Module 10 - File Access

Question 1

What do you call a program that runs all the time and keeps at least some of its data in permanent storage?

Answer 1

A persistent program

example: operating systems and web servers

Question 2

How do you read a line from a text file? Write the sample code.

Answer 2

use open function and assign to variable:
fin = open (‘words.txt’)
then use readline method:
fin.readline ( )

Question 3

What is a text file?

Answer 3

A sequence of characters stored on a permanent medium like a hard drive, flash memory, or CD-ROM

Question 4

How do you write a file?

Answer 4

You have to open it with mode ‘w’ as a second parameter:

fout = open(‘output.txt’, ‘w’)

Question 5

What are the two types of files? What is the difference?

Answer 5

Text file - contains data that has been encoded as text

Binary file - contains data that has not been converted to text

Question 6

What are the two types of access to data stored in a file?

Answer 6

Sequential access - file read sequentially from beginning to end, can’t skip ahead
Direct access - can jump directly to any piece of data in the file

Question 7

What is a filename extension and what does it indicate?

Answer 7

Short sequence of characters at the end of filename (after the period)
Indicate the type of data stored in the file

Question 8

What’s a file object? What’s the point of it?

Answer 8

Object associated with a specific file

Provides a way for a program to work with the file: file object referenced by a variable

Question 9

What’s the ‘open’ function used for and what does it do? What’s the syntax?

Answer 9

Used to open a file
It creates a file object and associates it with a file on the disk

file_object = open (filename, mode)

Question 10

What are the different modes of the ‘open’ function?

Answer 10

Mode is the second parameter in the function
Reading only - ‘r’
Writing - ‘w’
Appending - ‘a’

Question 11

What happens if the open function receives a filename that does not contain a path?

Answer 11

It assumes that file is in the same directory as the program

Question 12

If a program is running and a file is created, where is it created? Can you alter this?

Answer 12

In the same directory as the program
Can specify alternative path and file name in the open function argument - prefix the path string literal with the letter r

Question 13

What is a method?

Answer 13

A function that belongs to an object

Performs operations using that object

Question 14

How do you write data to a file?

Answer 14

File object’s write method

file_variable.write(string)

Question 15

How do you close a file?

Answer 15

File object’s close method

file_variable.close( )

Question 16

What’s the return value of the write method?

Answer 16

The number of characters that were added (i.e., ‘24’)

Question 17

What does ‘read’ do? How does it work?

Answer 17

Reads entire file contents into memory
Works only if file has been opened for reading
Contents return as a string

Question 18

What’s ‘read position’ do?

Answer 18

Marks the location of the next item to be read from a file

Question 19

What happens to the data if you open an existing file in ‘w’ mode?

Answer 19

It is overwritten

Use ‘a’ (append) to add to end of current contents

Question 20

What happens if you try to append a file that does not exist?

Answer 20

The ‘a’ mode will create a new file

Question 21

T or F: Numbers must be converted to strings before they are written to a file

Answer 21

True
Use str function
Then use int or float functions to convert back (to perform math)

Question 22

How can you use a while loop to read a file?

Answer 22

Use the readline method

while line != “ “

Question 23

How can you use a for loop to read a file?

Answer 23

for line in file_object:

___statements

Question 24

What is a record?

Answer 24

Set of data that describes one item

Question 25

What is a field?

Answer 25

Single piece of data within a record

Question 26

How do you write and read a record to/from a file?

Answer 26

Write record to sequential access file by writing the fields one after the other
Read record from sequential access file by reading each field until record complete

Question 27

What is the difference between write and writelines methods?

Answer 27

WriteLine() provides a new line character \n at the end of the string

Question 28

How do you remove the \n character at the end of the writelines method?

Answer 28

rstrip method

Question 29

What is the difference between read and readlines methods?

Answer 29

Reads entire file contents into memory vs. reads a line from the file (line returned as string ending in ‘\n’)

Question 30

Open the file hostdata.txt for reading.

Answer 30

open (‘hostdata.txt’, ‘r’)

DON’T FORGET QUOTES!

Question 31

How do you create a file object from a file?

Answer 31

Use the open function

It creates a file object, assigns it to a variable and associates it to the file

Question 32

A file named numbers.txt contains an unknown number of lines, each consisting of a single integer. Write some code that computes the sum of all these integers, and stores this sum in a variable name sum.

Answer 32

fin=open(‘numbers.txt’,’r’)
sum=0
for line in fin:
____sum+=int(line)

1. create file object (open function)
2. initialize sum variable
3. create for loop (it will iterate over every line in the file)
4. add each line to the sum variable (remember to CONVERT to integer!!)