Iteration Part 2. Review: Iteration [Part 1] Flow charts for two loop constructs. Review: Syntax of loops. while continuation_condition : statement1

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1 Review: Iteratin [Part 1] Iteratin Part 2 CS111 Cmputer Prgramming Department f Cmputer Science Wellesley Cllege Iteratin is the repeated executin f a set f statements until a stpping cnditin is reached. while lps are an iteratin cnstruct used when it is nt knwn in advance hw lng executin shuld cntinue. fr lps (an abstractin f while lps) are used when we have a fixed set f items in a sequence t iterate ver. If the stpping cnditin is never reached, the lp will run frever. It is knwn in this case as an infinite lp. The stpping cnditin might invlve ne r mre state variables, and we need t make sure that the bdy f the lp cntains statements that cntinuusly update these state variables. We can use the mdel f iteratin tables t understand the inner wrkings f a lp. Its clumns represent the state variables and the rws represent their values in every iteratin Review: Syntax f lps while cntinuatin_cnditin : Cmparing the syntax f bth lp cnstructs. a blean expressin denting whether t iterate thrugh the bdy f the lp ne mre time. Flw charts fr tw lp cnstructs while cntinuatin _cnditin while lp bdy A variable that takes its values frm the items f the sequence. fr var in sequence: A sequence f items: characters in a string, items in a list, the result f range, etc. Still elements in sequence fr lp bdy fr

2 Review: sumbetween with while lp In[6]: sumbetween(4,8) Out[6]: 30 # sumbetween(4,8) returns 30 sumbetween(4,4) returns 4 sumbetween(4,3) returns 0 Using the iteratin table t reasn abut a prblem. step l hi sumsfar def sumbetween(l, hi): '''Returns the sum f the integers frm l t hi (inclusive). Assume l and hi are integers.''' sumsfar = 0 initialize accumulatr T ntice: while l <= hi: Rw 0 in the table shws the sumsfar += l initial values f all state l += 1 update accumulatr variables. return sumsfar Rw 1 shws values after the updates in the lp bdy. ccumulatr 11-5 Tday s tpics Nested fr lps Hw t interrupt lps with cde? Swapping tw variable values Simultaneus assignment in Pythn 11-6 Nested lps fr printing A fr lp bdy can cntain a fr lp. Outer lp fr i in range(2, 6): fr j in range(2, 6): print i, 'x', j, '=', i*j Tw nested lps: the uter and inner lp. Inner lp # print the multiplicatin table frm 2 t 5 2 x 2 = 4 2 x 3 = 6 T ntice: 2 x 4 = 8 Variable i in the uter lp is set initially t value 2. 2 x 5 = 10 Variable j in the inner lp is set initially t value 2. 3 x 2 = 6 Variable j keeps changing its value: 3, 4, 5, meanwhile i desn t change. 3 x 3 = 9 When i becmes 3, j restarts its cycle: 2, 3, 4,5, and s 3 x 4 = 12 it repeats, until i has taken values ver all items f the... list 2, 3, 4, Nested lps fr accumulatin def isvwel(char): return char.lwer() in 'aeiu' verse = "Tw rads diverged in a yellw wd" fr wrd in verse.split(): cunter = 0 fr letter in wrd: if isvwel(letter): cunter += 1 print 'Vwels in', wrd, '->', cunter Vwels in Tw -> 1 Vwels in rads -> 2 Vwels in diverged -> 3 Vwels in in -> 1 Vwels in a -> 1 Vwels in yellw -> 2 Vwels in wd -> 2 Using nested lps fr successive accumulatins. T ntice: The accumulatr variable cunter is set t 0 every time the inner lp starts. Outer lp iterates ver a list f wrds. Inner lp iterates ver characters in a string. 11-8

Flw Chart fr nested fr lps Exercise: print wrds What is printed?

4-9 11-9 11-10 Exercise: Nested Lps with graphics Here's a picture invlving a grid f randmly clred circles with radius = 50 n a 800x600 canvas.

(50, 50) (150, 50) (250, 50) Interrupting Lps Smetimes we want t interrupt a lp withut iterating ver all elements f a sequence.

3 Flw Chart fr nested fr lps Exercise: print wrds What is printed? fr letter in ['g','p','d','s']: fr letter2 in ['ib', 'ump']: print letter + letter2 A flw chart diagram t explain the cde executin fr the example in Exercise: Nested Lps with graphics Here's a picture invlving a grid f randmly clred circles with radius = 50 n a 800x600 canvas. This picture is created using tw nested fr lps and the Clr.randmClr() functin. Hw wuld yu d that? (50, 50) (150, 50) (250, 50) Interrupting Lps Smetimes we want t interrupt a lp withut iterating ver all elements f a sequence. Examples: When we have fund an element we re lking fr When we re accumulating a value thrugh a fr lp and reached sme desired value There are tw situatins when we can d this: Within a functin, via a return statement Within a blck f cde, via a break statement (50, 350) (50, 450) (50, 550) Imprtant: - We will nt cver break in this lecture. - lways exits the bdy f a functin. - We ll use break when we want t exit a lp, but nt the functin

4 Returning early frm a lp In a functin, return can be used t exit the lp early (e.g., befre it visits all the elements in a list). def iselementof(val, elts): '''Returns if val is fund in elts; therwise''' fr e in elts: if e == val: return # return (and exit the functin) # as sn as val is encuntered return # nly get here if val is nt in elts In [1]: sentence = 'the cat that ate the muse liked the dg that played with the ball' Premature return dne wrng (1) def iselementofbrken(val, elts): '''Faulty versin that returns if val is fund in elts; therwise''' fr e in elts: if e == val: return else: return The functin always returns after the 1 st element withut examining the rest f the list. In [2]: iselementof('cat', sentence.split()) Out[2]: # returns as sn as 'cat' is encuntered In[]: iselementofbrken(2, [2, 6, 1]) Out[]: In [3]: iselementof('bunny', sentence.split()) Out[3]: In[]: iselementofbrken(6, [2, 6, 1]) Out[]: Premature return dne wrng (2) def sumhalvesbrken2(n): '''Brken versin f returns sum f halves f n''' sumsfar = 0 while n > 0: sumsfar = sumsfar + n # r sumsfar += n n = n/2 return sumsfar # wrng indentatin! # exits functin after first # lp iteratin. Smetimes we # want this, but nt here! Wrng indentatin within the lp. Functin returns after first iteratin In [4]: sumhalvesbrken2(22) Out[4]: 22 Exercises [in the ntebk] In the ntebk we ll write the fllwing functins that return early. cntainsdigits cntainsdigit('the answer is 42') cntainsdigit('76 trmbnes') cntainsdigit('the cat ate the muse') cntainsdigit('ne tw three') areallpsitive areallpsitive([17, 5, 42, 16, 31]) areallpsitive([17, 5, -42, 16, 31]) areallpsitive([-17, 5, -42, -16, 31]) areallpsitive([]) indexof indexof(8, [8,3,6,7,2,4]) indexof(7, [8,3,6,7,2,4]) indexof(5, [8,3,6,7,2,4]) Returns Hint String bjects have a methd called isdigit, try it ut

5 lngestcnsnantsubstring Swapping Values in Pythn T swap the values f tw variables, a third variable is needed. lngestcnsnantsubstring('strng') returns 'str' lngestcnsnantsubstring('strengths') returns 'ngths' lngestcnsnantsubstring('lightning') returns 'ghtn' lngestcnsnantsubstring('prgram') returns 'Pr' lngestcnsnantsubstring('adbe') returns 'd' def lngestcnsnantsubstring(s): '''Returns the lngest substring f cnsecutive cnsnants. If mre than ne such substring has the same length, returns the first t appear in the string. ''' Nte: This is hard! Draw iteratin tables first! What state variables d yu need? Imagine yu have a list f numbers that yu want t srt by swapping tw adjacent (neighbr) items every time ne is smaller than the ther. This is a famus algrithm knwn as the bubble srt, and is usually implemented via nested fr lps. If yu re curius read this page. Yu ll learn hw t implement bubble srt in CS 230. Start f list nums = [3, 2, 1, 4] After 1 st swap nums = [2, 3, 1, 4] After 2 nd swap nums = [2, 1, 3, 4] After 3 rd swap nums = [1, 2, 3, 4] If we want t d the first swap f 3 and 2, can we write the fllwing? nums[0] = nums[1] nums[1] = nums[0] Try it ut t see what happens. The slutin in this case wuld lk like this: tempval = nums[0] nums[0] = nums[1] nums[1] = tempval Simultaneus assignment in Pythn In Pythn, we can assign values t many variables at nce, here are sme examples, that yu shuld try in the cnsle: a, b = 0, 1 a, b, c = 1, 2, 3 a, b = "AB" a, b = [10, 20] a, b = (15, 25) a, b, c, d = [1, 2, 3, 4] Swapping thrugh simultaneus assignment a, b = b, a num[0], num[1] = num[1], num[0] It is pssible t assign values t multiple variables in ne statement. The reasn that these assignments wrk is that there is an equal number f variables and values n each side. Even the string AB is a sequence f tw characters. Try a different number f variables r values n bth sides t see what errrs yu get. D these statements wrk? Variable update rder matters def sumhalvesbrken(n): sumsfar = 0 while n > 0: n = n/2 # updates n t early! sumsfar += n return sumsfar In [3]: sumhalvesbrken(22) Out[3]: 19 Imprtant: If update rules invlve rules where state variables are dependent n ne anther, be very careful with the rder f updates. step n sumsfar This table is the slutin t slide

Simultaneus update example: Greatest Cmmn Divisr algrithm The greatest cmmn divisr (gcd) f integers a and b is largest integers that divides bth a and b Eg: gcd(84, 60) is 12 Euclid (300 BC) wrte

6 Simultaneus update example: Greatest Cmmn Divisr algrithm The greatest cmmn divisr (gcd) f integers a and b is largest integers that divides bth a and b Eg: gcd(84, 60) is 12 Euclid (300 BC) wrte this algrithm t cmpute the GCD: Given a and b, repeat the fllwing steps until b is 0. Let the new value f b be the remainder f dividing a by b Let the new value f a be the ld value f b this is a perfect pprtunity fr a while lp. step a b Simultaneus update (e.g., gcd) step a b step a b step a b Neither f the fllwing tw gcd functins wrks. Why? def gcdbrken1(a, b): a = b b = a % b def gcdbrken2(a, b): b = a % b a = b Fixing simultaneus update step a b step a b def gcdfixed1(a, b): preva = a prevb = b a = prevb b = preva % prevb step a b def gcdfixed2(a, b): preva = a prevb = b b = preva % prevb a = prevb Pythn s simultaneus assignment is an even mre elegant slutin! def gcdfixed3(a, b): a, b = b, a % b # simultaneus assignment f a # pair f values t a pair # f variables (parens ptinal) T ntice: - Functins 1&2 use temprary variables t stre values befre updates. - The third functin assigns multiple values in ne step Test yur knwledge 1. The sumbetween slutin in 11-5 has an iteratin table with three state variables. Hw will the iteratin table lk like if the slutin is written with a fr lp (see Ntebk Lecture 9). 2. If we want t print ut the entire multiplicatin table fr 1 t 10, hw many times will the print statement in 11-7 be executed. 3. What wuld be the value f cunter in 11-8, if we mve the assignment statement befre the uter fr lp? 4. What results will be printed in 11-8 if the cunter assignment statements mves within the inner lp? 5. Fr the exercise in 11-11, try t draw a flw chart diagram as the ne in 11-9, befre writing cde t slve the prblem. 6. What is an alternative way f writing the functin in 11-4, which leads t the same gtcha? 7. Only by reasning abut the prblems in (n need t write cde yet), which f them needs t be slved with the accumulatr pattern? 8. If yu write 0, 1, 2 in the Pythn cnsle, what kind f type will Pythn assign t this sequence f numbers? Hw des that help fr simultaneus assignments? 11-24

CS1150 Principles of Computer Science Loops

CS1150 Principles of Computer Science Loops CS1150 Principles f Cmputer Science Lps Yanyan Zhuang Department f Cmputer Science http://www.cs.uccs.edu/~yzhuang CS1150 UC. Clrad Springs Annuncement HW1 graded HW2 due tnight HW3 will be psted sn Due