Strings and Basic I/O

Text values and observable program behavior

Gregory S. DeLozier, PhD

Breaking Changes

Earlier code	Change needed
`print x`	`print(x)`
Variables `input`, `number`, `string`	Rename: now keywords
Ordinary `__input`, `__output` variables	Rename: now special globals

Earlier programs may need edits.

A Program with Text

__input = "Ada";
name = input("Name? ");
print("Hello, " + name + "!");

Hello, Ada!

Literals and Values

Source	Value
`"42"`	String
`42`	Integer
`""`	Empty string
`"a\nb"`	Text containing a newline

The String Token

(r'"(?:\\[^\r\n]|[^"\\\r\n])*"', "string")

An escaped character or an ordinary character

token["value"] = decode_string(value, line, column)

Decoding the String

escapes = {"n": "\n", "t": "\t", "r": "\r", '"': '"', "\\": "\\"}

Ordinary character: copy it. Backslash: consume two characters.

Source characters	Value characters
`\n`	One newline
`\"`	One quotation mark
`\\n`	A backslash followed by `n`
`\q`	Syntax error

A String in an Expression Tree

"ha" * 3 + "!"

A String in an Expression Tree

Operators and Types

Expression	Result
`"ha" * 3`	`"hahaha"`
`3 * "ha"`	`"hahaha"`
`"3" + "4"`	`"34"`
`3 + 4`	`7`
`"ha" * 3.0`	Type error

Input Returns Text

name = input();
name = input("Your name? ");

Typing 42 produces "42".

Text to Number

Expression	Result
`number("42")`	Integer `42`
`number("-3")`	Integer `-3`
`number(" .5 ")`	Float `0.5`
`number("5.")`	Float `5.0`

Number to Text

answer = 21 * 2;
print(answer);
print("Answer: " + string(answer));

42
Answer: 42

Expression Forms

input_expression ::= "input" "(" [ expression ] ")"
number_expression ::= "number" "(" expression ")"
string_expression ::= "string" "(" expression ")"

answer = number(input()) * 2;

The Conversion Node

number("21")

{
    "tag": "number_conversion",
    "expression": {
        "tag": "string", "value": "21"
    }
}

Print Is Still a Statement

print_statement ::= "print" "(" expression ")"

Source	Accepted?
`print(42)`	Yes
`print 42`	No
`value = print(42)`	No

Capturing Output

print(42);
saved = __output;
print("Captured: " + saved);

Variable after the program	Value
`saved`	`"42"`
`__output`	`"Captured: 42"`

Supplied Input Is Used Once

__input = "Ada";
name = input();

After input	Value
`name`	`"Ada"`
`__input`	`""`

The Input Decision

A Repeatable Test

__input = "21";
answer = number(input()) * 2;
print(answer);

Expected: __input is "", __output is "42".

Errors at Different Stages

Example	Stage
Unterminated string	Tokenization
`number()`	Parsing
`number("hello")`	Evaluation
`"ha" * 2.0`	Evaluation

The Example and Tests

./vertex example.v < /dev/null
bash run-tests.sh

Supplied input, clearing, captured output, conversions

Practice

Supply "18.5", add 2, and print "Total: 20.5".

Check the result and confirm that the input was consumed.