Free Recall

• Get out a sheet of paper or open a text editor
• For 2 minutes write down whatever comes to mind about the last class
  • This could be topics you learned
  • Questions you had
  • Connections you made

Setting Up CS Circles

• Sign up for a free account on CS Circles
• This is mandatory
• Next set jhaugh as your “guru”
  • Click “Edit my profile” in the top right menu
  • Set the guru field to jhaugh
• I will now be able to see your progress and give you credit in Canvas

Chapter 1: Variables

• Complete all exercises in chapter 1
• I will give you around 15-20 minutes for this

Different Types For Different Situations

• Data comes in many different shapes and sizes
• To try to categorize these many languages employ the use of types
• Types tell us useful information about what is held inside a variable
• So far we have encountered two different types:
  • Integers: 1
  • Strings: "hello"
• Another which we will often see is floating-point number often abbreviated as float
• Floats are numbers with a decimal point, e.g. 3.14

Inspecting Types

• You can ask Python what the type of a variable is by using the type() function:

  x = 3
  y = 3.14
  z = "hello"
  print(type(x))
  print(type(y))
  print(type(z))

  Visualize

• Outputs:

  <class 'int'>
  <class 'float'>
  <class 'str'>

Explaining Types

• Types are useful for both the programmer and the computer
• The programmer can easily get a sense of what is held in a variable
• The computer can make sure that the types of variables match when necessary
• For now we will focus on 3 types:
  • Integers: Whole numbers
  • Floats: Numbers with a decimal point
  • Strings: Sequence of characters

Test Your Understanding

What type will each variable in the following code have?

x = 3.0
y = 4.256
z = ""
print(type(x))
print(type(y))
print(type(z))

Visualize

Converting Between Types

• Many times you will be given data of one type but you need it to be another type
• This often happens between strings and other types
• Luckily, Python makes this fairly easy
• There is a function for each type we have discussed so far to convert to that type
• These functions are:
  • int(): Converts from another type to integer
  • float(): Converts from another type to float
  • str(): Converts from another type to string

Int() Function

Let’s try out the int() function:

x = "33"
y = 3.14
z = 3.65
a = "hello"
print(int(x))
print(int(y))
print(int(z))
print(int(a))

Float() Function

Next the float() function:

x = "33"
y = 3
z = "3.14"
a = "hello"
print(float(x))
print(float(y))
print(float(z))
print(float(a))

Str() Function

Finally, the str() function:

x = 3
y = 3.14
z = "hello"
a = 'world'
print(str(x))
print(str(y))
print(str(z))
print(str(a))

First Step To Useful Programs

• So far all are programs have been static

• For example:

  c = 10
  f = (c * (9/5)) + 32
  print(f)

  Visualize

• The program above converts celsius to fahrenheit

• Pretty useful but what if we want to convert a number other than 10?

• Right now we would have to change the program!

• There is a better way…

User Input

c_str = input()
c = float(c_str)
f = (c * (9/5)) + 32
print(f)

Visualize

• The input() function asks the user for a value
• This value is returned as a string
• Thus, to use it in our calculation we must convert it to a float
• CS Circles handles user input slightly differently then if you were running Python directly.
  • You need to enter your input before the program is run
  • CS Circles then handles giving this to your program
  • We will see later that normally you give this input during the run of the program

String Input

What does the following code do?

msg = input()
print(msg)

Visualize

Exercise: Sourdough Starter Ratios

• Sourdough starters are made up 3 ingredients:
  • Starter from before
  • Flour
  • Water
• When “feeding” you need to follow a prescribed ratio of starter to flour to water
• This is usually given something like “1:5:5”
  • This means 1 part starter, 5 parts flour, 5 parts water
  • For example say I had 40 grams of starter
  • With the ratio above I would then need to use 200 grams flour and 200 grams water

Exercise: Sourdough Starter Ratios

• Your task is to create a program which asks for 3 inputs
• These inputs are all given as ints
  • How much starter? (the 40 from before)
  • What is the flour ratio? (the 5 from before)
  • What is the water ratio? (the other 5 from before)
• Your program then needs to print how much flour and water are needed
• Note: You need to use the int() and input() functions to gather data
• For example to gather how much starter the user wants to use you might do the following:

starter = int(input())