ECS15, Lecture 13. Midterm solutions posted. Topic 4: Programming in Python, cont. Staying up to speed with Python RESOURCES. More Python Resources

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1 Midterm solutions posted ECS15, Lecture 13 Topic 4: Programming in Python, cont. Don t forget, you have one week since the material returned to request a re-grade for whatever reason. We will re-evaluate your work (potentially deducting points for misgrade that worked in your favor). You have until Tues, Feb 24 to request a regrade. (Submit your exam with a cover letter explaining the issue.) Staying up to speed with Python RESOURCES Python Primer, Introduction. Python Primer, Chap 1, Chap 2, Chap 3. Links to simple example programs (under Lectures tab of course homepage.) Python Programming for the Absolute Beginner (the recommended text). More Python Resources the command line help function: >>> help() Search the web!!! e.g. sort in python e.g. NameError: name 'e' is not defined (in other words, type in the error message to a search engine).

2 Variable: an object you define who s value may change Types of variables int float str Functions Highlights/Review: Typically take an argument Nested functions: e.g., float(raw_input( Enter a real number\n )) Methods (Functions that operate on particular variable types) Highlights/Review: Syntax Syntax: The proper grammer (if not grammatically correct, the code cannot be understood). Be careful with use of single and double quotes. Though they are often interchangeable: print test phrase print test phrase Don t forget Python is case sensitive myexamplevariable not equal MyExampleVariable Variables can be a scalar or an array Scalar (left) Array (right) scalar: a single element s = 10 # a scalar of type int s = This is a phrase # a scalar of type str * array: a collection of items of the same type, and where the elements of the collection are variable (changeable). s = [1, 5, 10, 9, 7] # array of type int s = [ This, is, a, phrase ] # array of type str s = [ This is a, phrase ] # array of type str lists: very similar to array (python considers a str as list of letters; len(s) returns the length of the phrase s.) We will explore the difference between lists and arrays shortly.

3 Tuples: Note: the structure tuple is like an array, but immutable (contents can t be changed unless the whole list/tuple redefined). This is like ROM. In a list or array can change each element, e.g. array[i]=2. We will likely not need tuples but they are covered in Chap 2 of Primer. Highlights/Review: Methods Methods: functions that can be invoked by different types of objects, e.g. (from Chap 1 of Primer): >>> exstring = This is a test string exstring.strip() exstring.lower() exstring.upper() exstring.replace( sub1, sub2,n) (Note: >>> help(str) lists all built in string methods) You will need some of these methods for Lab 6 Example program: ENCRYPTION With all the private data streaming over and web forms we want a cypher that encrypts a string. # This program converts all A s in a string to 1 s, # B s to 2 s and C s to 3 s s = raw_input( Enter a phrase: \n ) m = s.replace( A, 1 ) m = m.replace( B, 2 ) m = m.replace( C, 3 ) Primer Chap 1: Indexing and slicing strings String is considered a list of length m = len(str) s = This is an example string. s[0] = T s[1] = h s[4] = s[-1] =. s[-2] = g # the blank space # negative index reads from the right This may also be useful for Lab 6

4 Selecting a subset from a string: Let the variable s be a scalar of type str The function len(s) returns the length of s s[i:j:k] # start at position i, up to (but not including) j, in increments of k. What happens if you type >>> s[len(s)] s[i:j:] default) s[::] # the same as s[i:j] and as s[i:j:1] (k=1 by # selects to whole string What does this following command do? (Recall s[::] selects the whole string.) s = This is an example string. >>> b = s[::-1] >>> print b.gnirts elpmaxe na si siht b = s[::-1]

5 Also in Chapter 2 What does the following assign to b? (# if not possible returns empty list) >>> b = s[3:20:2] >>> b = s[3:20:-2] >>> b = s[20:3:-2] In other words, be careful: if s[i:j:k] make sure j > i if s[i:j:-k] make sure j < i Another useful general function: range() >>>"range(10)" [0,1,2,3,4,5,6,7,8,9]" >>>"range(10,21)" [10,11,12,13,14,15,16,17,18,19,20]" >>>"range(1,10,2)" [1,3,5,7,9]" (As with string slicing str[1:20] selects elements 1 to 19) Converting a string to a list A string is a list of individual characters: Easy: Reversing the string:.gnirts elpmaxe na si siht But what if want to reverse word order? string. example an is This (By default python considers a string a list of characters) (Converting a list to a string in Chap 2) Using the split() method that operates on strings. exstring = This is a test string exstring.split() = [ This, is, a, test, string ] (Note len(exstring) = 21, len(exstring.split()) = 5) Need to group the string into words not letters! (converting strings to lists and lists to strings next) split(sep), sep = (blank space) by default. exstring.split( ) = [ This, is, a, test, string ] exstring.split( i ) = [ Th, s, s a test str, ng ]

6 Using Different Separator Characters. So, now reversing the word order is easy! exstring = This is a test string. m = exstring.split() = [ This, is, a, test, string. ] m[::-1] = ['string.', test', a', 'is', 'This'] (Note indexing and slicing lists analogous to strings) paragraph = Here is an example of a string with multiple sentences. Multiple sentences grouped together are defined as a paragraph in the English language. Let s break the paragraph down into different granularity. >>> len(paragraph) # Returns the number of characters in the whole statement in quotes. (paragraph is considered a list of individual characters.) sentence = paragraph.split(. ) # period separates We could do something more sophisticated using loops. Chapter 3. words = paragraph.split( ) # blank space separates Different Separator Characters; different granularity. Highlights, Primer Chap 2 len(paragraph) # returns the number of characters in the full paragraph. (Recall a str is a list of characters by default in python.) sentence = paragraph.split(. ) len(sentence) # returns number of sentences # i.e. sentence[0] is the first sentence of the paragraph. # i.e. sentence[1:3:] extracts the second and third sentences. words = paragraph.split( ) len(words) # returns the number of words. # i.e. words[1] is the second word of the paragraph # i.e. words[-1] is the last word of the paragraph Chap 2: More on tuples, lists, arrays and dictionaries array manipulation, sub-setting and indexing is essentially identical to string indexing and slicing (a few slides back) See Chap 2, 4.3 for a table of methods for arrays (sorting, extending, inserting, length, number of occurrences.)

7 Converting a list to a string (Recall exstring.split(sep) converts a string to a list.) converting a list to a string >>> examplestring=.join(examplelist) exlist = [ This, is, an, example ].join(exlist) returns Thisisanexample.join(exList) returns This is an example 1.join(exList) returns This 1is 1an 1example General use: insertbetweenelements.join(list) And note the unconventional syntax for this method Additional useful string and list functions Let exstring = This is an example string Counting number of characters (including white space): len(exstring) Counting number of words: len(exstring.split()) Counting number of white spaces: exstring.count( ) Counting number of for instance a s exstring.count( a ) More highlights, Primer Chap 2 Types of command flow in computer programs: Chap 2: More on tuples, lists, arrays and dictionaries array manipulation, sub-setting and indexing is essentially identical to string indexing and slicing (a few slides back) See Chap 2, 4.3 for a table of methods for arrays (sorting, extending, inserting, length, number of occurrences.) With a little sophistication we can often turn sequential into a looping (repeating) set of code instructions: The IF, FOR, and WHILE control statements next.

8 Conditional control statements (switches and circuits) need Logical operators. Table 3.1 in primer. Conditional structures, Chap 3.3 >>>"if"condition:" """"""""""""""""""code"block"to"execute" """""""which"can"be"on"multiple"lines" """""""but"with"each"line"indented" Can combine into compound operators. Valid comparisons: (Fine print: variables being compared should be of same type.) (a==b) and (b < c) NOT[(a==b) and (b < c)] (remember the midterm EC) Note the SYNTAX! Must have a : at end of the condition Each line to be executed if the condition holds must be indented IDLE environment will automatically indent the next line when following a : Putting it all together: Example: sort a list of numbers nums = [3,6,10,7,5,2] Any ideas on the approach? let s write it in psuedocode How many operations does it take?