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COMP 1000: Think Like a Computer: The PreTeXt Interactive Edition

Section 9.12 Exercises

Checkpoint 9.12.1.

Write a program that will print out the length of each item in the list as well as the first and last characters of the item.

Checkpoint 9.12.2.

Write code to determine how many ’t’s are in the following sentences.

Checkpoint 9.12.3.

Although Python provides us with many list methods, it is good practice and very instructive to think about how they are implemented. Implement a Python function that works like the following:
  1. count
  2. in
  3. reverse
  4. index
  5. insert
Solution.
def count(obj, lst):
    count = 0
    for e in lst:
        if e == obj:
            count = count + 1
    return count
def is_in(obj, lst):  # cannot be called in() because in is a reserved keyword
    for e in lst:
        if e == obj:
            return True
    return False
def reverse(lst):
    reversed = []
    for i in range(len(lst)-1, -1, -1): # step through the original list backwards
        reversed.append(lst[i])
    return reversed
def index(obj, lst):
    for i in range(len(lst)):
        if lst[i] == obj:
            return i
    return -1
def insert(obj, index, lst):
    newlst = []
    for i in range(len(lst)):
        if i == index:
            newlst.append(obj)
        newlst.append(lst[i])
    return newlst
lst = [0, 1, 1, 2, 2, 3, 4, 5, 6, 7, 8, 9]
print(count(1, lst))
print(is_in(4, lst))
print(reverse(lst))
print(index(2, lst))
print(insert('cat', 4, lst))

Checkpoint 9.12.4.

Write a Python function that will take a list of 100 random integers between 0 and 1000 and return the maximum value. (Note: there is a builtin function named max but pretend you cannot use it.)
Solution.
import random
def max(lst):
    max = 0
    for e in lst:
        if e > max:
            max = e
    return max
lst = []
for i in range(100):
    lst.append(random.randint(0, 1000))
print(max(lst))

Checkpoint 9.12.5.

Write a function sum_of_squares(xs) that computes the sum of the squares of the numbers in the list xs. For example, sum_of_squares([2, 3, 4]) should return 4+9+16 which is 29:

Checkpoint 9.12.6.

Create a list of 100 random integers between 0 and 1000, and write code for the function called countOdd(lst) to count every odd number in the list
Solution.
import random
def countOdd(lst):
    odd = 0
    for e in lst:
        if e % 2 != 0:
            odd = odd + 1
    return odd
# make a random list to test the function
lst = []
for i in range(100):
    lst.append(random.randint(0, 1000))
print(countOdd(lst))

Checkpoint 9.12.7.

Create a list of 100 random integers between -1000 and 1000, and write code for the function called sumEven(lst) to sum up all the even numbers in the list
Solution.
import random
def sumEven(lst):
    sum = 0
    for e in lst:
        if e % 2 == 0:
            sum += e
    return sum
# make a random list to test the function
lst = []
for i in range(100):
    lst.append(random.randrange(-1000, 1000))
print(sumEven(lst))

Checkpoint 9.12.8.

Create a list of 100 random integers between -1000 and 1000, and write code for the function called sumNegatives(lst) to sum up all the negative numbers in the list
Solution.
import random
def sumNegatives(lst):
    sum = 0
    for e in lst:
        if e < 0:
            sum += e
    return sum
lst = []
for i in range(100):
    lst.append(random.randrange(-1000, 1000))
print(sumNegatives(lst))
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