Chapter 4 Flashcards
T or F : An integer can’t be factored into more primitive parts
True
What is a string
A data structure which consists of smaller pieces of data
What is the form of a subscript operator
“String”[integer operator- position of a particular character in the string]
Ie:
»> name = “Alan Turing”
»> name[0] # Examine the first character
‘A’
subscript operator in loops: count- controlled loop
IN: data = “Hi there!”
IN: for index in range(len(data)):
IN: print(index, data[index])
OUT:
0 H
1 i
2
3 t
4 h
5 e
6 r
7 e
8 !
How to use a subscript operator to obtain a substring
Slicing- place colon in the subscript; an integer value can appear on either side of the colon
»> name = “myfile.txt” # The entire string
»> name[0:]
‘myfile.txt’
»> name[0:1] # The first character
‘m’
»> name[0:2] # The first two characters
‘my’
»> name[:len(name)] # The entire string
‘myfile.txt’
»> name[−3:] # The last three characters
‘txt’
»> name[2:6] # Drill to extract ‘file’
‘file’
Testing for a substring with the in operator
When used with strings, the left operand of in is a target substring and the right
operand is the string to be searched
* Returns True if target string is somewhere in search string, or False otherwise
* This code segment traverses a list of filenames and prints just the filenames
that have a .txt extension:
»> fileList = [“myfile.txt”, “myprogram.exe”, “yourfile.txt”]
»> for fileName in fileList:
if “.txt” in fileName:
print(fileName)
myfile.txt
yourfile.txt
Caesar cipher
replaces each character in plain text with a character a given distance away
How to decrypt Caesar cipher
Apply a method that uses the same distance value but looks to the left of each character for replacement value
ord and chr
ord - returns the ordinal position in the ASCII sequence
chr- is the inverse function
Block cipher
- uses plaintext character to compute 2+ encrypted characters
- uses an invertible matrix
What number system do the arithmetic operations use
Decimal number system- base 10 number system
binary number system
- used to represent info in a digital computer
- its called the base 2 number system (0 and 1)
T or F: The digits use in each system are counted from 1 to n - 1, where n is the system’s base
FALSE: The digits used in each system are counted from 0 to n − 1, where n is the system’s base
Positional notation
In positional notation, a digit has a positional value, determined by raising the base to the power specified by the position
(base^position)
converting binary to decimal
- Each digit or bit in binary number has positional value that is power of 2
- We occasionally refer to a binary number as a string of bits or a bit string
-to determine the integer quantity that a string of bits represents: multiply the value of each bit (0 or 1) by its positional value and add the results.
code of converting binary to decimal
- A positional value is computed by using the ** operator
“““
File: binarytodecimal.py
Converts a string of bits to a decimal integer.
”””
bitString = input(“Enter a string of bits: ”)
decimal = 0
exponent = len(bitString) − 1
for digit in bitString:
decimal = decimal + int(digit) * 2 ** exponent
exponent = exponent − 1
print(“The integer value is”, decimal)
Enter a string of bits: 1111
The integer value is 15
Enter a string of bits: 101
The integer value is 5
How are integers converted from decimal to binary
- One algorithm uses division and subtraction instead of multiplication and
addition - Repeatedly divides the decimal number by 2
- After each division, the remainder (either a 0 or 1) is placed at the beginning of a
string of bits - Quotient becomes the next dividend in the process
- Process continues while the decimal number is greater than 0
Convert from octal to binary
Start by assuming that each digit in the octal number represents 3 digits in the corresponding binary number
Convert binary to octal
Begin at the right and factor the bits into groups of 3 bits each
convert hex to binary
replace each hex digit with the corresponding 4 bit binary number
convert binary to hex
factor the bits into groups of 4 and look up the corresponding hex digits
What is a method
- behaves like a function
- always called with a given data value called an object
- syntax:
<an>.<method>(<argument‐1>,..., <argument‐n>)
</method></an>
T or F: In Python, there are a variety of different data values
False: In Python, all data values are objects
How do you view a complete list/documentation of string methods
enter dir(str) at a shell prompt
and enter
help(str.<method‐name>) to receive documentation on an individual method
What is a text file?
- software object that stores data on permanent medium such as disk or CD
Advantages of a text file
- data set can be much larger
- data can be input much more quickly and less chance of error
- data can be used repeatedly with the same program or with different programs
T or F: All data output to or input from a text file must be strings
True: number must be converted to string before output
Data can be output to a text file using a ___ object
file
* To open a file for output:
»> f = open(“myfile.txt”, ‘w’)
* If file does not exist, it is created
* If it already exists, Python opens it; when data are written to the file and the file is
closed, any data previously existing in the file are erased
How do you open a file for input
method read
open(filename, mode)
Opens a file at the given filename and returns a file
object. The mode can be ‘r’, ‘w’, ‘rw’, or ‘a’. The last
two values mean read/write and append
f.close()
Closes an output file. Not needed for input files.
f.write(aString)
outputs aString to a file.
f.read()
Inputs the contents of a file and returns them as a
single string. Returns “” if the end of file is reached.
f.readline()
Inputs a line of text and returns it as a string,
including the newline. Returns “” if the end of file is
reached.
Absolute pathname
when the chain starts with the root directory
relative pathname
when chain starts from the current working directory
myfile.txt
current target directory
child/myfile.txt
child target directory
../myfile.txt
parent target directory
../sibling/myfile.txt
sibling target directory
chdir(path)
Changes the current working directory to path
getcwd()
Returns the path of the current working directory
list dir(path)
Returns a list of the names in directory named
path
mkdir(path)
Creates a new directory named path and places it
in the current working directory
remove(path)
Removes the file named path from the current
working directory.
rename(old, new)
Renames the file or directory named old to new.
rmdir(path)
Removes the directory named path from the
current working directory
sep
A variable that holds the separator character (‘/’ or
‘\’) of the current file system
exists(path)
Returns True if path exists and False otherwise
isdir(path)
Returns True if path names a directory and False
otherwise
isfile(path)
Returns True if path names a file and False
otherwise.
getsize(path)
Returns the size of the object names by path in
bytes.
normcase(path)
Converts path to a pathname appropriate for the
current file system; for example, converts forward
slashes to backslashes and letters to lowercase on
a Windows system.
