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Module 1a - Beginning Student Language

Table of Contents

Overview

This module introduces the Beginning Student Language (BSL) and the DrRacket programming environment. BSL is a simple language that is designed to help students learn the basics of programming. It is a subset of the Racket programming language.

Learning Objectives

Learn how to:

  • write expressions that operate on primitive data including numbers, strings, images and booleans.
  • write constant and function definitions.
  • write out the step-by-step evaluation of simple expressions including function calls.
  • use the stepper to automatically step through the evaluation of an expression.
  • use the Dr Racket help desk to discover new primitives.

Core Concepts

Expressions

An expression is a combination of operators and operands that can be evaluated to a value. It can formed by opening a parenthesis, followed by an operator, followed by any number of operands, and closed by a parenthesis. For example, the expression (+ 1 2) adds the numbers 1 and 2. The expression (+ 1 (* 2 3)) adds 1 to the product of 2 and 3.


Evaluation

The evaluation of an expression is the process of simplifying it to a single value. For example, the expression (+ 1 2) evaluates to 3.

There are two rules for evaluation. The first rule is to reduce operands to values. The second rule is to apply the operator to the values. So, say we have (+ 1 (* 2 3)). This expression is a call to a premitive function so, we first evaluate all the operands to values. The operands are 1 and (* 2 3). The first operand is already a value so we move on to the second operand. The second operand is a call to a primitive function so we evaluate it. The operands of the second operand are 2 and 3. These are already values so we apply the operator to the values. Which means, multiply 2 and 3 to get 6. Now we have the values 1 and 6. We apply the operator + to these values to get 7. So, the expression (+ 1 (* 2 3)) evaluates to 7.

(+ 1 (* 2 3))
(+ 1 6)

7

Strings and Images

In this section, I learned about the string and image primitives values. We can create a string by enclosing it in double quotes. Eg. "SelormDev". I also learned a couple of primitives that operate on strings. Examples are the string-length and string-append functions. The string-length function returns the length of a string and the string-append function concatenates two strings. There's also substring which sorts of slices part of a string with zero based indexing.

(string-length "SelormDev")
(string-append "Selorm" "Dev")
(substring "SelormDev" 0 6)

I also learned about the rectangle and circle functions that create images. The rectangle function takes in the width and height of the rectangle and the circle function takes in the radius of the circle. I also learned about the overlay function that overlays two images on top of each other.

(rectangle 100 100 "solid" "blue")
(circle 50 "solid" "red")
(overlay (rectangle 100 100 "solid" "blue") (circle 50 "solid" "red"))

Constant Definitions

A constant definition basicaly is storing a value in a constant variable. The syntax is (define constant-name value). For example, (define pi 3.14) defines a constant pi with the value 3.14. We can then use the constant in expressions. For example, (* pi 2) evaluates to 6.28.

(define pi 3.14)
(* pi 2)

Function Definitions

A function definition is a way to define a function that can be called later. The syntax is (define (function-name parameter-list) body). For example, (define (square x) (* x x)) defines a function square that takes in a parameter x and returns the square of x. We can then call the function by passing in a value. For example, (square 5) evaluates to 25.

(define (square x) (* x x))
(square 5)

Booleans and if Expressions