Collected expression syntax

Type categories

The data types we are looking at for expression syntax are:

integer
real
("real" and "integer" expressions are collectively known as "numeric expressions")
string
boolean
list

This file discusses syntax only. So for example, it will tell you that 2+3 is an integer expression, but it won't tell you enough to know that it evaluates to 5.

Literals

Rather than defining the syntax of literals precisely, I just want to give some examples.

You know what an integer literal looks like. A sequence of digits, optionally preceded by a minus sign. Two examples: 38, -38

A real literal additionally has a decimal point in it somewhere, or a power of ten in scientific notation (see insert), or both.

Scientific notation in Python (and also most other programming languages):
An 'e' means "times ten to the power of ...". For example, "3e6" is three million, or 3 X 10⁶, or 3000000. Similarly, 2.123e5 is 212300.

A string literal is a sequence of characters enclosed in single quotes. There are various "escapes" to indicate hard-to-type characters, such as "\n" for newline, and "\'" (backslash single-quote) for a single quote itself.

A list literal begins with a left square bracket, has zero or more comma-separated literals (of any type), then concludes with a right square bracket.

Variable invocation

A variable name can be used as an expression of whatever type has been assigned to the variable.

Built-in functions

A built-in-function can be used by stating its name, then a left parenthesis, zero or more comma-separated parameters, and a right parenthesis. The required types of the parameters and the resulting type of the built-in-function expression depend upon the built-in function being used.

Arithmetic (numeric) operators

If X and Y are numeric expressions (of either real or integer type, not necessarily both the same), then

X+Y is a numeric expression
X*Y is a numeric expression
X-Y is a numeric expression
X/Y is a numeric expression
-X is a numeric expression
(X) is a numeric expression

The type of the expression is "integer" if the one or two subexpressions are both of type integer; the type of the expression is "real" if one or more of the subexpressions are of type real.

If X and Y are of integer type, then X/Y requests integer division, and yields the quotient. The remainder of the integer division is available as the expression "X%Y" (without the quotes, of course). In a typical spreadsheet program this is written "MOD(X,Y)".

String operators

If S and T are string expressions and X is an integer expression, then

(S) is a string expression
S+T is a string expression
S*X is a string expression
len(S) is an integer expression

Boolean operators

If X and Y are numeric expressions and P and Q are boolean expressions, then

X<Y is a boolean expression
X<=Y is a boolean expression
X==Y is a boolean expression (written X=Y in gnumeric)
X>=Y is a boolean expression
X>Y is a boolean expression
X != Y is is a boolean expression meaning "not equal to" (It is written X<>Y in gnumeric and some other programming languages -- mnemonic: think of it as "less than or greater than" -- and it's "#" in some other spreadsheet programs)
not P is a boolean expression (written "not(P)" in gnumeric)
P and Q is a boolean expression (written "and(P,Q)" in gnumeric)
P or Q is a boolean expression (written "or(P,Q)" in gnumeric)

Expression-if

If P is a boolean expression and X and Y are numeric expressions, then
if(P,X,Y) is a numeric expression in gnumeric (and just about all spreadsheet programs).

In recent versions of Python only, if P is a boolean expression and X and Y are numeric expressions, then

X if P else Y

is a numeric expression. P is evaluated first; if P evaluates to true, then X is evaluated; if P evaluates to false, then Y is evaluated.

This is not nearly as commonly used as "expression-if" in spreadsheets, which is why Python made do without it for so long. It often produces hard-to-understand computer programs and I advise against using it in this course, or, in most cases, in "real life".

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