File I/O

Dr. Bernard Chen Ph.D.University of Central Arkansas

Spring 2012

File

Files: named storage compartment on your computer that are managed by operating system

The built-in “open” function creates a Python file object, which serves as a link to a file in your computer

Open Function

After calling open, you can read or write the associated external file

Two major types of open file function:

1. Read file (‘r’)2. Write file (‘w’)

Read/Write file Open function take two variables, first on

is the file name you want to deal with, another one is read or write of the file

input = open ('file1.txt','r') Variable name Keyword file name read file

output = open (‘output_file.txt’, ‘w’)Variable name Keyword file name write file

Read/Write file

In the end of the program, remember to close the file via close() function

Input.close() Output.close()

Read in File1

Read File

After connect with the file through open function, you need “readlines()” to read the contents in the file

readlines() is the function that read entire file into list of line strings

Read File

input = open ('file1.txt','r')input.readlines()

But this is not good enough, because the contents we read is in text format, we need to convert it into numbers and store those in a list, so that we can do some computations on those numbers

Read File1

Read File

After we read in those files into list ‘all’, we can easily calculate average, max, min…

Class Practice

Download file1 into the Python folder

Read in the file and print out the max value

Write file We have a “write” function for write

things on a file Things you want to write MUST be

string, so if you want to write a number, you have to convert it by str()

output = open (‘output_file.txt’, ‘w’)output.write(“Hello World!!!”)output.write(str(12))

Write file write number form 0 to 10 into a file

output = open (‘output_file.txt’, ‘w’)for i in range(11):

output.write(str(i))output.write(‘\n’)

output.close()

Class Practice Write the following half pyramid into a

new created file named “pyramid.txt”

***************

Read File

Now, think about different file type we have…

Read in File2

Read in File2

Since all numbers are in one line, we cannot read this file like we did in file1

We need to “separate” the numbers through space

Read in File2

As the result, we need the function “split(str)”

This method returns a list of all the words in the string, using str as the separator (splits on all whitespace if left unspecified)

split() examples

>>> sentence = "the cat sat on the mat" >>> print sentence.split() ['the', 'cat', 'sat', 'on', 'the', 'mat']

>>> sentence = "the,cat,sat,on,the,mat“>>> print sentence.split(',') ['the', 'cat', 'sat', 'on', 'the', 'mat']

Class Practice

>>> sentence = "the cat sat on the mat"

>>> print sentence.split(‘t’)

Read in File2

input=open('file2.txt','r')

for line in input.readlines(): all=line.split()

for i in range(len(all)): all[i]=int(all[i])

Read in File3

2D list: lists inside of list Here’s the way to create a 2D list (Just like

what we saw last week)

aa=[1,2,3]bb=[4,5,6]cc=[7,8,9]

matrix=[]matrix.append(aa)matrix.append(bb)matrix.append(cc)

Access 2D list With one index, you get an entire row:>>> matrix[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

>>> matrix[0][1, 2, 3]

>>> matrix[1][4, 5, 6]

>>> matrix[2][7, 8, 9]

Access 2D list With two index, you get a item within the row:

>>> matrix[0][0]1>>> matrix[0][1]2>>> matrix[1][1]5>>> matrix[1][2]6>>> matrix[2][2]9

Len() function on Matrix len() function can tell the size of the list

By the same token:

>>>len(matrix)3>>>len(matrix[0])3

Matrix If we have another matrix looks like:>>> matrix[[10, 20, 30, 40, 50, 60, 70, 80], [11, 12, 13, 14, 15, 15],

[15, 25, 35, 45, 55, 65, 75, 85, 95]]

>>> matrix[0][10, 20, 30, 40, 50, 60, 70, 80]

>>> matrix[1][1]12

>>> matrix[0][8]error

Matrix

>>> len(matrix)3

>>> len(matrix[0])8

>>> len(matrix[1])6

>>> len(matrix[2])9

Append function If we have a matrix=[[1,2,3],[4,5,6],[7,8,9]]

after>>> matrix.append(3)

the matrix looks like:

therefore,>>> matrix[3]3

Append function If we have a matrix=[[1,2,3],[4,5,6],[7,8,9]]

after>>> matrix[1].append(3)

the matrix looks like:

therefore,>>> matrix[1][3]3

Append function If we have a matrix=[[1,2,3],[4,5,6],[7,8,9]]

after>>> matrix.append([1,2,3])

the matrix looks like:

therefore,>>> matrix[3][1,2,3]

Read File3

input=open('file3.txt','r')

matrix=[]for line in input.readlines(): matrix.append(line.split())

Print out the average score of each studentinput=open('file3.txt','r')

matrix=[]for line in input.readlines(): matrix.append(line.split())

for i in range(len(matrix)): sum=0 for j in range(len(matrix[i])): sum+=int(matrix[i][j]) avg=sum/len(matrix[i]) print avg

Write the average score of each student to fileinput=open('file3.txt','r')output=open('avg.txt','w')

matrix=[]for line in input.readlines(): matrix.append(line.split())

for i in range(len(matrix)): sum=0 for j in range(len(matrix[i])): sum+=int(matrix[i][j]) avg=sum/len(matrix[i]) print avg output.write(str(avg)+'\n')

input.close()output.close()