1.3. Solutions to Python Exercise

Note

The solution to each task is not exclusive. Depending on the purpose of our programming, we may wish our scripts to be intuitive or concise or fast or flexible. There should be many other ways to accomplish the following programming tasks.

1.3.1. Task 1 (Containers)

For a list which contains three sublists precipitation = [[0, 0, 3.0], [1.1, 4.8, 0], [0, 0, 0]], write a program to find the maximum for each sublist and print it as a dictionary whose key is sublist_n (where n is the index of sublist) and value is the corresponding maximum.

Hint: use max() to obtain the maximum of a list. You could also try to use dictionary comprehension to accomplish this task within one line of code.

# Your solution goes here.

precipitation = [[0, 0, 3.0], [1.1, 4.8, 0], [0, 0, 0]]

print({"sublist_{}".format(x): max(precipitation[x]) for x in range(0, len(precipitation))})

{'sublist_0': 3.0, 'sublist_1': 4.8, 'sublist_2': 0}

Note

len() is a Python built-in function that returns the number of elements in a list. We may use help(len) to see the description and usage of this function.

1.3.2. Task 2 (Control flows)

Write a program to print all prime numbers within the range range(25, 50).

Hint: a prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. You may wish to use the following statements:

• use for num in range(25, 50) to iterate over all numbers;

• for a specific number, use for i in range(2, num) to iterate over potential factors except 1;

• use (num % i) == 0 to determine whether i is truly a factor of num (if it is true, num is not a prime number); and

• use break to stop one loop.

# Your solution goes here.

for num in range(25, 50):
    if num > 1:  # all prime numbers are greater than 1
        for i in range(2, num):  # check for factors
            if (num % i) == 0:  # not a prime number, so break inner loop
                break
        else:
            print(num)

29
31
37
41
43
47

Note

It is not common to use for-else loops. else here means that the following algorithms will be run when the for loop ends. Normally, if we want to run specific algorithms after loops, we just need to script them after loops. However, if break exists in the loop, the algorithms in else will not execute when break happens, because the loop do not reach its end. Thus, for-else loops is useful only when there exists break statements within loops.

In addition, please mind the indents before each line. Python relies on indents to identify code blocks, so a wrong slip in indents may cause errors or even unanticipated results. You may try adding indents before else: and print(num) to see what will happen.

1.3.3. Task 3 (Functions)

Following Task 2, define a function that returns the list of prime numbers for a specified range. Then, use this function to print the prime numbers within the ranges range(25, 50) and range(50, 100) respectively.

Hint: in the function, you may define an empty list prime_num = [] at first and use the append method to add prime numbers.

# Your solution goes here.

def findprime(rangelist):
    prime_num = []
    for num in rangelist:
        if num > 1:
            for i in range(2, num):
                if (num % i) == 0:
                    break
            else:
                prime_num.append(num)
    return prime_num


print(findprime(range(25, 50)))
print(findprime(range(50, 100)))

[29, 31, 37, 41, 43, 47]
[53, 59, 61, 67, 71, 73, 79, 83, 89, 97]

1.3.4. Bonus Task

Still remember the example quicksort() programming at the beginning of Python Tutorial? Try to comprehend how this function sorts a list. This function was run in a recursive manner (i.e. the function was used within its own codes). This brings effects quite alike as loops; you could try adding a printing function print(left, middle, right) to see what happened in each step.