Revenge of the Strings
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And now presenting a cool trick that can be done with strings:
def shout(string): for character in string: print "Gimme a "+character print "'"+character+"'" shout("Lose") def middle(string): print "The middle character is:",string[len(string)/2] middle("abcdefg") middle("The Python Programming Language") middle("Atlanta")
And the output is:
Gimme a L 'L' Gimme a o 'o' Gimme a s 's' Gimme a e 'e' The middle character is: d The middle character is: r The middle character is: aWhat these programs demonstrate is that strings are similar to lists in several ways. The shout procedure shows that for loops can be used with strings just as they can be used with lists. The middle procedure shows that that strings can also use the len function and array indexes and slices. Most list features work on strings as well.
The next feature demonstrates some string specific features:
def to_upper(string): ## Converts a string to upper case upper_case = "" for character in string: if 'a' <= character <= 'z': location = ord(character) - ord('a') new_ascii = location + ord('A') character = chr(new_ascii) upper_case = upper_case + character return upper_case print to_upper("This is Text")with the output being:
THIS IS TEXTThis works because the computer represents the characters of a string as numbers from 0 to 255. Python has a function called ord (short for ordinal) that returns a character as a number. There is also a corresponding function called chr that converts a number into a character. With this in mind the program should start to be clear. The first detail is the line:
if 'a' <= character <= 'z':which checks to see if a letter is lower case. If it is than the next lines are used. First it is converted into a location so that a=0,b=1,c=2 and so on with the line:
location = ord(character) - ord('a'). Next the new value is found with
new_ascii = location + ord('A'). This value is converted back to a character that is now upper case.
Now for some interactive typing exercise:
>>> #Integer to String ... >>> 2 2 >>> repr(2) '2' >>> -123 -123 >>> repr(-123) '-123' >>> #String to Integer ... >>> "23" '23' >>> int("23") 23 >>> "23"*2 '2323' >>> int("23")*2 46 >>> #Float to String ... >>> 1.23 1.23 >>> repr(1.23) '1.23' >>> #Float to Integer ... >>> 1.23 1.23 >>> int(1.23) 1 >>> int(-1.23) -1 >>> #String to Float ... >>> float("1.23") 1.23 >>> "1.23" '1.23' >>> float("123") 123.0
If you haven't guessed already the function
repr can convert a integer to a string and the function
int can convert a string to an integer. The function float can convert a string to a float. The
repr function returns a printable representation of something. Here are some examples of this:
>>> repr(1) '1' >>> repr(234.14) '234.14' >>> repr([4,42,10]) '[4, 42, 10]'The
intfunction tries to convert a string (or a float) into a integer. There is also a similar function called
floatthat will convert a integer or a string into a float. Another function that Python has is the
evalfunction takes a string and returns data of the type that python thinks it found. For example:
>>> v=eval('123') >>> print v,type(v) 123 <type 'int'> >>> v=eval('645.123') >>> print v,type(v) 645.123 <type 'float'> >>> v=eval('[1,2,3]') >>> print v,type(v) [1, 2, 3] <type 'list'>If you use the
evalfunction you should check that it returns the type that you expect.
One useful string function is the
split function. Here's the example:
>>> import string >>> string.split("This is a bunch of words") ['This', 'is', 'a', 'bunch', 'of', 'words'] >>> string.split("First batch, second batch, third, fourth",",") ['First batch', ' second batch', ' third', ' fourth']Notice how
splitconverts a string into a list of strings. The string is split by spaces by default or by the optional second argument (in this case a comma).
#This program requires a excellent understanding of decimal numbers def to_string(in_int): "Converts an integer to a string" out_str = "" prefix = "" if in_int < 0: prefix = "-" in_int = -in_int while in_int / 10 != 0: out_str = chr(ord('0')+in_int % 10) + out_str in_int = in_int / 10 out_str = chr(ord('0')+in_int % 10) + out_str return prefix + out_str def to_int(in_str): "Converts a string to an integer" out_num = 0 if in_str == "-": multiplier = -1 in_str = in_str[1:] else: multiplier = 1 for x in range(0,len(in_str)): out_num = out_num * 10 + ord(in_str[x]) - ord('0') return out_num * multiplier print to_string(2) print to_string(23445) print to_string(-23445) print to_int("14234") print to_int("12345") print to_int("-3512")
The output is:
2 23445 -23445 14234 12345 -3512
Next: File IO Up: Non-Programmers Tutorial For Python Previous: More on Lists Contents Josh Cogliati email@example.com Wikibooks Version: Wikibooks Non-programmers Python Tutorial Contents