()[temp_index]))
Let's get five more lines:
for i in range(5):
print(f.readline()[:-1])
Suppose we now want to get the first line where the information is positive. We can do this using a while-loop: we'll keep reading in lines using readline while the temperatures are negative
f = open("january06.txt")
header = f.readline()
temp_index = header.split().index('Temp') #temp_index will be 3m subce header.split()[3] is "Temp"
f.readline() #Skip the next line, don't store it anywhere since it's not needed
line = f.readline()
cur_temp = float(line.split()[temp_index])
while cur_temp < 0:
line = f.readline()
cur_temp = float(line.split()[temp_index])
print(line.split())
What we accomplished is reading in the data up until a certain point, and then stopping. We already saw a mechanism that can accomplish the same kind of thing: if we use a return statement inside a function, we exit whatever loop we were in inside the function
def get_first_warm_day(filename):
f = open(filename)
header = f.readline()
temp_index = header.split().index('Temp')
f.readline()
for line in f:
cur_temp = float(line.split()[temp_index])
if cur_temp >= 0:
return line
print(get_first_warm_day("january06.txt").split())
A list contains the patient's report about their well-being for each day. Return the number of days it takes for the patient to say "I'm fine".
For example, for
reports = ["My leg hurts",
"I'm tired",
"I'm sleepy",
"Awesome!",
"Hi Doctor, I'm fine",
"Bad again"]
It takes 4 days for the patient to say "I'm fine".
Here is how to compute that using a while-loop:
reports = ["My leg hurts",
"I'm tired",
"I'm sleepy",
"Awesome!",
"Hi Doctor, I'm fine",
"Bad again"]
n_days = 0
while reports[n_days].find("I'm fine") == -1:
n_days += 1
print(n_days)
Here is how we can write a function with a for-loop in it to accompish the same thing:
def time_to_recover(reports):
n_days = 0
for report in reports:
n_days += 1
if reports[n_days].find("I'm fine") != -1:
return n_days
time_to_recover(reports)