ECS15, Lecture 14. Reminder: how to learn Python. Today is the final day to request a regrade of the midterm. Topic 4: Programming in Python, cont.

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1 ECS15, Lecture 14 Today is the final day to request a regrade of the midterm Topic 4: Programming in Python, cont. Turn in a short cover letter explaining the issue. Reminder: how to learn Python Type the code into IDLE and see what happens! You have to play with code to learn how to write proper code. RUN your programs & TEST OUT EACH LINE OF YOUR CODE!!!!!! Chap 1 on manipulating strings: Python treats a string as a list of individual characters: Easy: Reversing the string:.gnirts elpmaxe na si siht But what is want to reverse word order? string. example an is This Need to group the string into words not letters!

2 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 Selecting a subset from a string: s[i:j:k] # start at position i, up to (but not including) j, in increments of k. s[i:j:] # the same as s[i:j] and as s[i:j:1] (k=1 by default) s[::] # selects to whole string s = This is an example string. >>> b = s[::-1] >>> print b.gnirts elpmaxe na si siht Almost equivalent: b = s[len(s):0:-1] Explain the difference. What does the following assign to b? (# if not possible returns empty list) >>> b = s[3:20:2] >>> b = s[3:20:-2] (Not possible) >>> b = s[20:3:-2] But first: converting strings to lists** The split() string method Let exstring = This is a test string exstring.split() = [ This, is, a, test, string ] exstring.split( i ) = [ Th, s, s a test str, ng ] ** The value of an element in a list is mutable (can be updated). But the value of an element in a string is immutable (cannot be updated). ** Try s[1] = A for s of type str. Try s[1] = A when s is a well defined list, like s=[1, 5, 7, 10].

3 Now: converting lists to strings The.join() list method exlist = [ This, is, an, example ] So, now reversing the word order is easy! exstring = This is a test string. m = exstring.split() = [ This, is, a, test, string. ].join(exlist) returns Thisisanexample.join(exList) returns This is an example 1.join(exList) returns This 1is 1an 1example m[::-1] = ['string.', test', a', 'is', 'This'] (Note indexing and slicing lists analogous to strings) General use: insertbetweenelements.join(list) We could do something more sophisticated using loops. Chapter 3 In just a few slides! Using Different Separator Characters. 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.) Different Separator Characters; different granularity. 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. sentence = paragraph.split(. ) words = paragraph.split( ) # period separates # blank space separates 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

4 Scalar (left) Array (right) We now have two INDEPENDENT attributes for any variable 1) Type: str, int, or float 2) Scalar versus array/list Can have an array of int items Can have an array of float items Can have an array of str items samplearray = [item0, item1, item2, itemm] len(samplearray) = M samplearray[i] = itemi Examples 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 actually considers a str as list of letters; len(s) returns the length of the phrase s.) Highlights, Primer Chap 2 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.)

5 Types of command flow in computer programs: Conditional control statements (switches and circuits) need Logical operators. Table 3.1 in primer. 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. 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)] Conditional structures, Chap 3.3 >>>"if"condition:" """"""""""""""""""code"block"to"execute" """""""which"can"be"on"multiple"lines" """""""but"with"each"line"indented" 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 : Example of an if block if ((a == b) or (b > 10)): print This was an example of a simple if print We now know that either a==b or b > 10 raw_input( type enter to exit ) Print This line is outside the if statement and always prints. If the condition holds, then statement is true and executes.

6 if else block: Testing multiple conditions! Simple example of if else construction if""condition:" "block"code"1" "can"be"on"multiple"indented"lines" else:" "block"code"2" "can"also"be"on"multiple"(indented)"lines" gender= raw_input( Please enter your gender (M or F): ) if gender== F : print You have indicated you are female else: print You have indicated you are male (Note the : also needed after else ) (Note the : also needed after else ) Simple example of if elif else construction (elif is an abbreviation for else if ) LOOPS: Repeating instructions year = raw_input( Please indicate whether you are a Freshman, Sophmore, Junior or Senior: ) if year== Freshman : print Welcome to UC Davis. elif year== Sophmore : print Welcome back to UC Davis elif year== Junior : print You are a veteran by now else: print You must be a senior Repeat: for each element in a set or while a conditional statement is not yet satisfied.

7 Use of for for"eachvariable"in"lista:"" "code"block" # Program 1 Simple example programs with for loops # Print the numbers 0 to 9 for j in range(10): print j Again note syntax: : after the for statement all lines to execute need to be indented # Program 2 # Sum the numbers 0 to 9 and store as a float sum = float(0) for j in range(10): sum = sum + j # could also say sum += j print j, intermediate sum is, sum \n print The sum is, sum, \n More simple example programs with for loops (using strings and lists/arrays) # Print all the characters in a word on their own line exword = testing for j in exword: print j # Print each word in an array of words on a unique line exlist = [ This, is, an, example ] for j in range(len(exlist)): # Note the use of len() in range()! print exlist[j] print The for loop terminated, \n In pseudocode: While loop exvariable = 20 while (exvariable < 100): dosomething #maybe print In the loop\n exvariable = new_value #from raw_input() # or from a logic # or arithmetic update

8 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?