User guide

2.2. EXPRESSIONS 21

2.2.8 VALOF Expressions

An expression of the form VALOF C, wh er e C is a command, is evaluated by

executing the command C. On encountering a co m m an d of the form RESULTIS E

within C, execut i on terminates, returni n g the value of E as the result of the VALOF

expression. Valof expressions are often used as the bodies of functions.

2.2.9 Expression Precedence

So that the separator semicolon (;) can be omitted at the end of any line, there

is the restriction that inﬁ x ed operators may not occur as the ﬁrst token of a line.

So, if the ﬁrst token on a line i s !, + or -, these must be regarded as preﬁxed

operators.

The syntax of BCPL is speciﬁed by the diagrams in Appendix A, but a sum-

many of the precendence of expression op er a to r s is given in tab l e 2.1. The prece-

dence values are in the range 0 to 9, with the higher values signifying greater

binding power. The letters L and R denote the associativity of the operators.

For instance, the dyadic operator - is left associative an d so a-b-c is equiva-

lent to (v-i)-j, while b1->x,b2->y,z is right associative an d so is equivalent to

b1->x,(b2->y,z).

9 Names, Literals, ?,

TRUE, FALSE, BITSPERBCPLWORD

(E),

9L SLCT : Field selector constants

Function and method calls

8L ! % OF Dyadic

7 ! @ Preﬁxed

6L * / MOD REM Dyadic operators

5 + - ABS

4 = ~= <= >= < > Extended Relati on s

4L << >>

3 ~ NOT Bitwise and Boolean operators

3L &

2L |

1L EQV NEQV XOR

1R -> , Conditional expression

0 VALOF TABLE Valof and Tab l e expressions

Table 2.1: Operator preced en ce