Exercises

Here are some exercises for you to try on your own. Solutions are available at http://pragprog.com/titles/gwpy3/practical-programming.

1. Write a for loop to print all the values in the celegans_phenotypes list from ​Slicing Lists​, one per line. celegans_phenotypes refers to [’Emb’, ’Him’, ’Unc’, ’Lon’, ’Dpy’, ’Sma’].

2. Write a for loop to print all the values in the half_lives list from ​Operations on Lists​, all on a single line. half_lives refers to [87.74, 24110.0, 6537.0, 14.4, 376000.0].

3. Write a for loop to add 1 to all the values from whales from ​Storing and Accessing Data in Lists​, and store the converted values in a new list called more_whales. The whales list shouldn’t be modified. whales refers to [5, 4, 7, 3, 2, 3, 2, 6, 4, 2, 1, 7, 1, 3].

4. In this exercise, you’ll create a nested list and then write code that performs operations on that list.

   1. Create a nested list where each element of the outer list contains the atomic number and atomic weight for an alkaline earth metal. The values are beryllium (4 and 9.012), magnesium (12 and 24.305), calcium (20 and 40.078), strontium (38 and 87.62), barium (56 and 137.327), and radium (88 and 226). Assign the list to variable alkaline_earth_metals.

   2. Write a for loop to print all the values in alkaline_earth_metals, with the atomic number and atomic weight for each alkaline earth metal on a different line.

   3. Write a for loop to create a new list called number_and_weight that contains the elements of alkaline_earth_metals in the same order but not nested.

5. The following function doesn’t have a docstring, type annotations, or comments. Write enough of all three to make it easy for another programmer to understand what the function does and how, and then compare your solution with those of at least two other people. How similar are they? Why do they differ?

   ​	​def​ mystery_function(values):
   ​	​   ​result = []
   ​	​   ​​for​ sublist ​in​ values:
   ​	​       ​result.append([sublist[0]])
   ​	​       ​​for​ i ​in​ sublist[1:]:
   ​	​           ​result[-1].insert(0, i)
   ​
   ​	​   ​​return​ result

6. In ​Repetition Based on User Input​, you saw a loop that prompted users until they typed quit. This code won’t work if users type Quit, or QUIT, or any other version that isn’t exactly quit. Modify that loop so that it terminates if a user types that word with any capitalization.

7. Consider the following statement, which creates a list of populations of countries in eastern Asia (China, DPR Korea, Hong Kong, Mongolia, Republic of Korea, and Taiwan) in millions: country_populations = [1295, 23, 7, 3, 47, 21]. Write a for loop that adds up all the values and stores them in variable total. (Hint: Give total an initial value of zero, and, inside the loop body, add the population of the current country to total.)

8. You are given two lists, rat_1 and rat_2, that contain the daily weights of two rats over a period of ten days. Assume the rats never have exactly the same weight. Write statements to do the following:

   1. If the weight of rat 1 is greater than that of rat 2 on day 1, print "Rat 1 weighed more than rat 2 on day 1."; otherwise, print "Rat 1 weighed less than rat 2 on day 1.".

   2. If rat 1 weighed more than rat 2 on day 1 and if rat 1 weighs more than rat 2 on the last day, print "Rat 1 remained heavier than Rat 2."; otherwise, print "Rat 2 became heavier than Rat 1."

   3. If your solution to the previous exercise used nested if statements, then do it without nesting, or vice versa.

9. Print the numbers in the range 33 to 49 (inclusive).

10. Print the numbers from 1 to 10 (inclusive) in descending order, all on one line.

11. Using a loop, sum the numbers in the range 2 to 22 (inclusive), and then calculate the average.

12. Consider this code:

    ​	​from​ typing ​import​ List
    ​
    ​	​def​ remove_neg(num_list: List[float]) -> None:
    ​	​"""Remove the negative numbers from the list num_list.​
    ​
    ​	​ >>> numbers = [-5, 1, -3, 2]​
    ​	​ >>> remove_neg(numbers)​
    ​	​ >>> numbers​
    ​	​ [1, 2]​
    ​	​ """​
    ​
    ​	​for​ item ​in​ num_list:
    ​	​if​ item < 0:
    ​	num_list.remove(item)

    When remove_neg([1, 2, 3, -3, 6, -1, -3, 1]) is executed, it produces [1, 2, 3, 6, -3, 1]. The for loop traverses the elements of the list, and when a negative value (like -3 at position 3) is reached, it is removed, shifting the subsequent values one position earlier in the list (so 6 moves into position 3). The loop then continues on to process the next item, skipping over the value that moved into the removed item’s position. If there are two negative numbers in a row (like -1 and -3), then the second one won’t be removed.

    Rewrite the code to avoid this problem.

13. Using nested for loops, print a right triangle of the character T on the screen where the triangle is one character wide at its narrowest point and seven characters wide at its widest point:

    ​	T
    ​	TT
    ​	TTT
    ​	TTTT
    ​	TTTTT
    ​	TTTTTT
    ​	TTTTTTT

14. Using nested for loops, print the triangle described in the previous exercise with its hypotenuse on the left side:

    ​	T
    ​	TT
    ​	TTT
    ​	TTTT
    ​	TTTTT
    ​	TTTTTT
    ​	TTTTTTT

15. Redo the previous two exercises using while loops instead of for loops.

16. Variables rat_1_weight and rat_2_weight contain the weights of two rats at the beginning of an experiment. Variables rat_1_rate and rat_2_rate are the rate that the rats’ weights are expected to increase each week (for example, 4 percent per week).

    1. Using a while loop, calculate how many weeks it would take for the weight of the first rat to become 25 percent heavier than it was originally.

    2. Assume that the two rats have the same initial weight, but rat 1 is expected to gain weight at a faster rate than rat 2. Using a while loop, calculate how many weeks it would take for rat 1 to be 10 percent heavier than rat 2.