CEN217, Fall 2019
Lecture 19: While loops and File Input
Adapted from slides by Marty Stepp and Stuart Reges
File Input/output (I/O)
• name = open("filename")
• opens the given file for reading, and returns a file object
• name.read() - file's entire contents as a string
>>> f = open("hours.txt")
>>> f.read()
'123 Brett 12.5 8.1 7.6 3.2\n
456 Sarina 4.0 11.6 6.5 2.7 12\n
789 Nick 8.0 8.0 8.0 8.0 7.5\n'
File paths
• absolute path: specifies a drive or a top "/" folder
C:/Documents/smith/hw6/input/data.csv
• Windows can also use backslashes to separate folders.
• relative path: does not specify any top-level folder
names.dat
input/kinglear.txt
• Assumed to be relative to the current directory:
file = open("data/readme.txt")
If our program is inH:/hw6 ,
open will look for H:/hw6/data/readme.txt
split
You can use the split function to break a file apart
• str.split() - splits a string on blank space
• str.split(other_str) - splits a string on occurrences of the
other string
>>> f = open("hours.txt")
>>> text = f.read()
'1\n2\n45\n6\n'
>>> f = text.split()
['1', '2', '45', '6']
Looping through a file
• The result of split can be used in a for ... in loop
• A template for reading files in Python:
file = open("filename")
text = file.read()
text = text.split()
for line in text:
statements
16.2
File input question 23.5
19.1
7.4
22.8
18.5
• We have a file weather.txt: -1.8
14.9
• Write a program that prints the change in temperature between each
pair of neighboring days.
16.2 to 23.5, change = 7.3
23.5 to 19.1, change = -4.4
19.1 to 7.4, change = -11.7
7.4 to 22.8, change = 15.4
22.8 to 18.5, change = -4.3
18.5 to -1.8, change = -20.3
-1.8 to 14.9, change = 16.7
File input answer
# Displays changes in temperature from data in an input file.
def main():
input = open("weather.txt"))
lines = input.read().split()
prev = float(lines[0]) # fencepost
for i in range(1, len(lines)):
next = float(lines[i])
print(prev, "to", next, ", change =", (next - prev))
prev = next
Gas prices question
• Write a program that reads a file gasprices.txt
• Format: Belgium $/gal
US $/gal
date
8.20
3.81
3/21/11
8.08
3.84
3/28/11
...
• The program should print the average gas price over all data in the file for both countries:
Belgium average: 8.3
USA average: 3.9
Gas prices solution
def main():
file = open("gasprices.txt")
belgium = 0
usa = 0
count = 0
lines = file.read().split()
for i in range(0, len(lines), 3):
belgium += float(lines[i])
usa += float(lines[i + 1])
print("Belgium average:", (belgium / count), "$/gal")
print("USA average:", (usa / count), "$/gal")
