02-class.md

Code Club 2

We are continuing our stroll through the Prologue of HtDP2e.

(When we have finished this stroll, you will know much of the language. Then we will start over and talk about a more systematic way of designing programs.)

Exercises

• Compute the sum of 10, 20 and 30.

• Multiply 48 384 by 2 091.

• Divide a billion iby a thousand.

• Try evaluating 1/2 (just as written, not (/ 1 2)). What about 2/3? What about 2 / 3? What's going on?

• The speed of light is 3×10⁸ m/s; it is 93 million miles from the earth to the sun; and a mile is about 8/5 km. How long does it take the light from the sun to reach the earth?

  (Try to do this all in one expression.)

• Temperatures in Celsius, C, can be converted to Fahrenheit, F, with the following formula: C = (5/9)×(F−32). What is 37.5 degrees Celsius in Fahrenheit?

• What is the difference between 12 and (12)? What happens if you ask Racket to evaluate each? Can you explain the error message?

Recap of 01

We saw two ways to make expressions. Either:

1. A value; or

2. ( function arg₁ arg₂ ... )

where the args are also expressions. And so far, "a value" has meant "a number". (This is not the whole truth!)

Overview of today

• Other kinds of value.

• Other functions (on those new kinds of values).

• (Perhaps) Definitions.

Other kinds of values

1. Numbers (42)
2. Strings ("Hello, World.")
3. Truth (#true, #false) (these are called "Booleans", after George Boole)
4. Images

Other kinds of function

Try the following. Before each, try to say what you expect to happen.

"Hello, World."

(string-length "Hello, World.")

(string-append "Hello" "World.")

(string-append "Hello" ", " "World" ".")

(string-append "Hello" 42)

(string-length "Hello, World.")

(string=? "Hello" "World")

(string=? "Hello" "hello")

(string=? "Hello" "Hello ")

(number->string 42)

(= 1 2)

(= 2 2)

(> 2 1)

(and #true #false)

(or #true #false #false)