Python Programming Exercises 3 - was@bi

Python Programming Exercises 3

Notes: These exercises assume that you are comfortable with the contents of the two previous sets of exercises including variables, types, arithmetic expressions, logical expressions, conditional statements and defining functions. If you did not finish the previous set of exercises you really should finish those before moving on to this set.

1. Lists are declared using square brackets with the elements of the list separated by commas. Lists can hold an arbitrary number of values which can be of different types. We use the len(? ? ?) function to discover the number of elements in a list. Try these examples: >>> len([]) >>> len([0, 1, 2, 3, 4]) >>> len(["a", 1, []])

2. Lists can be added together and multiplied by an integer: >>> x = [1, 2, 3] + [4, 5, 6] >>> y = ["a"] * 3 We can use the inline versions of addition and multiplication if the list has already been declared: >>> x += [7, 8, 9] >>> y *= 2

3. We can append individual items to an existing list using the append(? ? ?) method:

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>>> x = [] >>> x.append(1) >>> x.append(2)

Multiple items can be added using the extend(? ? ?) method: >>> x = [] >>> x.extend([1, 2])

4. You must be careful with assignment of lists because, unlike variables that contain simple values, the contents of the list is not copied, just a reference to the list: >>> x = [] >>> y = x >>> x.append(1) >>> x == y

If it is not your intention to have two variables pointing to the same list, then you must use the copy(? ? ?) method: >>> x = [] >>> y = x.copy() >>> x.append(1) >>> x != y

It is very important you understand the difference between these two examples!

5. We can find out if a list contains a value using the in operator, just like we did last time for strings: >>> x = [1, 1, 1, 2, 3, 3, 5] >>> 2 in x >>> 4 in x and find the number of occurances of a value with count(? ? ?):

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>>> x.count(1)

Write a function called append non dup(mylist, element) that accepts two arguments, a list (mylist) and another variable (element) that can be of any type. The function should only append element to mylist if it is not already present in the list (so the list does not contain any duplicate values). Does your function return the list? Does it matter to the result? Is returning the list a good idea?

6. The ordering of elements in a list does not change unless we reorder them explicitly. This is important because we want to use integers to index elements in the list directly. Programming languages tend to start counting from zero, so the first element in the list is found at index 0, the second element at index 1, etc: >>> x = [1, 2, 3, 4, 5] >>> x[0] >>> x[1]

In Python you can additionally access elements with negative numbers, for example, the last element in the list has index -1. This is useful because then we do not need to calculate the length of the list to calculate the correct index: >>> x[-1] >>> x[len(x) - 1] # unnecessary!

Your answer from the previous question prevents duplicates from being appended to a list. Rewrite your function to reject consecutive duplicates, i.e. it should work like this: x = [] append_non_dup(x, 1) append_non_dup(x, 2) append_non_dup(x, 2) append_non_dup(x, 1)

if x == [1,2,1] : print("correct!")

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7. Elements can be removed from a list by value using the remove(? ? ?) method or by index using the del keyword: >>> x = [1,1,2,2,3,3] >>> x.remove(3) # removes first 3 in the list >>> del x[-1] # removes the element at index -1 (the last one) What happens if you try to remove a value not present in the list or using an index outside of the range of the list?

8. As well as single values we can access subsets of the list. In Python these are called slices. Assume we have a list called x: >>> x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

and two integers to use as indices: slice start and slice end: >>> slice_start = 3 >>> slice_end = 6 We can extract the slice from index slice start (inclusive) to the end of the list: >>> x[slice_start : ]

From the start of the list up to index slice end (exclusive): >>> x[ : slice_end]

And, finally, from slice start (inclusive) to slice end (exclusive): >>> x[slice_start : slice_end]

Make sure you understand how this works by doing a few more examples yourself. The following code helps to demonstrate why the start of the slice is inclusive and the end of the slice is exclusive (because using the same index will neatly split the list in two non-overlapping lists):

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>>> x = [0,1,2,3,4,5] >>> x == x[:3] + x[3:]

9. All Python scripts have a predefined list called argv in the sys module, which contains all command line arguments given to a script when it is run: import sys

print(len(sys.argv), "command line arguments") print(sys.argv) Copy the code above to a file (I have called my version "argv.py") and run it with some arguments: python3 argv.py hello world 1 2 3

Change the script above to only print out the command line arguments after the name of the script (to understand what I mean, run the example).

10. There are two methods to sort a list into order. For both methods to work all elements in the list must be orderable (i.e. values that can be compared with (==, =). The sort(? ? ?) method will sort a list in place, whereas the sorted(? ? ?) function returns a new list: >>> x = [2,5,1,3,4] >>> y = x.copy() >>> y.sort() >>> z = sorted(x) Both methods accept the reverse parameter to sort in reverse order: >>> sorted([2,5,1,3,4], reverse=True)

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