Exploring Expect

Expressions

Strings can be treated as expressions having mathematical values. For example, when the string "1+1" is treated as an expression, it has the value 2. Tcl does not automatically treat strings as expressions, but individual commands can. I will demonstrate this in a moment. For now, just remember that expressions are not complete commands in themselves.

Expressions are quite similar to those in the C language. The following are all valid expressions:

1 + 5
1+5
1+5.5
1e10+5.5
(5%3)*sin(4)
1 <= 2
(1 <= 2) || (2 != 3)

All of the usual mathematical operators are available including +, -, *, /, and % (modulo). Many functions exist such as sin, cos, log, and sqrt. Boolean operators include || (or), && (and), and ! (not). The usual comparison operators are available (<= (less than or equal), == (equal), != (not equal), etc.). They return 0 if the expression is false and 1 if it is true.

Whitespace may be used freely to enhance readability. A number of numeric forms are supported including scientific notation as well as octal (any number with a leading 0) and hexadecimal (any number with a leading 0x). Functions such as floor, ceil, and round convert from floating-point to integer notation.

Precedence and associativity follow C rules closely. For instance, the expression "1-2*3" is interpreted as "1-(2*3)" because multiplication is of higher precedence than addition. All binary operators at the same level of precedence are left-associative. This means, for instance, that the expression ...