COS 100: Introduction to Programming

Interim 2020

Lab 2: Making decision

In-class practice labs will not directly impact your grade (although you'll get your standard participation point for staying on task), and will serve to help you with homework projects.

Lab 2A: Is my number divisible by 3?

• Goal: To start experimenting with if statements.
• Task: Follow the steps.
  1. Prompt the user for an integer, and report whether it is divisible by 3. Use an if-else construction. Sample runs:

     Integer, please: 30
     Divisible by 3, yay!
     
     Integer, please: 23
     Not divisible by 3, sad face.
     

  2. Modify your program so if the number is not divisible by 3, you report the remainder. Use an if-else construction nested inside the "not divisible" case. Sample runs:

     Integer, please: 30
     Divisible by 3, yay!
     
     Integer, please: 23
     The remainder is two.
     
     Integer, please: 7
     The remainder is one.
     

  3. Modify your program to use if-elif-else construction instead of two nested if-else constructions. Maintain the same program behavior.
• Before moving on: Take a moment to meditate upon the benefits of the third step above. This is known as refactoring code: even though no new functionality has been added to the program, the code has been changed to make it easier to read/understand/maintain.

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Lab 2B: What letter grade do I have in this class? (chained ifs)

• Goal: To get some practice with chained if statements (a special case of nested if statements, reformatted).
• Task:
  • Prompt the user to input the percentage they have in a class (as a float number between 0 and 100, without the percent sign).
  • Tell them what letter grade they are getting.
  • Use high school standard 90-80-70-60 cutoffs for A, B, C, and D, respectively.
• Hints:
  • Use a chain of if ... elif ... elif ... elif ... elif ... else.
  • (I'm not telling you exactly how many elifs you need.)
  • Note that the last block is just else rather than elif; it catches all situations that don't match any of the previous cases.
  • Once one of the if conditions is true, it will execute the associated block of code and then skip all the way to after the entire chain.
• Before moving on: Have the professor or TA look at your code for feedback on your formatting. There are multiple legal ways to format this, but really only one standard way. Get into the right habit now.

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Challenge Lab 2C: Use if only, or else!

• Goal: To think about the expressive power of if alone.
• Task: Make a copy of the files above and rewrite them to only use if, without using elif or else.
• Before moving on: Promise yourself to not do this again. Even though elif and else are technically unnecessary (i.e., you can accomplish your tasks without them), these constructs do make code much more organized and understandable.