The String Data Structure and Intro to for Loops

Transcription

1 The String Data Structure and Intro to for Loops June 16, 2015 Brian A. Malloy Slide 1 of 15

2 1. A string is an indexed data structure consisting of a sequence of characters The characters can be anything represented by the ASCII code The characters are stored in contiguous memory, i.e., right next to each other You can index into the string and access individual characters Example: x = cat print x[0] Slide 2 of 15

3 2. Refers to the fact that you can reference a particular character in a string using an integer called an index. Some valid indexes for cat are 0, 1, and 2 Slide 3 of 15

4 2.1. Index vs Value With strings, 2 important concepts: 1. The index 2. The value stored at that index Slide 4 of 15

5 2.2. Indexes Enable Character Access The 0 th item in a string is the first character in the string The length of a string x is len(x) The length of cat is 3 len( cat ) is 3 The length of antelope is 8 The last character in a string x is len(x)-1, because indexing starts at 0 For example, if x = cat, then x[2] is t If x = cat, then x[3] is undefined Slide 5 of 15

6 3. Indexes can be Negative The index, 1, refers to the last item in the string If x = cat, then x[ 1] is t If x = cat, then x[ 3] is c Slide 6 of 15

7 4. You can isolate parts of a string, called slice if x = Hello World, then x[0 : 5] is Hello if x = Hello World, then x[: 5] is Hello if x = Hello World, then x[6 :] is World Slide 7 of 15

8 5. Python uses the C language formatting The format specifier is % Syntax: <F ORMAT > % (<value>) Provides precise control over output Example: 1 color = blue 2 print "The dog was %s" % color 3 print "The %s dog was pretty" % color Slide 8 of 15 The dog was blue The blue dog was pretty

9 5.1. Formatting: strings, ints, floats There are 3 specifiers we might use: strings %s integers %d floats %f Can specify column width: %10i means use 10 columns to print an int right justified %-10i use 10 columns for an int left justified Compare the output for the following 4 column tables Slide 9 of 15

10 5.2. Table with no format 1 count = 0 2 print "No.", "Square", "cube", "Tenth" 3 while count < 25: 4 print count, count**2, count**3, count**10 5 count += 5 No. Square cube Tenth Slide 10 of 15

11 5.3. Table with format 1 count = 0 2 format = "%-10s %-10s %-10s %-10s" 3 print format % ("No.", "Square", "cube", "Tenth") 4 while count < 25: 5 print "%-10i %-10i %-10i %-10i" % (count, count**2, count**3, count**10) 6 count += 5 No. Square cube Tenth Slide 11 of 15

12 5.4. Intimidation Don t be intimidated by the format string that I used to specify columns Take each format one-at-a-time %10s would mean use 10 columns for a string, right justified But we want the numbers left justified, so we use: %-10s Since there are 4 columns, we repeat 4 times Slide 12 of 15

13 6. You can change the string: s = dog but cannot change individual characters Assume we write a file immutable.py: 1 x = antelope 2 x = cat 3 x[0] = b File "immutable.py", line 3, x[0] = b TypeError: str object does not support item assignment Slide 13 of 15

14 7. Built-in functions included len returns the number of characters in the string Comparison operators: bat < cat Conversion: s = 747 x = int(s) Slide 14 of 15

15 8. We can always iterate with a while loop because while is the most general loop form. for loops are more convenient than while, but we cannot always use for. Let s compare for and while; both do the same thing: (1) s = banana s = banana (2) count = 0 for x in s: (3) while count < len(s): print x (4) print s[count] (5) count += 1 Slide 15 of 15