CPSC120B
Fundamentals of Computer Science

Reading Questions

Quiz

• Note that on the quiz for today there are questions with answers that are identical values but different types.
• For example, 2 vs 2.0
• The difference is important, 2 is an int and 2.0 is a float
• Being able to tell the difference in the output of a program is important to preventing type errors

Idle

• On the daily schedule there is a link to the lab activity for today
• You could do all of these in the web browser using the textbook, but your code would be lost when you close the web page
• So let’s first go over how to write programs outside of the web browser
• Launch Idle
• Create a new file (File > New)
• Add some Python code that uses turtle
• Save (File > Save)
• Run it (Run > Run Module)

Hacker Rank

• First homework assignment is posted
• It uses the website Hacker Rank which will test your code
• This is great because you get immediate feedback on the correctness of your programs
• So you don’t have to wait 2 days for me to grade them
• So you can fix your problems as soon as you discover them and learn more quickly

Types

• We have used variables to store information in the computer’s memory
• Everything thing in a computer’s memory is represented with bits, off or on
• Different types of values are represented in memory differently
• Note that the values "2", 2.0, and 2 are all stored in memory differently even though they all represent the same thing
• Strings can be text, "two" or "2", the literal value has quotes
• Floats can be real numbers, 1.5 or 2.0, the literal value has a decimal point
• Ints can be integers, 1 or -2, the literal value has no decimal point
• Can convert between types with the functions str, float, and int
• When converting a string to a float or int float will produce an error if the string does not represent a literal
• When converting to a float to an int, the decimal values will be truncated i.e. rounded to zero
• When performing aritmetic operations, if both operands are ints the result will be an int, otherwise the result will be a float
• Unless the operator is /, in which case the result will always be a float
• Aritmetic operators with strings and numbers will produce errors
• Except for adding two string, concatenate, and multiplying a string and an integer, repeating

Input

• Can rewrite our programs so that instead of changing the value of a variable at the top of the program, it will prompt the user for a value
• The input function stops the program until the user hits the enter key
• The function evaluates to whatever the user typed, as a string

• So the age in seconds program

  age_years = 31
  seconds_in_year = 31536000
  age_seconds = age_years * seconds_in_year
  print("my age in seconds is", age_seconds)

• Becomes

  age_years = input('enter your age in years: ')
  seconds_in_year = 31536000
  age_seconds = age_years * seconds_in_year
  print("my age in seconds is", age_seconds)

• The string inside the parentheses of the input function are what are printed while waiting for the user to type something, it is the prompt
• Note, the prompt doesn’t include a space, so it usually looks better if you add one
• But the program doesn’t work, recall the function evaluates to a string and multiplying a string by a number repeats the string

• So to fix, need to convert the string to an integer

  age_years = int(input('enter your age in years: '))
  seconds_in_year = 31536000
  age_seconds = age_years * seconds_in_year
  print("my age in seconds is", age_seconds)

Activity