Python Lists, Tuples, Dictionaries, and Loops

Transcription

1 Python Lists, Tuples, Dictionaries, and Loops What you need to Know For this lecture you need to know: 1. How to write and run basic python programs 2. How to create and assign data to variables 3. How to use the IF command in python Introduction You have learned how to define and use variables. A Variable is a named place in memory where data can be stored. Most variables have a unique name and hold a single value. However, there are times when you want to identify a set of data items with the same name. For examples, if you wanted to store sales numbers for days of the month, you could create a series of variables named sales1, sales2, sales30 but this is not a very efficient way to store data. As long as you can store the sales value by day number you can locate the sales value. Most programming languages have a storage type that can hold multiple values under a single name. Each item that will be stored in this structure so you can find it again by using an Index. The Index is an identifier used to locate a place in the data structure. In Python this is called a List or Tuple. For example, suppose you had a List named Sales and it contained a number of boxes, each of which has a number, and which can hold some sales data. You might assign sales numbers this way: Sales=[456.66, ,122.44,567.55] The variable named Sales now contains all the numbers listed on the right side of the equal sign. Each element of this List is automatically identified with an index number. Each element can be treated as a variable if you supply both the List name Sales and the index. Note that the index number starts at Zero. X = Sales[0] Sales[1] = There are four items in the Sales list. The first item (456.66) is at index position zero and the last item (567.55) is at index position 3. In the above lines of code the value at position zero is assigned to the variable X and the value at position 1 is changed to Understanding how Lists, Tuples, and Dictionaries work is critical in order to create more interesting programing in Python. Lists, Tuples, Dictionaries Lists In Python a List is an ordered collection of items that can have different kinds of indexes and that is expandable. You can create a list several different ways; by pre-defining the items and slots in the list or by creating an empty list and appending the items you want to add to the list. The important thing is to let Python know that you will be using a List. To pre-define the list and assign values you write the List Name on the left side of an equal sign and on the right side enclose the items you wish to put into the list with [ ] symbols. Each item is separated by a comma: Sales = [456.66, , , , ] This statement does two things; it creates a List variable and assigns data to this list. This command will automatically give each item an Index value starting at zero. For example:

2 Sales[0] is Sales[1] is Sales[2] is Sales[3] is Sales[4] is The index values are 0 to 4 and are used to identify the position of each data item in the List. You can use the List name and the index enclosed in [] symbols and treat each element as a variable, which means it can appear on either side of the equal sign: print( Sales for the first day are, Sales[0] ) Sales[1] = In the first line the print() method prints out the value of Sales[0]. In the second line the value of Sales[1] is changed from to Suppose you wish to add items to this list? Since the list List was created with a set number of values (5 numbers at the index positions 0 to 4) you must use the Append() method. This is a method that is automatically attached to any List you create. This method will append the item to the end of the list. For example: NewSales = [12, 44, 55] print(newsales) NewSales.append(99) print(newsales) This program will print: [12, 44, 55] [12, 44, 55, 99] The append() method has added the value 99 to the end of the list. The print() method will print an entire list. To remove an item from a list you can use the pop() method. This will remove an item from a specific position and return the item or it will return the last item on the list of you don t specify an index. For example: MyList = [40, 50, 60, 70, 80] MyList.append(100) x = MyList.pop(1) y = MyList.pop() This program will print: [40, 50, 60, 70, 80] [40, 50, 60, 70, 80, 100] # after append(100) [40, 60, 70, 80, 100] # after pop(1) [40, 60, 70, 80] # after pop() Notice that the pop(index) command will remove the item at the index (remember that Lists start the index at zero) which the pop() command removes the last item in the list. You can also inset new items into a list at specific positions using the insert( index, value). Existing items are shifted to the right:

3 MyList = [40, 50, 60, 70, 80] MyList.insert(2, 100) This program will print: [40, 50, 60, 70, 80] [40, 50, 100, 60, 70, 80] With the value 100 inserted at index position 2 and everything shifted over. You can put a variety of data items in lists including Strings and even other lists. An example of storing names in a list is: Employees = [ Smith, Edwards, Jones, Franks ] print(employees) You still index the elements the same way: print(employees[1] ) will print Edwards. Another useful method for Lists is the sort method which will rearrange a list in descending sort order. Employees = [ Smith, Edwards, Jones, Franks ] print(employees) Employees.sort() print(employees) This will print [ Smith, Edwards, Jones, Franks ] [ Edwards, Franks, Jones, Smith ] # before sort # after sort You can sort numbers in the same way. Lists can contain both numbers and Strings and most of the List methods still work (except for sort() ) MyData = [ Smith, 55, Jones, 45.55, False] Ranges There are times when you want to create a list of numbers or use a list of numbers. This can be done by manually creating a list containing integer values but Python has a built-in method to return a set of values. This is called a range. The range method has the following syntax: range( start number, one greater than end number) for example, range(0,10) will return a set of numbers from 0 to 9. This can be converted into a list by using: x = list( range(0,10) ) print(x) which will print [0,1,2,3,4,5,6,7,8,9] You can also use ranges in loops:

4 for i in range(0,10): print( value is,i) will print value is 0 value is 1 value is 2 value is 9 Tuples A Tuple looks similar to a list with one important exception; Tuples cannot be modified once they are created. A Tuple is used to store a set of values under a single name but these values cannot be change. Tuples are defined using () rather than []. However, the elements in a Tuple are referenced as read-only using the index notation. For example: MyTuple = (34, 22, 44) print(mytuple) print( MyTuple[1] ) This will print (34, 22, 44) 22 #index item 1 If you try to modify an element in a Tuple Python will return an error: MyTuple[0] = 9999 # this returns an error Tuples are used to store fixed sets of data that won t be changed. For example, X and Y index values or RGB colors. Dictionaries The final structured data item in Python is called a Dictionary. This term refers to the way that index work in an ordered list of items. In both Lists and Tuples the index value is a number starting at zero. We can access an item in a List or Tuple by using the name of the List/Tuple and the index. This works ok, but there are times when the index value itself has informational content. That is, meaningful index are good for locating items in a list. Python has a way to store ordered data and use strings as index values. Defining a Dictionary is different from a List or Tuple because you must also specify the index value along with the data item value. For example, suppose you wanted to store sales data totals my month. It would be useful if the month name was the index value. You don t have to remember that index 0 is January, 2 is March, etc. A Dictionary is defined using the {} symbols which enclose the index string and the value. MonthSales = {"Jan":456.77, "Feb":123.44, "Mar":567.33} You access the elements in this dictionary using the string as the index: print(monthsales[ Jan ] ) MonthSales[ Jan ] = Adding items to a Dictionary is not done using append() but by just setting a new value MonthSales[ Apr ] = print(monthsales)

5 This will print: { Jan :456.77, Feb :123.44, Mar :567.33, Apr : } Using Lists, Tuples, and Dictionaries Lists, Tuples, and Dictionaries are used in a wide variety of applications. The main purpose of such data structures is to be able to handle multiple items that are related in some way. A List of names may be used to organize Students or Employees and a Dictionary may be used to handle pay amounts. The main operations on Lists, Tuples, and Dictionaries are Create Add to and Remove items in list Locate items in list Split and combine lists Loop through lists The last activity will be illustrated when we learn how to create loops in Python. Creating: MyList = list() # create empty list MyList = [123, 555, 9999] MyTuple = (0, 255,0) MyNames = {} # create empty Dictionary MyNames = { Jan :45.55, Feb :23.44, Mar :98.33} Adding and Removing: MyList.append(876.33) # add element to end of list MyList.pop(0) # remove element at position 0 del( MyList[0] ) # remove element at position 0 MyNames[ Apr ] = # adds element to MyNames del( MyNames[ Apr ] ) # removes element from MyNames You can use the in command to locate items in a list. This operator returns True or False if the value (List) or index (Dictionary) is in the list. For example: MyList = [100, 300, 400, 600] if 100 in MyList: print( Value is in list ) x = MyList.index(100) # get index for item 100 print( Value is, MyList[x] ) In this example, a list is created and the if command returns True if the value 100 is one of the values in the list. If this value is in the list then the message Value is in list is printed. Next, the index value of this value is returned into x (or at least the first value that matches if there are more than one elements with the value of 100 ) and used to print the value at this index. Dictionaries are similar but the index string is being searched: MonthSales = { Jan :100, Feb : 200, Mar : 300} if Jan in MonthSales: print( Found index ) print( Value is, MonthSales[ Jan ] You can also split and combine Lists by using ranges of index values:

6 MyList = [100, 200, 300, 400, 500] print( MyList[0:2]) Will print: [100, 200] Notice that the syntax of this command (which is called Splitting the List) is: ListName[ startindex: endindex+1] The endindex+1 is the index value one past where you want to split the list. That is if you want to print index items 0, 1, 2 you would use: print(mylist[0:3] ) because 3 is one greater than the index you wish to print. This range option in the List index field can also be used to split a list and get a copy of a subsection of the List. MyList = [100, 200, 400, 500, 800] List2 = MyList[0:3] print(list2) This would print: [100, 200, 400] Since 400 is at position 2 while the split command is [0:3] Loops Looping in a computer program means your program will execute commands more than once. This is often used as part of the programming logic. A loop must include two components: 1. A way to indicate what lines of code are part of the loop 2. A way to control when the loop ends This second item is important otherwise you may end up writing programs that go into loops and never get out. Python has two kinds of loops, Logical Loops and Data Loops. The first type using a Boolean expression and the while statement. The second uses some data structure and the for statement. Both kinds of loops use the : symbol and tabs to indicate what lines of code will be part of the loop. For example, suppose you want to subtract a number from a value until the value reaches a particular level. WaterLevel = 38 DrainAmount =.5 Count = 0 while (WaterLevel > 10): WaterLevel = WaterLevel DrainAmount Count = Count + 1 print( Water level is, WaterLevel, it took, Count, times through loop ) The command: while( WaterLevel > 10): is the Boolean expression which is being evaluated each time the loop executes. The basic logic is:

7 1. The Boolean expression is evaluated at the start of the loop. 2. If the Boolean expression is True then the loop code is executed 3. If the Boolean expression is False then the loop code is skipped. Notice that the WaterLevel variable is initially set to 38. If this variable was set to 10 (which is not greater than 10) then the loop would be skipped entirely and it would never run. The while(waterlevel>10) Boolean expression is False and so the loop is never run. The second type of loop is a Data loop and it is typically used with a List, Tuple, or Dictionary. This uses the in command to sequence over each item in the List/Tuple/Dictionary MyList = [133, 444, 234, 900] for x in MyList: print( Value is, x) Will print: Value is 133 Value is 444 Value is 234 Value is 900 The For statement is followed by a variable (in this case named x ). The for loop will successively get each value in the list and put it into the X variable. This variable is then used in the code identified as part of the loop. Tuples work the same was as Lists. A Dictionary can be iterated in the same way but what is put into the X value are the index values: MyNames = { Smith : , Jones :234.44, Edwards :890.00} for namex in MyNames: print( Index is, namex, and value is,mynames[namex] ) Since each index is returned into the namex variable this can be used with the Dictionary name to retrieve the value at that location. Using Loops Sample program One Loops are used in many different ways in a computer program. As an example, consider a user-interface program that asks the user to enter how much money they want to withdraw from their checking account. This program has a method named GetAvailable() which will return the available balance the user has in their account. For the purpose of this program the method will return a fixed number, but in a real program this method would likely connect to a central computer and use the user s PIN code to retrieve the balance. Program Description: This program is used to withdraw money from an account. The user is prompted to enter their PIN number and the amount they wish to withdraw. If the PIN is correct and the amount they wish to withdraw is less than the daily balance and less than or equal their checking amount then they can have the money. If they are trying to withdraw more than the daily amount an error message is displayed and the user is asked to exit or try a new amount. If the amount is more than they have in their account they are asked to exit or try a new amount. Program Outline: 1. Ask user to enter PIN code. 2. Read PIN code

8 3. Verify PIN code (call VerifyPin( pin ) ) 4. If PIN code is bad then a. Ask user to either re-enter or exit. b. Get input from user. c. If user chooses re-enter then go back to step 1 d. If user chooses exit then end program 5. If code is good then a. Get Daily Limit (getdailylimit() method) b. Get Checking Amount (getcheckingamount() method) c. Ask user for withdrawal amount d. Get input from user. e. If withdrawal amount > Daily Limit or > checking amount then i. Print error message (either over daily limit or over checking amount) ii. Ask user to either re-enter amount or exit iii. Get input from user iv. If user chooses to re-enter then go back to step 5d v. If user chooses to exit the end program. f. If withdrawal amount valid then i. Deduct amount from checking. ii. Give money to user ( issuemoney( amount) method ) 6. End Program This program outline references four methods that we are not going to write but will be written by another programmer. We can write our own versions of these methods and hard-code in numbers for the purpose of testing, but when our module is integrated with the main program we will have to change our code. Program Code: (Note: the symbol indicates that the following line should be on the same line) author = 'dcraig' import sys # Stub methods for testing purposes - actual methods will be external to this program def VerifyPin( pin ): return True def GetDailyLimit(pin): return def GetCheckingAmount(pin): return def IssueMoney( amount ): return True ################# Main program ################## # Ask user to enter Pin loop

9 EnterPinFlag = False Pin = "" while(enterpinflag==false): Pin = input("enter Pin and press Enter ") if( VerifyPin(Pin)==True): EnterPinFlag = True else: ans = input("invalid Pin - Continue or Exit - Enter C or X ") if( ans=="x"): sys.exit() # Get Daily Limit and Checking Amount DailyLimit = GetDailyLimit(Pin) CheckingAmount = GetCheckingAmount(Pin) # Ask user for amount to withdraw AmountFlag = False CheckingFlag = False AmountWith = 0 while(amountflag==false or CheckingFlag==False): withdrawstr = input("enter Amount to withdraw ") AmountWith = float(withdrawstr) if(amountwith<dailylimit and AmountWith>0.0): AmountFlag = True else: ans = input("error: Over daily withdraw limit -Try again or exit Enter T or X ") if(ans=="x"): print("have a nice day") sys.exit() if(amountwith<=checkingamount): CheckingFlag = True else: ans = input("error: You don't have enough in checking -Try again or exit- Enter T or X ") if(ans=="x"): print("have a nice day") sys.exit() print("you are withdrawing",amountwith) if( IssueMoney(AmountWith)==True): print("transaction Complete") else: print("error - problem with computer")