6.11. Exercises: Loops

Loops simplify repetitive tasks!
Image showing how Bart Simpson could use Python to write his sentences on a chalkboard.

Image from Bart’s Blackboard, Season 2, Episode 11 (Jan 24, 1991)

6.11.1. for Practice

Note

If your teacher added you to a Trinket course, complete the exercises there.

Otherwise, use the links below to code in your own free account.

  1. Construct for loops that accomplish the following tasks: (repl.it code or Trinket code)

    1. Print the numbers 0 - 20, one number per line.
    2. Print only the ODD values from 3 - 29, one number per line.
    3. Print the EVEN numbers 12 down to -14 in descending order, one number per line.
    4. Print the numbers 50 down to 20 in descending order, but only if the numbers are multiples of 3.
  2. Given the string 'LaunchCode', code two for loops to do the following: (repl.it or Trinket)

    1. Print out each character of the string, one letter per line. Do this WITHOUT using index values.
    2. Now use index values to print each character of the string—in reverse order—to a new line. Recall that you can access a single character from a string with the syntax var_name[index], where index is an integer value, and var_name is the variable used to store the string.
  3. Given the string gibberish = 'Vna#hewzB*rQhT%yq^lv %iPwgOexWo &C^oUoGSdtJLj', print every fifth character, including the first character. Use index values and range(start, stop, step).

    Hint: Instead of figuring out the stop value by counting all of the characters in gibberish yourself, make Python do it for you! Recall that len(gibberish) returns the length of the string stored in the variable.

    repl.it file or Trinket file

Check your solutions.

6.11.1.1. Bonus

Repeat the previous problem, but:

  1. Replace range(start, stop, step) with range(len(gibberish)).
  2. Use an if statement and the modulus (%) operator to check if the index is divisible by 5.
  3. If True, print the character. If False, do not print the character.
  4. The output should be the same as before.

6.11.2. while Practice

Define three variables for a spacecraft—one for the starting fuel level, another for the number of astronauts aboard, and the third for the altitude the spacecraft reaches. Assign each variable an initial value of 0.

While loop starter code: repl.it or Trinket.

  1. Construct while loops to do the following:

    1. Ask the user for the starting fuel level. The loop should continue until the user enters a value between 5000 and 30000. If the user submits a number outside of the range, print "Invalid entry."
    2. Use a second loop to prompt the user for the number of astronauts (up to a maximum of 7). Validate the entry by having the loop continue until the user enters an integer from 1 - 7. For numbers outside of the range, print "Invalid entry."
  2. Use a third while loop to update the fuel and the altitude of the spacecraft. Each iteration, decrease the fuel level by 100 units for each astronaut aboard. Also, increase the altitude by 50 kilometers.

    Hint: The loop should end when there is not enough fuel to boost the crew another 50 km, so the fuel level might not reach 0.

  3. After the loops finish, print the result using the phrase, The spacecraft gained an altitude of ___ km and has ___ kg of fuel left. Fill in the blanks with the altitude and fuel level values.

  4. If the altitude is 2000 km or higher, add "Orbit achieved!" to the output. Otherwise add, "Failed to reach orbit."

Sample Output

Enter starting fuel level (5000 - 30000):  22000
Enter the number of astronauts (1 - 7):  3
The spacecraft gained an altitude of 3650 km and has 100 kg of fuel left. Orbit achieved!

Check your solutions.

6.11.3. The Accumulator Pattern

Accumulator starter code: repl.it or Trinket.

Use two input statements to prompt the user for a start_value and an end_value. Both inputs should be integers.

  1. Use a loop to add up all of the numbers from start_value to end_value. Use the variable total as the accumulator. Print "The sum of the numbers from ___ to ___ is ___." Fill in the blanks with the values for start_value, end_value, and total.
  2. Define a variable to hold the string 'It was a bright cold day in April, and the clocks were striking thirteen.' Use the accumulator pattern to build a new string. It should contain all of the characters in the original string, but without any vowels. For this task, y does NOT count as a vowel. Print the new string.

Check your solutions. Challenge ———

If our spacecraft gets hijacked by space pirates, the astronauts can activate a self-destruct sequence to provide some drama for the viewers at home.

In order to prevent a rogue astronaut from activating the code, it takes two crew members to begin the countdown. Each person must enter a different code, after which the computer will “zip” them together before overloading the engines.

In a new code file, construct a loop that combines two strings together, alternating the characters from each source. For now, be careful to make both strings the same length.

Examples

  1. If string = "1234" and other_string = "5678", then the output will be "15263748".
  2. If code_1 = "ABCDEF" and code_2 = "notyet", then the output will be "AnBoCtDyEeFt".
  3. If ka = "LoOt" and blam = "oku!", then the output will be "LookOut!".

Check your solutions.