Changes

:In previous labs, you learned some programming tools in order to make your Python scripts '''more functional''' and allowed your Python script to run differently based on different data or situations. These tools included '''objects/variables''', '''condition statements''' and '''loops'''. The utilization of these basic tools not only apply to Python scripts, but basically all programming languages including interpreted (including '''Perl scripts''', '''Bash Shell scripts''', '''JavaScript''', etc ) and compiled languages (including '''C''', '''C++''', '''Java''', etc).

:A very simple definition of using '''functions''' is to create and reuse '''smaller programs within a larger program'''. In programming languages such as '''C''', '''C++''' and '''Java''', commonly used functions are pre-packaged in '''libraries'''. This relates to dependency issues that were discussed when compiling C programming code in your OPS235 course: if a supporting library is missing, the program would not be able to run the called function.

:Functions may be designed :* '''not to accept arguments or return a value''', designed * to '''not accept arguments but not return a value''', designed * to '''accept arguments and not return a value''', * or designed to '''both accept arguments and return a value'''. In this investigation, will we will focus of creating functions that either do NOT return a value, or return a value.

print(text)

:#Create the '''~/ops435/lab3/lab3c.py''' script. The purpose of the script is to have a single function that can perform addition, subtraction, or multiplication on a pair of numbers. But the function will allow us to choose exatly what operation we are performing on it when we call it. If the operate function does NOT understand the operator given, it should return an error message(e.g. calling the function to 'divide' two numbers).

# Will output return 30

# Will output return 5

# Will output return 95

# Will output return -10

# Will output return 25

# Will output return 1000

# Will output return Error: function operator can be "add", "subtract", or "multiply"

# Will output return Error: function operator can be "add", "subtract", or "multiply"

'# Will return 9.6G'

:'''Lists''' are one of the most powerful '''data-types''' in Python. A list is a series of '''comma separated values found between square brackets'''. Values in a list can be anything: '''strings''', '''integers''', '''objects''', even '''other lists'''. In this section, you will introduce lists and how to use them effectively, you will further user lists in later labs. It is important to realise that although lists may appear very similar to arraysin other languages, they are different in a number of aspects including which functions are used to manipulate lists as opposed to which functions are used to manipulate arraysthe fact that they don't have a fixed size.

:#Start the ipython3 shellCreate a new Python file for testing things in this section.:<source>ipython3</source>You will now create #Create a few lists with different values: list1 contains only '''integers''', list2 contains only '''strings''', list3 contains a combination of both '''integers and strings'''.<br><br>:#Issue the following from the ipython shell:<source lang="python">

</source>List are constructed similar to arrays. The best way to get access individual '''elements''' from in a list is using the list '''index'''.<br>The index is a number starting from 0 to ('''number_of_items - 1'''), the list index starts counting at '''0'''.<br><br>:#Issue the following Inspect specified elements in the ipython shell to obtain stored datayour lists:<source lang="python">print(list1[0] ) # First element in list1print(list2[1] ) # Second element in list2print(list3[-1] ) # Last element in list3</source>:#Issue the following to retrieve ranges (slices) of items from a list: <source lang="python">list1[0:5] # Starting with index 0 and stopping before index 5list2[2:4] # Starting with index 2 and stopping before index 4list3[3:] # Starting with index 3 and going to the end</source>Lists can also contain other lists. This means data can be contained in: lists of strings, lists of integers, or lists contains a combination of strings and integers.<br><br>:#Issue of the following to create a list that contains lists:<source lang="python">list4 = [ [1, 2, 3, 4], ['a', 'b', 'c', 'd'], [ 5, 6, 'e', 'f' ] ]</source>The list just created only has 3 index locations. Each index points the individual list stored as the list element. <source>list4[0]list4[1]list4[2]</source>:#To access a list inside another list, a second index is needed. Spend some time trying out the syntax and try and navigate to a specific spot in the list.<source lang="python">list4[0][0] # First element in first listlist4[0][-1] # Last element in first listlist4[2][0:2] # First two elements in third list

:#You can use different elements also retrieve ranges of items from existing lists to create new lists. To demonstrate, issue the followinga list (these are called slices):<source lang="python">first_only_list = [ print(list1[0:5], ) # Starting with index 0 and stopping before index 5print(list2[02:4], ) # Starting with index 2 and stopping before index 4print(list3[03:] ]first_only_list) # Starting with index 3 and going to the end

# Create the list called "my_list" below this command (here, not within any function defined below).# That makes it a global variable. We'll talk about that in another lab.

# Returns all of the entire list global variable my_list unchanged

# Returns a single string that is the first item in the listglobal my_list

# Returns a list that includes the first and last items in the listglobal my_list

# Returns a list that includes the second and third items in the listglobal my_list

if __name__ == '__main__': # This section also referred to as a "boiler platemain code"

run ./ lab3e.py

:::'''Sample Run 2 (with importfrom another script):'''<source>

# Will print [100, 200, 300, 'six hundred']

# Will print 100

# Will print [100, 'six hundred']

# Will print [200, 300]

:::3. Exit the ipython3 shell, download Download the checking script and check your work. Enter the following commands from the bash shell.<source>

:::4. Before proceeding, make certain that you identify any and all errors in lab3e.py. When the checking script tells you everything is OK before proceeding - proceed to the next step.

:#Start the ipython3 shell:<source>ipython3</source>:#Let's perform a simple change to a list element. Issue Try the following in the ipython shellcode:<source lang="python">

print(courses[0])

print(courses[0])print(courses)</source>It might be useful to change a list element-by-element, but there are other more efficient methods of changing a list (for example: using functions). You will now use the '''dir()''' and '''help()''' functions to see what functions and attributes are available for manipulating lists. The '''help()''' function will also give us tips on how to use those functions. For example, you can use issue help(list-name) in order to see what functions are available to use for that list specific list.<br><br>:#Issue the following:<source lang="python">dir(courses)help(courses)</source>Below are some examples of using built-in functions to '''manipulate''' lists. Take your time to see how each function can be a useful tool for making changes to existing lists.<br><br>:#Issue the following:<source lang="python">help(courses.append)

print(courses)

print(courses)

print(courses)

print(courses)print(sorted_courses)

</source>:#In addition to using functions to manipulate lists, there are functions that are useful to provide '''information''' regarding the list such as number of elements in a list, the smallest value and largest value in a list.<br><br> :#Issue the following:<source lang="python">

length_of_list = len(list_of_numbers) # Returns the length of the listsmallest_in_list = min(list_of_numbers) # Returns the smallest value in the listlargest_in_list = max(list_of_numbers) # Returns the largest value in the list</source>In addition to manipulating and obtaining characteristics of a list, it # Notice how the long line below is also useful wrapped to be able to '''perform searches''' for values within lists and obtain the location of values for elements contained within a list. The '''indexfit on one screen:print()''' function allows searching inside a list for a value, it will return the index number of the first occurence. <source lang="pythonList length is ">number = 10help+ str(list_of_numbers.indexlength_of_list)+ list_of_numbers.index(number) # Return index of the number searched for</source>One common annoyance that can occur when performing searches are '''error messages''' when performing a search for an ", smallest element that happens NOT to exist in the list. A good way to prevent those type of errors is to use an '''if''' statement to check to see if the value for an element is in a list, then the appropriate search can be performed for that existing element value.<br><br>:#Issue the following:<source lang="python">list_of_numbers = [ 1, 5, 2, 6, 8, 5, 10, 2 ]number = 7if number in list_of_numbers: # Returns True if value in list, returns False if item not in list number_index = list_of_numbers.index(number) print('index is: ' + str(number_index)smallest_in_list)+else: # If ", largest element in the statement list is False, the else will run print(" + str(numberlargest_in_list) + ' is not in list_of_numbers')

:This last section demonstrates an extremely useful for lists: the ability to quickly '''loop through every value in the list'''. '''For loops''' have a set number of times they loop. The '''for''' loop will execute all indented code for each item (element) in the list. Using loops with list allow for efficient processing of stored data.

::Let's take a moment to understand how the '''for''' loop works. This The following '''for''' loop will store the value of each list element from list_of_numbers within a variable named '''item''' and run code indented below the loop for each item.<br><br>:#Issue the following in the ipython shellRun this from a temporary Python file:<source lang="python">

print(list_of_numbers)print(new_list_of_numbers)</source>The above is just one example of a quick use of for loops mixed with lists. But be careful when passing lists into functions. When you give a function a list as an argument, it is the actual list reference and NOT a copy. This means a function can completely change the list without making a new list. While you do have to be careful this is can also be useful, a function can modify any given list, without have to return or store it.<br><br>

print(list_of_numbers)

# Appends new item to end of list which is with the value (last item + 1)

:::*The script program should have a function called '''add_item_to_list(ordered_list)'''<dd><dl>This function takes a single argument which is a list name itself. It will then look at the value of the last existing item in the list, it will then append a new value that is one unit bigger (i.e. '''+1''' and modifying that same list without returning any value).</dl></dd>:::*The script should have a function called '''remove_items_from_list(ordered_list, items_to_remove)'''<dd><dl>This function takes two arguments: a list, and a list of numbers to remove from the list. This function will then check if those items exist within that list, and if they exist, then they will be removed. This function will modify the list without returning any value.</dl></dd>

from lab3f import * [1/1899]print(my_list)# Will print [1, 2, 3, 4, 5]

print(my_list)# Will print [1, 2, 3, 4, 5, 6, 7, 8]

print(my_list)# Will print [2, 3, 4, 7, 8]

:::2. Exit the ipython3 shell, download Download the checking script and check your work. Enter the following commands from the bash shell.<source>

:::3. Before proceeding, make certain that you identify any and all errors in lab3f.py. When the checking script tells you everything is OK before proceeding - proceed to the next step.

:# Assume that the following command was issued at the ipython3 promptlist has been defined: '''mylist = [ 'apple', 1, 'grape', 2, 'banana', 3, ]'''<br>Based on the command issued abovethat, what are the results of will the following commandscontain?<blockquotesource lang="python">'''mylist[0]''' , '''mylist[3]''' , '''mylist[-1]''' , '''mylist[0:1]'''</blockquotesource>:# Assume that the following command was issued at the ipython3 promptlist has been defined: '''combined_list = [ [7, 5], ['x', 'y'], [ 5, 'f' ] ]'''<br>Based on the command issued abovethat, what are the results of will the following commandscontain?<blockquotesource lang="python">'''combined_list[0]''' , '''combined_list[1]''' , '''combined_list[1][0]''' , '''combined_list[2][0:2]'''</blockquotesource>