CS计算机代考程序代写 python Digression: Scoring Matrices

Digression: Scoring Matrices

Lecture 13
File Processing Part I

L13 File Processing Part I – 2

Objectives

• To understand how to format strings

• To understand the basic text file processing
concepts in Python.

• To learn how to read and write text files in
Python and string formatting

L13 File Processing Part I – 3

Revision

• What is printed when the code is executed ?

L13 File Processing Part I – 4

String Formatting

>>> amount = 1.50

>>> print(amount)

1.5

• If the code about is meant to represent an amount in dollars
and cents, we conventionally do not use fractional dollars but
rather dollars with 2 digits for cents

• Use the format method

>>> print(“${0:0.2f} change”.format(amount))

$1.50 change

• The first part is the string to be printed, called the template
string. The part between {} is the format specifier (where the
value is to be inserted and how it should look).

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String Formatting

“${0:0.2f} change”.format(amount)

• The template contains a single specifier slot
with the description: 0:0.2f

• Form of description:
:

• Index tells which parameter to insert into the
slot (there can be more than one). In this case,
amount (numbered from 0)

L13 File Processing Part I – 6

String Formatting

Looking at 0.2f
• The formatting specifier has the form:
.

• f means “fixed point” number

• tells us how many spaces to use to display
the value. 0 means to use as much space as
necessary.

• is the number of decimal places.
>>> “Compare {0} and {0:0.20f}”.format(3.14)

‘Compare 3.14 and 3.1400000000000001243’

L13 File Processing Part I – 7

String Formatting Example
# Print out a child’s multiplication table 0..10
def multiplication_table() :
for i in range(11) :

for j in range(11) :
print(“{0:0d} x {1:0d} = {2:0d}”.format(i, j, i*j))

print()

A basic child’s
multiplication table to
illustrate string
formatting.

Needs modification to
properly resemble table,
e.g. numbers along top
and down left hand side

L13 File Processing Part I – 8

Multiline Strings
• You sometimes need strings that span more than one line. Two

ways to do this:
• Embedded ‘\n’ in single string

– “Twas brillig, and the slithy toves\nDid gyre
and gimble in the wabe\nAll mimsy were the
borogoves\nAnd the mome raths outgrabe.”

• Multiline string:
“””Beware the Jabberwock, my son!
The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
The frumious Bandersnatch!”””

• ”’ single quotes are also available
• “hello\tworld”
• ‘\t’ is the tab character

L13 File Processing Part I – 9

Files: Multi-line Strings

• A file is a sequence of data that is stored in
secondary memory (disk drive).

– Files don’t disappear when program ends

• Files can contain any data type, but the easiest to
work with are text.

• A file usually contains more than one line of text.

• Python uses the standard newline character (\n) to
mark line breaks.

L13 File Processing Part I – 10

File Processing

• The process of opening a file involves associating a
file on disk with variable in memory.

• We can manipulate the file by manipulating this
variable.

– Read from the file

– Write to the file

L13 File Processing Part I – 11

File Processing

• When you’ve finished working with the file, it needs
to be closed.

– Closing the file causes any outstanding
operations and other bookkeeping for the file to
be completed.

• In some cases, not properly closing a file could result
in data loss.

– Think of safely ejecting your memory stick

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File Processing – Reading

• Reading a file into a program, e.g. word
processor
– File opened
– Contents read into RAM
– File closed
– Changes to the file are made to the copy

stored in memory, not on the disk.

L13 File Processing Part I – 13

File Processing – Writing

• Saving a file, i.e. data in RAM onto file

– The original file on the disk is reopened in a mode
that will allow writing (this actually erases the
old contents unless specifically appending)

– File writing operations copy the version of the
document in memory to the disk

– The file is closed

L13 File Processing Part I – 14

File Processing in Python

• Working with text files in Python

– Associate a disk file with a file object using the open
function

= open(, )

– is a string with the actual file name on the
disk. The can be ‘r’, ‘w’ or ‘a’ depending
on whether we are reading, writing or appending the
file. You can also add ‘+’ in mode to mention both
read and write operations.

– infile = open(“numbers.dat”, “r”)

L13 File Processing Part I – 15

File Methods

• .read() – returns the entire remaining
contents of the file as a single (possibly large, multi-
line) string. Watch out for final \n

• .readline() – returns the next line of the
file. This is all text up to and including the next
newline character

• .readlines() – returns a list of the
remaining lines in the file. Each list item is a single
line including the newline characters.

L13 File Processing Part I – 16

File Processing
# printfile.py

# Prints a file to the screen.

def main():

fname = input(“Enter filename: “)

infile = open(fname,’r’)

data = infile.read()

infile.close()

print(data)

• First, prompt the user for a file name
• Open the file for reading
• The file is read as one string and stored in the

variable data

L13 File Processing Part I – 17

File Processing

• readline can be used to read the next line from a
file, including the trailing newline character
infile = open(someFile, “r”)
for i in range(5):

line = infile.readline()
print(line[:-1])

• This reads the first 5 lines of a file
• Slicing is used to strip out the newline characters at

the ends of the lines

L13 File Processing Part I – 18

File Processing Loop

• Python treats the file itself as a sequence of
lines!
infile = open(someFile, “r”)
for line in infile:

# process the line here
infile.close()

• Most efficient way to read through (and
process) file
– Multiple calls to readline() is inefficient

L13 File Processing Part I – 19

File Processing

• Opening a file for writing prepares the file to
receive data

• If you open an existing file for writing, you
wipe out the file’s contents. If the named file
does not exist, a new one is created.
outfile = open(“mydata.out”, “w”)

outlife.write()

May use writelines() for writing sequence
(list) of strings

L13 File Processing Part I – 20

Summary

• We learned how to format a string for output that is
more readable and looks nice

• We learned how to read files

– All at once
– Single lines
– All the lines, line-by-line

• We learned how to write into files

Lecture 13�File Processing Part I
Objectives
Revision
String Formatting
String Formatting
String Formatting
String Formatting Example
Multiline Strings
Files: Multi-line Strings
File Processing
File Processing
File Processing – Reading
File Processing – Writing
File Processing in Python
File Methods
File Processing
File Processing
File Processing Loop
File Processing
Summary