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2.21 Exercises

  1. Variables, print, and the String Format Operator. Start the interactive interpreter. Assign values to some variables (strings, numbers, etc.) and display them within the interpreter by typing their names. Also try doing the same thing with the print statement. What is the difference between giving just a variable name versus using it in conjunction with print? Also try using the string format operator ( % ) to become familiar with it.
  2. Program Output. Take a look at the following Python script:
    #!/usr/bin/env python
    1 + 2 * 4
    1. What do you think this script does?
    2. What do you think this script will output?
    3. Type the code in as a script program and execute it. Did it do what you expected? Why or why not?
    4. How does execution differ if you are running this code from within the interactive interpreter? Try it and write down the results.
    5. How can you improve the output of the script version so that it does what you expect/want?
  3. Numbers and Operators. Enter the interpreter. Use Python to add, subtract, multiply, and divide two numbers (of any type). Then use the modulus operator to determine the remainder when dividing one number by another, and finally, raise one number to the power of another by using the exponentiation operator.
  4. User Input with raw_input().
    1. Create a small script to use raw_input() built-in function to take a string input from the user, then display to the user what he/she just typed in.
    2. Add another piece of similar code, but have the input be numeric. Convert the value to a number (using either int() or any of the other numeric conversion functions), and display the value back to the user. (Note that if your version of Python is older than 1.5, you will need to use the string.ato*() functions to perform the conversion.)
  5. Loops and Numbers. Create some loops using both while and for.
    1. Write a loop that counts from 0 to 10 using a while loop. (Make sure your solution really does count from 0 to 10, not 0 to 9 or 1 to 10.)
    2. Do the same loop as in part (a), but use a for loop and the range() built-in function.
  6. Conditionals. Detect whether a number is positive, negative, or zero. Try using fixed values at first, then update your program to accept numeric input from the user.
  7. Loops and Strings. Take a user input string and display string, one character at a time. As in your above solution, perform this task with a while loop first, then with a for loop.
  8. Loops and Operators. Create a fixed list or tuple of five numbers and output their sum. Then update your program so that this set of numbers comes from user input. As with the problems above, implement your solution twice, once using while and again with for.
  9. More Loops and Operators. Create a fixed list or tuple of five numbers and determine their average. The most difficult part of this exercise is the division to obtain the average. You will discover that integer division truncates and that you must use floating point division to obtain a more accurate result. The float() built-in function may help you there.
  10. User Input with Loops and Conditionals. Use raw_input() to prompt for a number between 1 and 100. If the input matches criteria, indicate so on the screen and exit. Otherwise, display an error and reprompt the user until the correct input is received.
  11. Menu-Driven Text Applications. Take your solutions to any number of the previous five problems and upgrade your program to present a menu-driven text-based application that presents the user with a set of choices, e.g., (1) sum of five numbers, (2) average of five numbers,...s; (X) Quit. The user makes a selection, which is then executed. The program exits when the user chooses the “quit” option. The great advantage of a program like this is that it allows the user to run as many iterations of your solutions without necessarily having to restart the same program over and over again. (It is also good for the developer who is usually the first user and tester of their applications!)
  12. The dir() Built-In Function.
    1. Start up the Python interpreter. Run the dir() built-in function by simply typing dir() at the prompt. What do you see? Print the value of each element in the list you see. Write down the output for each and what you think each is.
    2. You may be asking, so what does dir() do? We have already seen that adding the pair of parentheses after dir causes the function to run. Try typing just the name dir at the prompt. What information does the interpreter give you? What do you think it means?
    3. The type() built-in function takes any Python object and returns its type. Try running it on dir by entering type(dir) into the interpreter. What do you get?
    4. For the final part of this exercise, let us take a quick look at Python documentation strings. We can access the documentation for the dir() function by appending .__doc__after its name. So from the interpreter, display the document string for dir() by typing the following at the prompt: print dir.__doc__. Many of the built-in functions, methods, modules, and module attributes have a documentation string associated with them. We invite you to put in your own as you write your code; it may help another user down the road.
  13. Finding Out More About the sys Module with dir().
    1. Start the Python interpreter again. Run the dir() command as in the previous exercise. Now import the sys module by typing import sys at the prompt. Run the dir() command again to verify that the sys module now shows up. Now run the dir() command on the sys module by typing dir(sys). Now you see all the attributes of the sys module.
    2. Display the version and platform variables of the sys module. Be sure to prepend the names with sys to indicate that they are attributes of sys. The version variable contains information regarding the version of the Python interpreter you are using, and the platform attribute contains the name of the computer system that Python believes you are running on.
    3. Finally, call the sys.exit() function. This is another way to quit the Python interpreter in case the keystrokes described above in problem 1-4 do not get you out of Python.
  14. Operator Precedence and Grouping with Parentheses.

    Rewrite the mathematical expression of the print statement in Section 2.4, but try to group pairs of operands correctly, using parentheses.

  15. Elementary Sorting.
    1. Have the user enter three numeric values and store them in three different variables. Without using lists or sorting algorithms, manually sort these three numbers from smallest to largest.
    2. How would you change your solution in part (a) to sort from largest to smallest?
  16. Files. Type in and/or run the file display code in Section 2.15. Verify that it works on your system and try different input files as well.

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