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Chapter 5

Chapter 5. Hashes and databases. File contains more than numbers. Name Score Johnny 8.65 Juan 9.12 Joseph 8.45 Stacey 7.81 Aideen 8.05 Zack 7.21 Aaron 8.31. Who won the surfing contest?. scores=[ ] result_f =open(“results.txt”) for line in result_f :

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Chapter 5

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  1. Chapter 5 Hashes and databases

  2. File contains more than numbers Name Score Johnny 8.65 Juan 9.12 Joseph 8.45 Stacey 7.81 Aideen 8.05 Zack 7.21 Aaron 8.31

  3. Who won the surfing contest? scores=[ ] result_f=open(“results.txt”) for line in result_f: (name, score)=line.split() scores.append(float(score)) result_f.close() scores.sort(reverse=True) print(“The top scores are: “) ??? print(scores[0]) print(scores[1]) print(scores[2])

  4. Rewrite the program scores=[ ] names=[ ] result_f=open(“results.txt”) for line in result_f: (name, score)=line.split() scores.append(float(score)) names.append(name) result_f.close() scores.sort(reverse=True) names.sort(reverse=True) print(“The top scores are: “) ??? print(names[0]+’ with ‘ +str(scores[0])) print(names[1]+’ with ‘ +str(scores[1])) print(names[2]+’ with ‘ +str(scores[2]))

  5. Associate name and score Name Score Johnny 8.65 Juan 9.12 Joseph 8.45 Stacey 7.81 Aideen 8.05 Zack 7.21 Aaron 8.31 Need a different data structure!!!

  6. Hash A variable that has exactly two columns and (potentially) many rows of data key – value A data structure that associates a name with a value Other names in different programming languages: mapping dictionary association array key-value list

  7. Using a hash # Start with an empty hash: scores={} # Add data # Put the key inside the square brackets # put the value to the right of the assignment op scores[8.45]=‘Joseph’ # New row of data is added to the hash # No explicit “append()” method like arrays

  8. Using a hash Can use a for loop to iterate over each of the rows in a hash: for key in scores.keys(): print(scores[key]+’ had a score of ‘+str(key)) keys() – built-in method that returns an array of the keys of the hash. values() – built-in method that returns an array of the values of the hash.

  9. Using a hash Use the square brackets to refer to a value associated with a key: print(scores[9.12]) items() – Another hash method that returns key-value pair. (one can use it with the for loop) for score,surfer in scores.items(): print(surfer + ‘ had a score of ‘ + str(score))

  10. The new program scores={} result_f=open(‘results.txt’) for line in result_f: (name,score)=line.split() ??? scores[score]=name result_f.close() print(“The top scores were”) for each_score in scores.keys(): print(‘Surfer ‘+scores[each_score]+’ scored ‘+each_score)

  11. Sorting data in a has No sort() method BUT There is a function called sorted() sorted() – bulit-in function that has the ability to sort any data structure for each_score in sorted(scores.keys(),reverse=True): print(‘Surfer ‘+scores[each_score]+’ scored ‘+each_score)

  12. The new program scores={} result_f=open(‘results.txt’) for line in result_f: (name,score)=line.split() Hurray!!! scores[score]=name result_f.close() print(“The top scores were”) for each_score in sorted(scores.keys(), reverse=True): print(‘Surfer ‘+scores[each_score]+’ scored ‘+each_score)

  13. When data gets complex Data file (competition id, name, country, average score, board type, age): 101;Johnny 'wave-boy' Jones;USA;8.32;Fish;21 102;Juan Martino;Spain;9.01;Gun;36 103;Joseph 'smitty' Smyth;USA;8.85;Cruizer;18 104;Stacey O'Neill;Ireland;8.91;Malibu;22 105;Aideen 'board babe' Wu;Japan;8.65;Fish;24 106;Zack 'bonnie-lad' MacFadden;Scotland;7.82;Thruster;26 107;Aaron Valentino;Italy;8.98;Gun;19 Problem: Find and display a surfer’s data based on competition ID

  14. Printing surfer’s data line=“101;Johnny ‘wave-boy’ Jones;USA;8.32;Fish;21” Display: ID: 101 Name: Johnny ‘wave-boy’ Jones Country: USA Average: 8.32 Board type: Fish Age: 21

  15. Printing surfer’s data line=“101;Johnny ‘wave-boy’ Jones;USA;8.32;Fish;21” s={} #create an empty hash (s[‘id’],s[‘name’],s[‘country’],s[‘average’].s[‘board’],s[‘age’])=line.split(“,”) print(“ID: “+s[‘id’]) print(“Name: “+s[‘name’]) print(“Country: “+s[‘country’]) print(“Average: “+s[‘average’]) print(“Board type: “+s[‘board’]) print(“Age: “+s[‘age’])

  16. Processing ALL lines in data file Problem: Write a function that takes the surfer ID as a parameter, searches the file one line at a time for a matching ID, and then returns the found data to the caller. How to return the data ??? 1. as a string 2. as a hash Which one should we use???

  17. Processing ALL lines in data file Problem: Write a function that takes the surfer ID as a parameter, searches the file one line at a time for a matching ID, and then returns the found data to the caller. How to return the data ??? 1. as a string 2. as a hash Which one should we use??? Answer: A hash – It allows the calling code to simply pick out the information it needs without further processing.

  18. find_details function def find_details(id2find): surfers_f=open(“surfing_data.csv”) for each_line in surfers_f: s={} (s[‘id’],s[‘name’],s[‘country’],s[‘average’].s[‘board’],s[‘age’])=line.split(“,”) if id2find == int (s[‘id’]): surfers_f.close() return(s) surfers_f.close() return ({})

  19. Testing the function lookup_id=int(input(“Enter the id of the surfer: “)) surfer=find_details(lookup_id) if surfer: print(“ID: “+s[‘id’]) print(“Name: “+s[‘name’]) print(“Country: “+s[‘country’]) print(“Average: “+s[‘average’]) print(“Board type: “+s[‘board’]) print(“Age: “+s[‘age’])

  20. Using data from a database

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