Strings

1. Strings

2. Strings • A string is a sequence of characters. • A string can have 0 characters (“”) or it can be as big as the memory on the computer running your program.

3. Character Sets • There are 2 basic sets of characters in the computer world • ASCII • 8 bits • size of set is 2 ** 8 • big enough • Unicode • 32 bits • Size of set is 2 ** 32 • big!!!!!

4. ASCII character set • http://www.asciitable.com/ • Unicode is pretty complex. • don't go there until you have to.

5. Assigning Strings to Variables • Make a variable hold a string by enclosing the characters to be assigned in double or single quotes. e.g. userName = “billy” • Note that: aNumber = “129” • Is not the same as aNumber = 129

6. Converting Strings • Convert a string to an int aNumber = int(aString) or aNumber = int(“562”) • Convert a int to a string aString = str(aNumber) • There's a float()version of int(), too

7. Indexing Strings • You can pick out individual characters from a string. • >>> someStr = "Billy" • >>> print someStr[0] • B • >>> print someStr[1] • i • >>> print someStr[2] • l • >>> print someStr[3] • l • >>> print someStr[4] • y • >>> print someStr[5] • Traceback (most recent call last): • File "<pyshell#30>", line 1, in <module> • print someStr[5] • IndexError: string index out of range • >>>

8. Indexing Strings • String indexes start at 0 (not at 1) • If you try to access a string at an index that is too high, you generate an IndexError.

9. String Formatting • Format Specifiers: special character sequences inside strings that are meant to be replaced by other strings or floats or integers • fName = “Billy” • print “Hello %s” % fName • >>> Hello Billy

10. String Formatting (cont’d) This thing is called a tuple • lName = Billson • print “Hello %s %s” % (fName, lName) • >>> Hello Billy Billson • The format specifiers are like placeholders which are replaced by the variables inside the brackets

11. String Formatting (cont’d) • The order of the variables in the tuple controls the order in which they appear in the string • print “Hello %s %s” % (lName, fName) • >>> Hello Billson Billy

12. String Formatting (cont’d) • The items in the tuple don’t have to be variables • print “Hello %s %s” % (“Mr.”, “Mac”) • >>> Hello Mr. Mac • …but they almost always are…

13. String Formatting (cont’d) • There are format specifiers for integers as well. • age = 1052 • print “%s %s is %d years old” % \ (“Mr.”, “Mac”, age) • >>> Mr. Mac is 1052 years old • For integers we use %d

14. String Formatting (cont’d) • There are format specifiers for float/decimal values too • discount = 1052.97 * .094538 • print “Discount: $%f” % discount • >>> Discount: $99.545678 • Since this represents money, 2 decimal places would be better: • print “Discount: $%.2f % discount • >>> Discount $99.55

15. String Formatting (cont’d) • We can also add padding to a string • >>>print "X%20sX" % fName • X billyX • Python adds enough spaces to make fName be twenty chars wide • fName is right-justified in this example. Want left-justified? Use a ‘-’ sign. • >>> print “X%-20sX” % fName • Xbilly X

16. String Methods • The following methods can be used on any string. Assume we have a string called s. • s.center(n) • produces a new string with s centered in a field of width n. The field is padded with spaces if n is greater than the length of s. • >>> knight = "Nee“ • >>> print "X%sX" % knight.center(10) • X Nee X

17. Stripping White Space • s.strip() returns s with any leading or trailing whitespace (tabs, newlines, or spaces) removed • s.lstrip() – as above but only the leading whitespace is removed • s.rstrip() – as above but only the trailing whitespace is removed

18. strip() examples • >>> knight = " Nee " • >>> print "X%sX" % knight.strip() • XNeeX • >>> print "X%sX" % knight.lstrip() • XNee X • >>> print "X%sX" % knight.rstrip() • X NeeX • >>>

19. Lower & Upper Case • s.lower() and s.upper() convert s to lower and upper case, respectively • >>> print name.upper() • BILLY • >>> print name.lower() • billy • >>>

20. Ex2.6 - alignIt • Ask the user for 5 first and last names. • Use string formatting to print out the first and last names (one per line) so that the last names are all lined up (table format) underneath each other. • Billy Billson • Margaret Wilson • Dmitry Dillson • Kay Killson • Yolanda Yillson

21. Ex 2.7 boxIt.py • Pgm asks user to put in 5 strings • Pgm converts strings to upper case • Pgm strips any leading or trailing whitespace • Pgm then prints all 5 strings left justifed in a box of asterisks. • Box must have a row of asterisks at the top and at the bottom.

22. Ex 2.4a boxIt.py (advanced)IGNORE this PLS • Talk to me for some extras needed for this exercise. (It’s complicated!) • User types in 5 strings • Pgm loops until user enters 5 unique strings • Unique means regardless of whitespace or capitalization • Pgm then prints all 5 strings left justifed in a box of asterisks. • Box must have a row of asterisks at the top and at the bottom.

23. Escape sequences • Putting a backslash “\” before some characters gives them special meanings. • “\n” - means print a new line • “\t” - means print a tab >>> print "Billy is a\nbad,bad\t\t\tboy" • Billy is a • bad,bad boy • >>>

24. More escape sequences • If “\” has a special meaning when it’s in front of an “n” or a “t” (& other things), how do we actually print a “\”? • >>> print "Billy is a \\nasty boy" • Billy is a \nasty boy • >>>

25. More escape sequences • Use \’ to print a single quote • Use \” to print a double quote • >>> print "Billy\'s a \"rude\" boy" • Billy's a "rude" boy • >>>