Python Strings
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| string definition | denoted str
used to represent and manipulate text data or, a sequence of characters, including blanks, punctuation, and various symbols.
A string value is represented as a sequence of characters that is enclosed within "quotes".
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| string operators | ==, !=, <, >, and so on
returns True/False
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| addition | makes new string
when applied to two strings, evaluates to a new string that is the concatenation (i.e., the joining) of the two strings
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| multiply | 'string' * 'string' = error
'string' * 2 = 'stringstring'
'string ' * 2 = 'string string'
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| length of string | len(s)
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| Character of string s at index i | s[i]
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| Concatenation of n copies of s | s*n
n*s
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| Concatenation of string s and string t | s + t
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| False if string x is a substring of string s, and true otherwise | x not in s
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| True if string x is a substring of string s, and false otherwise | x in s
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| indexing operator [] | takes a nonnegative index i and returns a string consisting of the single character at index i
Negative indexes access the characters from the back (right side) of the string. For example, the last character can be retrieved using negative index -1.
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| index | the character's offset (i.e., position in the string) with respect to the first character.
The first character has index 0, the second has index 1 (because it is one away from the first character), the third character has index 2, and so on.
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| Can you change a string? | NO. Strings are immutable. Can make new string only.
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| forgetting quote delimiters | no quotes, text will be treated as a name.
>>> hello
Traceback (most recent call last):
File “<pyshell#35>”, line 1, in <module>
hello
NameError: name ‘hello’ is not defined
Error message reports that name hello is not defined.
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| What to do if string text contains quotes | If the text contains a single quote, we can use double quote delimiters, vice versa.
If the text contains both quote types, then the escape sequence \’ or \” is used to indicate that a quote is not the string delimiter but is part of the string value.
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| How do I not have the escape sequence show up? | the print() function will interpret any escape sequence in the string and omit the string delimiters
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| escape sequence | in a string is a sequence of characters starting with a \ that defines a special character and that is interpreted by function print()
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| multi-line strings | use triple quotes '''
encode the new line characters explicitly
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| new line character | The escape sequence \n stands in for a new line character, also When it appears in a string argument of the print() function, the new line escape sequence \n starts a new line
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| slice | The expression s[0:2] evaluates to the slice of string s starting at index 0 and ending before index 2.
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| slice containing first or last character | first s[:3]
last s[2:]
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| string method s.find() | When it is invoked on string s with one string input argument target, it checks whether target is a substring of s.
it returns the index (of the first character) of the first occurrence of string target;
otherwise, it returns -1.
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| string method s.count() | The method count(), when called by string s with string input argument target, returns the number of times target appears as a substring of s.
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| string method s.replace(a, b) | The function replace(), when invoked on string s, takes two string inputs, old and new, and outputs a copy of string s with every occurrence of substring old replaced by string new.
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| replace (a, b) and assignment statement | the string is not changed (immutable) by replace.
to use the replace changes must assign string to something
ergo, make new string
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| string method s.capitalize () | Method capitalize(), when called by string s, makes the first character of s uppercase;
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| string method s.upper() | method upper() makes all the characters uppercase.
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| string method s.split() | the method split() uses the blank spaces in string ‘this is the text’ to create word substrings that are put into a list and returned.
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| split() with delimiter | The delimiter string is used in place of the blank space to break up the string. For example, to break up the string
>>> x = ‘2;3;5;7;11;13’
into a list of number, you would use ‘;’ as the delimiter:
>>> x.split(‘;’)
[‘2’, ‘3’, ‘5’, ‘7’, ‘11’, ‘13’]
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| string method translate() | mapping is constructed using a table that is called not by the string class str itself:
>>> table = str.maketrans(‘abcdef’, ‘uvwxyz’)
>>>‘fad’.translate(table)
‘zux’
ch6
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| string method s.lower() | A copy of string s converted to lowercase
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| string method s.strip() | A copy of string s with leading and trailing blank spaces removed
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| print() | a function used to print values onto the screen.
input is an object
output it prints a STRING REPRESENTATION of the object's value.
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| print() default separater | takes an arbitrary number of input objects, not necessarily of the same type. The values of the objects will be printed in the same line, and blank spaces (i.e., characters ‘ ’) will be inserted between them as a default separater.
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| print() defined separater | The print() function takes an optional separation argument sep:
>>> print(n, r, name, sep=‘;’)
5;1.66666666667;Ida
The argument sep=‘;’ specifies that semicolons should be inserted to separate the printed values.
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| how separater works | In general, when the argument sep=<some string> is added to the arguments of the print() function, the string <some string> will be inserted between the values.
common used for (;), (\n)
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| end=<some string> | end=<some string>
string <some string> is printed after all the arguments have been printed,
eliminating the implied \n
>> for name in [‘Joe’, ‘Sam’, ‘Tim’, ‘Ann’]:
print(name, end=‘! ’)
Joe! Sam! Tim! Ann!
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| printing using format () | >>> hour = 11
>>> minute = 45
>>> second = 33
>>> ‘{0}:{1}:{2}’.format(hour, minute, second)
‘11:45:33’
characters outside {} are printed as is.
{0}, {1}, and {2} are placeholders where the objects will be printed
no {number}, defaults L2R
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| lining up data | >>> ‘{0:3},{1:5}’.format(12, 354)
‘ 12, 354’
The 0 refers to the first argument of the format() function (12), In this case, 3 indicates that the width of the placeholder should be 3.
right justified
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| precision | >>> ‘{:8.4}’.format(1000 / 3)
‘ 333.3’
Compare this with the unformatted output:
>>> 1000 / 3
333.3333333333333
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| type format binary | Outputs the number in binary
>>> n = 10
>>> ‘{:b}’.format(n)
‘1010’
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| type format unicode | >>> n = 10
>>> ‘{:c}’.format(n)
‘\n’
c Outputs the Unicode character corresponding to the integer value
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| type format decimal | >>> n = 10
>>> ‘{:d}’.format(n)
‘10’
d Outputs the number in decimal notation (default)
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| type format base 16 lowercase | >>> n = 10
>>> ‘{:x}’.format(n)
‘a’
x Outputs the number in base 16, using lowercase letters for the digits above 9
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| type format base 16 uppercase | >>> n = 10
>>> ‘{:X}’.format(n)
‘A’
X Outputs the number in base 16, using uppercase letters for the digits above 9
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| type format base 8 | o Outputs the number in base 8
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