files and strings (class slides)

CSc 110 - files and strings

String methods

• string.strip(chars) – removes any of the characters in chars from the beginning or end of string, returns a string
• string.split(chars) – splits string at the chars, returns a list
• string.lower(string) – forces all characters to lowercase, returns a string

Write a function

1. Its name is sum_all
2. It takes a string file_name as argument
3. It reads the file with file_name in read mode
4. It iterates over each line in the file, stripping the line breaks, casting each line as integer (assume each line is a single number), summing each number to a total variable
5. Return the total
6. Download the text file for the test case below

assert sum_all("numbers.txt") == 15

Write a function – solution

def sum_all(file_name):
  total = 0
  for line in open(file_name, "r"):
    number = int(line.strip("\n"))
    total += number
  return total

def main():
  assert sum_all("numbers.txt") == 15

main()

Submit code for attendance

Submit your sum_all functions to Gradescope for attendance.

Name your file sum_numbers.py

Modes

• To read a file:

  • Use ‘r’ for reading the contents of a file

• To write to a file: use

  • Use ‘a’ to append to the existing file content

  • Use ‘w’ to write to a file, and replace existing content

Writing to a file

• After you have opened the file in either ‘a’ or ‘w’ mode

  a_file = open(file_name, mode)

• Use the write function to write text content to the file

  a_file.write('put this content in a file')

• When finished writing, close the file a_file.close()

Evaluate the code

words = open('words.txt', 'w')
words.write('The slow wolf')
words.write('jumped over')
words.write('the bear')
words.close()

Evaluate the code

words = open('words.txt', 'w')
words.write('The slow wolf\n')
words.write('jumped over\n')
words.write('the bear\n')
words.close()

Evaluate the code

words = open('words.txt', 'a')
words.write('The slow wolf\n')
words.write('jumped over\n')
words.write('the bear\n')
words.close()

Write a function

1. Its name is write_word_count
2. It takes a string file_name as argument, it opens the file in read mode
3. It writes a comma separated file (“out_” + original filename), with the lowercase word as first value and count second
4. Download test file

Test case:

write_word_count("alien.txt")
# writes out out_alien.txt with word counts

Write a function – solution

def count_words(file_name):
  f = open(file_name, "r")
  counts = {}
  for line in f:
    words = line.strip("\n").split(" ")
    for w in words:
      lower_case_w = w.lower()
      if lower_case_w not in counts:
        counts[lower_case_w] = 1
      else:
        counts[lower_case_w] += 1
  return counts

def write_word_count(file_name):
  count_dict = count_words(file_name)
  output_file = open("out_" + file_name, "w")
  for key, value in count_dict.items():
    output_file.write(key + "," + str(value) + "\n")
  output_file.close()
  
def main(): 
  write_word_count("alien.txt")
  
main()

Submit Attendance

Name your previous solution read_and_write.py and submit it to attendance on Gradescope