Module 4: Assorted topics: while-loops, I/O

First, examine the file testfile.txt to see that it's a file consisting of four lines of text. (From the poet Ogden Nash.)

We will look at a few different versions of reading from this file.

Here's the first:

with open('testfile.txt', 'r') as in_file:
    lines = in_file.read()

print(type(lines))
print(lines)
    

4.27 Exercise: Type up the above in my_file_read.py. What is the type of the variable lines? Note:

• We've used two Python reserved words:

  with open('testfile.txt', 'r') as in_file:
    

• Although file input/output (I/O) does not strictly require the with structure, it is useful because:
  • Files that are being accessed by one program are said to be in an "opened" state.
  • For another program to be able access the file, the first one has to "close" it (that is, signal that it's done with the file).
  • The with structure automatically takes care of it.

• The function call to open takes the name of the file and the kind of access, for example:

  with open('testfile.txt', 'r') as in_file:
    

  • 'r' for read-only access (we're not changing the file here)
  • 'w' for write, if we should choose to.

• The result of opening a file is to get a special kind of variable, what we've called in_file in this case:

  with open('testfile.txt', 'r') as in_file:
    

• It is this variable that's going to perform the reading and, in this case, get us all the text in one shot:

  with open('testfile.txt', 'r') as in_file:
      lines = in_file.read()
    

• Note that all the lines are returned as a single string.

• This means, it will be difficult to analyze string-by-string, if that's our goal.

• There is a way to take the single string and break it into separate lines, but let's instead find a way to read separate lines.

Accordingly, let's look at a way to read the file into a list of strings, where each line is one string in the list:

lines = []
with open('testfile.txt', 'r') as in_file:
    line = in_file.readline()
    while line != '':
        lines.append(line.strip())
        line = in_file.readline()

print(type(lines))
print(lines)
    

4.28 Exercise: Type up the above in my_file_read2.py. What is the type of the variable lines?

Note:

• Here, we're reading one line at a time and appending to a running list, which is the lines variable.

• The problem is, for any general file, we won't know in advance how many lines of text are in the file.

• A while-loop to the rescue!

• Thus, we keep reading from the file as long as a read operation produces a line:

  

Writing to a file:

• Suppose we've read a text file into a list of strings.

• Let's now write these to a new file:

  with open('testcopy.txt', 'w') as out_file:
      for line in lines:
          out_file.write(line + '\n')
    

• This time, we're opening a file called testcopy.txt for the purpose of writing to it:

  with open('testcopy.txt', 'w') as out_file:
      for line in lines:
          out_file.write(line + '\n')
    

• We've named our file variable out_file.

• That will let us use a function called write():

  with open('testcopy.txt', 'w') as out_file:
      for line in lines:
          out_file.write(line + '\n')
    

  Here, we're looping through the list, writing each string as one line in the file.

• Notice that we need to insert the '\n' at the end of each line.
  Recall:
  • '\n' represents an instruction to both output and files to "go to the next line right now".
  • Thus for example

    print('hello' + 'world')           # Prints helloworld on one line
    print('hello' + '\n' + 'world')    # Prints hello, and then world on the next line
      

  • So, to write strings to different lines, we have to tell the function that writes to files to go to the next line with an explicit '\n'.
  • It's similar with reading, if we read a whole file as one string, that string will contain the so-called linebreaks (the '\n' characters).

4.29 Exercise: In my_file_readwrite.py, combine the reading and writing so that the program as whole results in copying from testfile.txt to testcopy.txt.

Next, let's read from a file of numbers and perform some basic stats:

• First, examine the file data.txt and see that it's a collection of numbers, one per line.

• We'll read line by line as a string, and then convert to a floating-point number:

  data = []
  with open('data.txt', 'r') as in_file:
      line = in_file.readline()
      while line != '':
          s = line.strip()           # Remove leading/trailing whitespace
          x = float(s)               # Convert string to float
          data.append(x)             # Add to our list
          line = in_file.readline()  # Get the next line
  
  print(data)
    

4.30 Exercise: In my_file_data.py, add code to compute the average of the numbers and print it. Compute the total as you iterate in the while-loop.

4.31 Audio:

4.7    Extracting multiple data from each line

6.0 6.0 9.0
4 6 8  
24 16 2  
3 3.0 3
0.1 0.5 0.3
  

Average of 6.0 6.0 9.0 is: 7.0
Average of 4.0 6.0 8.0 is: 6.0
Average of 24.0 16.0 2.0 is: 14.0
Average of 3.0 3.0 3.0 is: 3.0
Average of 0.1 0.5 0.3 is: 0.3
  

k = 0
while k > 0:
    print(k)
    k = k - 1
      

m = 1
n = 2
while (m <= 10) or (n <= 20):
    print(m, 2*n)
    m = m + 1
    n = n + 1
      

x = 1
while x < 10:
    if x % 2 == 1:     # Test whether odd
        print(x)
    else:
        x = x + 1