Key Ideas

  • File Reading Methods
    • file_variable.readline() - in conjunction with a while loop to read an entire file (see examples below)
    • file_variable.readlines()
  • File Writing Methods
    • file_variable.write(some_string)
  • Let’s experiment with reading data from census.txt (you downloaded this file in Lesson 12-1), doing some calculations, and writing our calculation results to a new file.
census_file = open("census.txt", "r")   # file we're reading from
summary_file = open("summary.txt", "w") # file we're writing to

total_pop = 0

for one_line in census_file:		# the newline mark triggers our iteration
    values = one_line.split()
    total_pop += values[1]
    summary_file.write(str(total_pop) + "\n")	# experiment with this; "/n" is the newline character

# Experiment with not closing the files;
# the buffer resets when the program closes, 
# and we lose what we've written to summary_file.

# census_file.close()
# summary_file.close()

There is another way to iterate through a file using a while loop and the .readline() method:

census_file = open("census.txt", "r")
summary_file = open("summary.txt", "w")

input_line = census_file.readline()	# grabs first line, leaves a "bookmark" at the beginning of the second line
cumulative_population = 0
while input_line:     # while there are still lines in the file that haven't been read
    values = input_line.split()
    cumulative_population = cumulative_population + int(values[1])
    summary_file.write(str(cumulative_population) + "\n")           # our file write command (takes only one string)
    input_line = census_file.readline()	# grabs next line based on the "bookmark"

census_file.close()
summary_file.close()

HW:

  • Experiment with the code above until you completely understand it.
  • Begin Project 12