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Arrays (Lists) in Python

Arrays (Lists) in Python. one thing after another. Problem. Given 5 numbers, read them in and calculate their average THEN print out the ones that were above average. Data Structure Needed. Need some way to hold onto all the individual data items after processing them

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Arrays (Lists) in Python

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  1. Arrays (Lists) in Python one thing after another

  2. Problem • Given 5 numbers, read them in and calculate their average • THEN print out the ones that were above average

  3. Data Structure Needed • Need some way to hold onto all the individual data items after processing them • making individual identifiers x1, x2, x3,... is not practical or flexible • the answer is to use an ARRAY • a data structure - bigger than an individual variable or constant

  4. An Array (a List) • You need a way to have many variables all with the same name but distinguishable! • In math they do it by subscripts or indexes • x1, x2, x3 and so on • In programming languages, hard to use smaller fonts, so use a different syntax • x [1], x[0], table[3], point[i]

  5. Semantics • numbered from 0 to n-1 where n is the number of elements 0 1 2 3 4 5

  6. Properties of an array (list) • Heterogeneous (any data type!) • Contiguous • Have random access to any element • Ordered (numbered from 0 to n-1) • Number of elements can change very easily (use method .append) • Python lists are mutable sequences of arbitrary objects

  7. Syntax • Use [] to give initial value to, like x = [1,3,5] • refer to individual elements • uses [ ] with index in the brackets • most of the time you don’t refer to the whole array as one thing, or just by the array name (one time you can is when passing a whole array to a function as an argument)

  8. List Operations you know Python Programming, 2/e

  9. Indexing an Array • The index is also called the subscript • In Python, the first array element always has subscript 0, the second array element has subscript 1, etc. • Subscripts can be variables – they have to have integer values • k = 4 • items = [3,9,’a’,True, 3.92] • items[k] = 3.92 • items[k-2] = items[2] = ‘a’

  10. List Operations • Lists are often built up one piece at a time using append.nums = []x = float(input('Enter a number: '))while x >= 0: nums.append(x) x = float(input('Enter a number: ')) • Here, nums is being used as an accumulator, starting out empty, and each time through the loop a new value is tacked on. Python Programming, 2/e

  11. List Operations Python Programming, 2/e

  12. Using a variable for the size • It is very common to use a variable to store the size of an array • SIZE = 15 • arr = [] • for i in range(SIZE): arr.append(i) • Makes it easy to change if size of array needs to be changed

  13. Solution to starting problem SIZE = 5 n = [0]*SIZE total = 0 for ct in range(SIZE): n[ct] = float(input("enter a number “)) total = total + n[ct] cont'd on next slide

  14. Solution to problem - cont'd average = total / SIZE for ct in range(5): if n[ct] > average: print (n[ct])

  15. Scope of counter in a for loop • The counter variable has usual scope (body of the function it’s in) • for i in range(5): • counter does exist after for loop finishes • what‘s its value after the loop?

  16. Initialization of arrays • a = [1, 2, 9, 10] # has 4 elements • a = [0] * 5 # all are zero

  17. Watch out index out of range! • Subscripts range from 0 to n-1 • Interpreter WILL tell you if an index goes out of that range • BUT the negative subscripts work as they do with strings (which are, after all, arrays of characters) • x = [5]*5 • x[-1] = 4 # x is [5,5,5,5,4]

  18. Assigning Values to Individual Array Elements temps = [0.0] * 5 m = 4 temps[2] = 98.6; temps[3] = 101.2; temps[0] = 99.4; temps[m] = temps[3] / 2.0; temps[1] = temps[3] - 1.2; // What value is assigned? 7000 7004 7008 7012 7016 99.4 ? 98.6 101.2 50.6 temps[0] temps[1] temps[2] temps[3] temps[4]

  19. What values are assigned? SIZE =5 temps = [0.0]* SIZE for m in range(SIZE): temps[m] = 100.0 + m * 0.2 for m in range(SIZE-1, -1, -1): print(temps[m]) 7000 7004 7008 7012 7016 ? ? ? ? ? temps[0] temps[1] temps[2] temps[3] temps[4]

  20. Indexes • Subscripts can be constants or variables or expressions • If i is 5, a[i-1] refers to a[4] and a[i*2] refers to a[10] • you can use i as a subscript at one point in the program and j as a subscript for the same array later - only the value of the variable matters

  21. Variable Subscripts temps = [0.0]*5 m = 3 . . . . . . What is temps[m + 1] ? What is temps[m] + 1 ? 7000 7004 7008 7012 7016 100.0 100.2 100.4 100.6 100.8 temps[0] temps[1] temps[2] temps[3] temps[4]

  22. Random access of elements • Problem : read in numbers from a file, only single digits - and count them - report how many of each there were • Use an array as a set of counters • ctr [0] is how many zero's, ctr[1] is how many ones, etc. • ctr[num] +=1 is the crucial statement

  23. Parallel arrays • Sometimes you have data of different types that are associated with each other • like name (string) and GPA (float) • You CAN store them in the same array • ar = [“John”, 3.24, “Mary”, 3.9, “Bob”, 2.7] • You can also use two different arrays "side by side"

  24. Parallel arrays, cont'd for i in range(SIZE): name[i], gpa[i] = float(input(“Enter”)) • Logically the name in position i corresponds to the gpa in position i • Nothing in the syntax forces this to be true, you just have to program it to be so.

  25. Parallel Arrays Parallel arrays are two or more arrays that have the same index range and whose elements contain related information, possibly of different data types EXAMPLE SIZE = 50 idNumber = [“ “]*SIZE hourlyWage = [0.0] *SIZE parallel arrays

  26. SIZE = 50idNumber = [“ “] *SIZE // Parallel arrays holdhourlyWage =[0.0] *SIZE // Related information idNumber[0] 4562 hourlyWage[0] 9.68 idNumber[1] 1235 hourlyWage[1] 45.75 idNumber[2] 6278 hourlyWage[2] 12.71 . . . . . . . . . . . . idNumber[48] 8754 hourlyWage[48] 67.96 idNumber[49] 2460 hourlyWage[49] 8.97

  27. Selection sort - 1-d array Algorithm for the sort 1. find the maximum in the list 2. put it in the highest numbered element by swapping it with the data that was at that location 3. repeat 1 and 2 for shorter unsorted list - not including highest numbered location 4. repeat 1-3 until list goes down to one

  28. Find the maximum in the list # n is number of elements max = a[0] # value of largest element # seen so far for i in range(1, n): # note start at 1, not 0 if max < a[i]: max = a[i] # now max is value of largest element in list

  29. Find the location of the max max = 0 # max is now location of the # largest seen so far for i in range(1,n): if a[max] < a[i]: max = i # now max is location of the largest in # array

  30. Swap with highest numbered Remember element at right end of list is numbered n-1 a[max], a[n-1] = a[n-1], a[max]

  31. Find next largest element and swap max = 0; for i in range(1,n-1): # note n-1, not n if a[max] < a[i]: max = i a[max], a[n-2] = a[n-2], a[max]

  32. put a loop around the general code to repeat for n-1 passes for pss in range(n, 1, -1): max = 0 for i in range(1,pss): if a[max] <= a[i]: max = i a[max],a[pss-1] = a[pss-1],a[max]

  33. 2-dimensional arrays • Data sometimes has more structure to it than just "a list" • It has rows and columns • You use two subscripts to locate an item • The first subscript called “row”, second called “column”

  34. 2-dimensional arrays • syntax • a = [[0]*5 for i in range(4)] # 5 columns, 4 rows • Twenty elements, numbered from [0][0] to [4][3] • a = [[0]*COLS for i in range(ROWS)] • Which has ROWS rows and COLS columns in each row (use of variables to make it easy to change the size of the array without having to edit every line of the program)

  35. EXAMPLE -- Array for monthly high temperatures for all 50 states NUM_STATES = 50 NUM_MONTHS = 12 stateHighs = [[0]*NUM_MONTHS for i in range(NUM_STATES)] [0] [1] [2] . . stateHighs[2][7] . [48] [49] [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10][11] 66 64 72 78 85 90 99 105 98 90 88 80 row 2, col 7 might be Arizona’s high for August

  36. Processing a 2-d array by rows finding the total for the first row for i in range(NUM_MONTHS): total = total + a[0][i] finding the total for the second row for i in range(NUM_MONTHS): total = total + a[1][i]

  37. Processing a 2-d array by rows total for ALL elements by adding first row, then second row, etc. for i in range(NUM_STATES): for j in range(NUM_MONTHS): total = total + a[i][j]

  38. Processing a 2-d array by columns total for ALL elements by adding first column, second column, etc. for j in range(NUM_MONTHS): for i in range(NUM_STATES): total = total + a[i][j]

  39. Finding the average high temperature for Arizona total = 0 for month in range(NUM_MONTHS): total = total + stateHighs[2][month] average = round (total / NUM_MONTHS) average 85

  40. Passing an array as an argument • Arrays (lists) are passed by reference = they CAN be changed permanently by the function • Definition def fun1 (arr): • Call the function as x = fun1 (myarr)

  41. Arrays versus Files • Arrays are usually smaller than files • Arrays are faster than files • Arrays are temporary, in RAM - files are permanent on secondary storage • Arrays can do random or sequential, files we have seen are only sequential

  42. Using Multidimensional Arrays Example of three-dimensional array

  43. NUM_DEPTS = 5 # mens, womens, childrens, electronics, furniture NUM_MONTHS = 12 NUM_STORES = 3 # White Marsh, Owings Mills, Towson monthlySales = [[[0]*NUM_MONTHS for i in range(NUM_DEPTS)] for j in range(NUM_STORES)] monthlySales[3][7][0] sales for electronics in August at White Marsh 3 STORES sheets 5 DEPTS rows 12 MONTHS columns

  44. Example of filling a 3-d array def main(): NUM_DEPTS = 5 # mens, womens, childrens, electronics, furniture NUM_MONTHS = 12 NUM_STORES = 3 # White Marsh, Owings Mills, Towson monthlySales = [[[0]*NUM_MONTHS for i in range(NUM_DEPTS)] for j in range(NUM_STORES)] storeNames = ["White Marsh", "Owings Mills", "Towson"] deptNames = ["mens", "womens", "childrens", "electronics", "furniture"] for store in range(NUM_STORES): print (storeNames[store], end=" ") for dept in range(NUM_DEPTS): print (deptNames[dept], end = " ") for month in range(NUM_MONTHS): print("for month number ", month+1) monthlySales[store][dept] [month] = float(input("Enter the sales ")) print() print() print (monthlySales)

  45. Find the average of monthly_sales total = 0 for m in range(NUM_MONTHS): for d in range(NUM_DEPTS): for s in range(NUM_STORES): total += monthlySales [s][d][m] average = total / (NUM_MONTHS * NUM_DEPTS * NUM_STORES)

  46. Problem: student data in a file • The data is laid out as • Name, section, gpa • John Smith, 15, 3.2 • Ralph Johnson, 12, 3.9 • Bob Brown, 9, 2.5 • Etc.

  47. Read in the data inf = open(“students”,”r”) studs = [] for line in inf: data = line.split(“,”) studs.append(data) inf.close() #studs looks like [[“John Smith”,15,3.2], #[“Ralph Johnson”,12,3.9],[“Bob Brown”…]]

  48. Find the student with highest GPA max = 0 for j in range(1, len(studs)): if studs[max][2] < studs[j][2]: max = j #max is now location of highest gpa studs[max][0] is the name of the student studs[max][1] is the student’s section

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