String operators and methods in Java

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String operators and methods
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s[::-2] == 'do ea t h'
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s[::3] == 'ha m o'
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Figure 23 Sequence striding
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It is also possible to combine slicing indexes with striding, as Figure 24 illustrates
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s[-1:2:-2] == s[:2:-2] == 'do ea t'
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s[0:-5:3] == s[:-5:3] == 'ha m'
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Figure 24 Sequence slicing and striding
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Striding is most often used with sequence types other than strings, but there is one context in which it is used for strings:
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>>> s, s[::-1] ('The waxwork woman', 'namow krowxaw ehT')
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Stepping by -1 means that every character is extracted, from the end back to the beginning and therefore produces the string in reverse
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String Operators and Methods
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Iterable operators and functions
Since strings are immutable sequences, all the functionality that can be used with immutable sequences can be used with strings This includes membership testing with in, concatenation with +, appending with +=, replication with *, and augmented assignment replication with *= We will discuss all of these in the context of strings in this subsection, in addition to discussing many of the string methods Tables 28, 29, and 210 summarize all the string methods, except for two rather specialized ones (strmaketrans() and strtranslate()) that we will brie y discuss further on As strings are sequences they are sized objects, and therefore we can call
len() with a string as the argument The length returned is the number of
140
Sized
characters in the string (zero for an empty string) We have seen that the + operator is overloaded to provide string concatenation In cases where we want to concatenate lots of strings the strjoin() method
383
2 Data Types
offers a better solution The method takes a sequence as an argument (eg, a list or tuple of strings), and joins them together into a single string with the string the method was called on between each one For example:
>>> treatises = ["Arithmetica", "Conics", "Elements"] >>> " "join(treatises) 'Arithmetica Conics Elements' >>> "-<>-"join(treatises) 'Arithmetica-<>-Conics-<>-Elements' >>> ""join(treatises) 'ArithmeticaConicsElements'
The rst example is perhaps the most common, joining with a single character, in this case a space The third example is pure concatenation thanks to the empty string which means that the sequence of strings are joined with nothing in between The strjoin() method can also be used with the built-in reversed() function, to reverse a string, for example, ""join(reversed(s)), although the same result can be achieved more concisely by striding, for example, s[::-1] The * operator provides string replication:
>>> s = "=" * 5 >>> print(s) ===== >>> s *= 10 >>> print(s) ==================================================
As the example shows, we can also use the augmented assignment version of the replication operator When applied to strings, the in membership operator returns True if its lefthand string argument is a substring of, or equal to, its right-hand string argument In cases where we want to nd the position of one string inside another, we have two methods to choose from One is the strindex() method; this returns the index position of the substring, or raises a ValueError exception on failure The other is the strfind() method; this returns the index position of the substring, or -1 on failure Both methods take the string to nd as their rst argument, and can accept a couple of optional arguments The second argument is the start position in the string being searched, and the third argument is the end position in the string being searched
Strings also support the % operator for formatting This operator is deprecated and provided only to ease conversion from Python 2 to Python 3 It is not used in any of the book s examples
Strings
Table 28 String Methods #1
Syntax scapitalize() scenter(width, char) scount(t, start, end) sencode( encoding, err) sendswith(x, start, end) sexpandtabs( size) sfind(t, start, end) sformat()
Description
Returns a copy of str s with the rst letter capitalized; see also the strtitle() method Returns a copy of s centered in a string of length width padded with spaces or optionally with char (a string of length 1); see strljust(), strrjust(), and strformat() Returns the number of occurrences of str t in str s (or in the start:end slice of s) Returns a bytes object that represents the string using the default encoding or using the speci ed encoding and handling errors according to the optional err argument Returns True if s (or the start:end slice of s) ends with str x or with any of the strings in tuple x; otherwise, returns False See also strstartswith() Returns a copy of s with tabs replaced with spaces in multiples of 8 or of size if speci ed Returns the leftmost position of t in s (or in the start:end slice of s) or -1 if not found Use strrfind() to nd the rightmost position See also strindex() Returns a copy of s formatted according to the given arguments This method and its arguments are covered in the next subsection Returns the leftmost position of t in s (or in the start:end slice of s) or raises ValueError if not found Use strrindex() to search from the right See strfind() Returns True if s is nonempty and every character in s is alphanumeric Returns True if s is nonempty and every character in s is alphabetic Returns True if s is nonempty and every character in s is a Unicode base 10 digit Returns True if s is nonempty and every character in s is an ASCII digit Returns True if s is nonempty and is a valid identi er Returns True if s has at least one lowercaseable character and all its lowercaseable characters are lowercase; see also strisupper()
str format()