Programs and Assignments

How x = 2; print x; becomes output

Gregory S. DeLozier, PhD

The New Program

x = 2;
print x;
2

What must the interpreter remember?

Expressions and Statements

An expression produces a value.

x + 3 * 4

A statement performs an action.

x = 2
print x

A Program Contains Statements

program ::= statement_list

statement ::= assignment_statement
          | print_statement

A statement list contains at least one statement.

Separating Statements

Source Valid?
x = 2 Yes
x = 2; print x Yes
;;;x = 2;;;print x;;; Yes
x = 2 print x No
empty input No

Comments and Statement Separators

x = 2; // Keep the separator.
print x

The newline does not replace the semicolon.

x = 2 // ; is part of the comment.
print x

The second program has no separator between statements.

Statement List Grammar

statement_list ::= { ";" } statement
                 { ";" { ";" } statement }
                 { ";" }

Braces mean zero or more repetitions.

The Two Statement Forms

assignment_statement ::= <identifier> "=" expression

print_statement ::= "print" expression
x = 2
print x

Identifiers in Expressions

factor ::= <number>
       | <identifier>
       | "(" expression ")"
y = x + 3 * 4

What value does x have?

Expressions Carry Forward

term ::= unary { ("*" | "/") unary }
unary ::= "-" unary | factor
x = -1.5
y = 3 * -2

The Program Tree

program
  statement_list
    assign
      target: identifier x
      expression: number 2
    print
      identifier x

The tree preserves statement order.

The Environment

An environment maps names to values.

Name Value
x 2

The program begins with an empty environment.

Assignment Changes the Environment

x = 2
  1. Evaluate the expression 2.
  2. Associate the result with x.
{} becomes {"x": 2}
print x
  1. Look up x.
  2. Obtain 2.
  3. Print 2.

An unknown name causes an evaluation error.

Parent Environments

parent = {"x": 10, "y": 20}
environment = {"$PARENT": parent, "x": 3}

Search here, then outward. The first binding wins.

Lookup Result
x 3 from the current environment
y 20 from the enclosing environment
z Error: no binding in the chain

Assignment stays local: y = 7 leaves the parent’s y at 20.

One Environment for the Program

Statement Environment afterward Output
x = 2 {"x": 2}
print x {"x": 2} 2
y = x + 3 * 4 {"x": 2, "y": 14}
print y {"x": 2, "y": 14} 14

Assignment Is a Statement

Valid:

x = 2;
print x;

Invalid in Topic 2:

print (x = 2)

Real Parser Code: Go

case token.ADD, token.SUB, token.NOT,
     token.XOR, token.AND, token.TILDE:
    pos, op := p.pos, p.tok
    p.next()
    x := p.parseUnaryExpr()
    return &ast.UnaryExpr{OpPos: pos, Op: op, X: x}

Same move: consume the operator, parse the operand, build a tree node.

Topic 2

Vertex can now:

  • execute a nonempty statement list
  • assign values to names
  • use names in arithmetic expressions
  • print expression values

The next lecture follows these rules through the code.