Maintaining order. A small student database. Managing databases. Representing table of student data. Sorting and searching

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Leon Shaw
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1 Maintaining rder Srting and searching A small student database ID# First Last Year Majr 1 Jane Williams 2007 BISC 2 Ann Smith 2009 CS GPA Susan Jnes 2008 ARTS Claire French 2007 ENG Amy Clark 2009 CHIN Bra Thmas 2007 ECON 2.5 CS112 Scientific Cmputatin Department f Cmputer Science Wellesley Cllege Sme questins: 1. Hw many ECON majrs are in the class f 2009? 2. What are the names f all the ARTS majrs? 3. What is the average GPA f CHEM majrs in the class f 2007? Srting & Searching 17-2 Managing databases Representing table f student data Database management fcuses n develping ways t rganize and maintain large amunts f data, and ways t extract infrmatin frm this data The table f student data has the fllwing rganizatin: {{1 'Jane' 'Williams' 2006 'BISC' 3.3} {2 'Ann' 'Smith' 2008 'CS' 2.8} } Cmmn database peratins: Select a subset f the data that satisfies certain criteria Summarize the data, r sme prtin f it Srting & Searching 17-3 Srting & Searching

Selecting data frm studentdata table Select students frm the class f 2010: functin newtable = selectfrmtable1 (table) % cnstructs a new table that nly cntains data fr % students frm the class f 2010

2 Selecting data frm studentdata table Select students frm the class f 2010: functin newtable = selectfrmtable1 (table) % cnstructs a new table that nly cntains data fr % students frm the class f 2010 newtable = { }; fr index = 1:length(table) % preserve items whse year is 2010 if (table{index}{4} == 2010) newtable{+1,1} = table{index}; Srting & Searching 17-5 Time-ut exercises functin newtable = selectfrmtable1 (table) % cnstructs a new table that nly cntains data fr % students frm the class f 2010 newtable = { }; fr index = 1:length(table) % preserve items whse year is 2010 if (table{index}{4} == 2010) newtable{+1,1} = table{index}; (1) Mdify selectfrmtable1 t return a table that nly cntains data fr students wh are CS majrs (2) Mdify selectfrmtable1 t return a table that nly cntains data fr students wh wh have a GPA that is at least 3.0 Srting & Searching 17-6 Prjecting fields f data Anther cmmn database peratin, called prjectin, selects ne r mre fields (clumns) f data >> majrs = prjectfrmtable (studentdata, fields, 'majr') majrs = BISC CS ARTS ENG Cde fr prjectfrmtable functin prjectin = prjectfrmtable (table, fields, fieldname) % extracts a single field f data frm the input table, given a cell % array f all field names and desired single field name as input % determine clumn index frm field name fieldindex = find(strcmp(fields, fieldname)); numitems = length(table); % determine number f items prjectin = cell(numitems, 1); % create empty cell array fr index = 1:numItems % stre items in prjectin prjectin{index} = table{index}{fieldindex}; Srting & Searching 17-7 Srting & Searching

3 Hw d yu search a phnebk? MATLAB srt functin If yu expect t search fr values in certain field a lt, it pays t rder the data, either alphabetically r numerically But, hw can we srt the items in a table in the first place? And, nce the items are srted, hw can we search fr a particular value, in an efficient way? >> nums = [ ]; >> srtnums = srt(nums) srtnums = >> srtnums = srt(nums, 'desc') srtnums = >> [srtnums srtindices] = srt(nums, 'asc') srtnums = srtindices = Exercise: What des the expressin nums(srtindices) return? Srting & Searching 17-9 Srting & Searching Nw let s srt a cell array f strings >> wrds = {'early' 'clud' 'heights' 'a' 'black' 'great' 'frm 'desced'}; >> srtwrds = srt(wrds) srtwrds = 'a' 'black' 'clud' 'desced' 'early' 'frm' 'great' 'heights' >> wrds = {'early' 'Clud' 'heights' 'A' 'black' 'Great' 'frm' 'Desced'}; >> [srtwrds srtindices] = srt(wrds) srtwrds = 'A' 'Clud' 'Desced' 'Great' 'black' 'early' 'frm' 'heights' srtindices = Hmmmm. What s ging n here??? ASCII cde (again) When cmparing the rder f tw strings MATLAB uses the rder f characters in the ASCII cde in which all capital letters appear befre all lwercase letters Exercise: Write a functin that srts a cell array f wrds alphabetically, indepent f capitalizatin Srting & Searching Srting & Searching

4 Srting the table f student data First cnsider a simpler prblem Recall that the table f student data has the fllwing rganizatin: {{1 'Jane' 'Williams' 2006 'BISC' 3.3} {2 'Ann' 'Smith' 2008 'CS' 2.8} } Hw can we create a new table in which the students last names are rdered alphabetically? {{9 'Janne' 'Bentn' 2009 'ND' 3.3} {11 'Sally' 'Blake' 2009 'ND' 3.1} {10 'Kate' 'Bwer' 2006 'NEUR' 3.4} {5 'Amy' 'Clark' 2008 'CHIN' 3.5} } Srting & Searching girls = { {5 'Sally'} {3 'Jane'} {6 'Mary'} } srtedgirls = { {3 'Jane'} {6 'Mary'} {5 'Sally'} } Step 1. Write tw statements that prduce the fllwing cell array frm girls: names = {'sally' 'jane' 'mary'} Step 2. Write tw statements that prduce srtedgirls frm names and girls Srting & Searching srtbyfield functin newdata = srtbyfield(data, fields, fieldname, isstring) fieldvalues = prjectfrmtable(data, fields, fieldname); if isstring values = lwer(fieldvalues); values = zers(length(table), 1); fr i = 1:length(table) values(i) = fieldvalues{i}; [srtvalues srtindices] = srt(values); newdata = data(srtindices); cnvert strings t lwer case mve numbers frm cell array t vectr put riginal values in srted rder srt values, save indices get cell array f values with given field name Srting & Searching The search is n Nw that we have a field f the table in srted rder, hw can we search this field fr a particular value, in an efficient way? Let s play a game... I m thinking f a number between 1 and 100 What is it? Srting & Searching

An auxiliary functin T search a cell array fr a particular string, we need a way t cmpare tw strings t see which cmes first Kaplan prvides a functin alphabetcmpare fr this purpse searchtablebinary

5 An auxiliary functin T search a cell array fr a particular string, we need a way t cmpare tw strings t see which cmes first Kaplan prvides a functin alphabetcmpare fr this purpse searchtablebinary functin newtable = searchtablebinary (table, fields, fieldname, fieldvalue, isstring) % given input table f srted values in fieldname, searches fr all items with % a value in this field given by fieldvalue. isstring shuld be 1 fr string values % and 0 fr numbers. uses the binary search strategy newtable = {}; fieldindex = find(strcmp(fields, fieldname)); if isstring cmparefunctin = 'stringcmpare'; cmparefunctin = 'numcmpare'; Srting & Searching Srting & Searching Auxiliary functins functin cmpare = stringcmpare (item, index, value) % returns 1 if value is equal t item{index}, 2 if value is later % in the alphabet, and 3 if value is earlier in the alphabet cmpare = alphabetcmpare(lwer(item{index}), lwer(value)); functin cmpare = numcmpare (item, index, value) % returns 1 if value is equal t item{index}, 2 if value is larger % than item{index} and 3 if value is smaller if (item{index} == value) cmpare = 1; if (item{index} < value) cmpare = 2; cmpare = 3; Srting & Searching searchtablebinary cntinued functin newtable = searchtablebinary (table, fields, fieldname, fieldvalue, isstring) first = 1; % set initial regin f table t search last = length(table); while (last-first) > 1 middle = flr((first+last)/2) % calculate middle index val = feval(cmparefunctin, table{middle}, fieldindex, fieldvalue) if (val == 1 ) % fieldvalue is fund, s add item t table newtable{+1,1} = table{middle}; break; if (val == 2) % fieldvalue is at an index larger than middle first = middle; % fieldvalue is at an index smaller than middle last = middle; Srting & Searching

6 searchtablebinary cntinued functin newtable = searchtablebinary (table, fields, fieldname, fieldvalue, isstring) % if ne ccurrence f fieldvalue was fund, search abve and belw % this item fr ther items with the same fieldvalue if (length(newtable) >= 1) index = middle - 1; % check smaller indices while (index > 0) & (feval(cmparefunctin, table{index}, fieldindex, fieldvalue) == 1) newtable{+1, 1} = table{index}; index = index - 1; index = middle + 1; % check larger indices while (index <= length(table)) & (feval(cmparefunctin, table{index}, fieldindex, fieldvalue) == 1) newtable{+1, 1} = table{index}; index = index + 1; Srting & Searching

Data Structure Interview Questions

Data Structure Interview Questins A list f tp frequently asked Data Structure interview questins and answers are given belw. 1) What is Data Structure? Explain. Data structure is a way that specifies hw