File Reading/Writing Exercises #1

Example:

Suppose you were given the data file Fred.txt below, with one floating-point number per line (but with possible blank lines), and were asked to create the function Mean(input_filename, output_filename) that calculated the mean of all the numbers in the file whose name is in the argument input_filename and to put its answer into the file output_filename in the form:

The mean of the numbers in Fred.txt is 6.5

-5
13.5

11

Here is the code which will do that...

def Mean(input_filename, output_filename):
    '''Calculates and outputs the mean of the numbers in the input_filename'''
    # read the file's contents, and separate into words...
    get_it = open(input_filename, 'r')
    s = get_it.read()
    words = s.split()  # we now have an array of "words"
    get_it.close()

    n = 0  # counting the numbers
    the_sum = 0.0  # adding the numbers
    for a_word in words:
        if a_word != '':
            the_sum += float(a_word)  # REMEMBER: convert the string into a number 
            n += 1

    the_mean = the_sum / n

    # write the answer to the output file
    put_it = open(output_filename, 'w')
    s = 'The mean of the numbers in '+input_filename+' is '+str(the_mean)
    put_it.write(s)
    put_it.close()

... when called this way:  Mean('Fred.txt','Fred-answer.txt')

Exercises:

1. Create the function ReverseLines(input_filename, output_filename) which will reverse the string of each line of the input-file and write it to the output file.  Here's the result of calling ReverseLines('BeforeReverse.txt', 'AfterReverse.txt'):

2. Create the function Median(input_filename, output_filename) which will read the same type of files as Mean() in the example code above, and calculate the median of the numbers, and write the answer in the same way except replacing the word "mean" with "median" in the output file.  As a test, run it in on a file that has just the numbers:   6, 4, 5 and 3 (on separate lines), and make sure that you get 4.5 as the median in the output file.  Also, run it on a file containing just 4, 6 and 5 and make sure you get 5.

3. Imagine that you are correcting some arithmetic exercises that elementary school kids are doing.  They have been given a worksheet with a whole set of tasks like: "12 * 3 = ?" and "345 - 127 = ?" and have typed all the answers into a file that looks like this:

12 * 3 = 36
345 - 127 = 32
421 + 13 = 433
16 * 25 = 400

Notice that the second and third answers are wrong.  Write a function called CorrectingHomework(in_file, out_file) that reads all the lines that a student has written in in_file and writes to out_file every mistaken line, in the following form, and tallies up the number of correct answers divided by the total number of answers and provides the final grade as a percentage.

345 - 127 = 218 (not 32)
421 + 13 = 434 (not 433)

Correct: 2/4  Grade: 50%

Therefore, your function must perform the arithmetic and check whether the student's answer is correct, and if not, write the appropriate line in the out_file for each mistaken line.

Here's what you should know about the lines in the input file:

• There are only 3 types of arithmetic operators used:  "+",  "-"  and "*".
  
• All the numbers will be integers, positive or negative.
  
• There will always be a space before and after the operator, and before and after the "=".  This is so you can easily use the .split() method to separate the line into a list of "words" consisting of numbers and operations.
  
• Remember, you'll be reading these lines in string form...so convert the separate strings that represent numbers into numbers using the int() function before you try to do arithmetic on them.
  
• Test your function on the data file above (type it into a text file, and see if your results match mine), and then run the function on the big data file ('StudentArithmetic.txt') here (download it into the same directory as your Python code).
  
• Look at the data file first with a text editor (like gEdit) to make sure you understand its structure.
  
• After writing the results to 'StudentCorrections.txt', copy those same results into the Comments-to-Teacher.

4. Challenge problem: we are going to look at average SAT scores for the students at the high schools in NYC for 2012.  

• Create the function Top3SAT(infile,outfile) which will find the 3 schools with the highest SAT scores.
  
• sat-2012-clean-alpha.csv is the data file.  Download this file, into the same directory as your Python code.  Look at the file with a text editor to see its structure...
  
• This is a "CSV"-type data file ("Comma-Separated-Values").  Each line has 5 pieces of data, separated by commas.  Ignore any line that doesn't have 5 pieces of data. They are:
  • 1. School name (the list is in alphabetical order)
  • 2. number of students taking the SAT at that school
  • 3. Average critical reading score
  • 4. Average math score
  • 5. Average writing score
    
• For each school, you'll be calculating the average Total-SAT score (the sum of the three scores).
  
• You'll be writing out the lines corresponding to the 3 schools which had the highest Total-SAT scores.  The output file will therefore contain just the correct 3 lines from the input file, in order of decreasing Total-SAT score.
  
• Copy the names of the 3 schools into the Comments-to-Teacher.