Feb 152003

Eugene Volokh complains that his C programs used to crash, and his JavaScript still does, because the languages distinguish comparison (==) from assignment (=). (Volokh, at 14 the world’s youngest law professor, also worked as a programmer for several years as a preadolescent.) On the one hand it’s a lousy idea to use the single equal sign for assignment. The best-known operator should be reserved for the best-known operation, which is comparison. Many other languages keep the equal sign for comparison and use other symbols (:=, .=, =:) for assignment instead. Java inherits much of its syntax from C, including the equal and double equal signs, with the annoying consequence that if we assign a boolean variable, say x, the value false, then in this code snippet:

if (x == true)

doSomething() will not execute, whereas in this one:

if (x = true)

it will. The second snippet, though shorter, contains two bits of business: we assign the value true to x, and we then evaluate x. In Java this is done right to left, so by the time we arrive at the if, x is true.
This is a smaller problem in Java, however, because Java is strongly typed. If x is a String, or an Integer, or anything but a primitive boolean, the line if (x = true) will not compile.

But annoying and confusing as this is, it still beats languages, like Visual Basic, that use the same symbol for comparison and assignment. Consider this legal statement:

a = b = c

Even if we assume an execution order of right to left this is ambiguous. It might mean “assign the value of c to b, then assign the value of b to a.” Or it might mean “assign true to a if b and c are equal, otherwise assign false to a.” No way to tell.

The first language, to my knowledge, to distinguish comparison from assignment was APL (A Programming Language) in the 1950s, in which a left arrow (<â €”) indicates assignment. There is a famous, perhaps apocryphal, story of Ken Iverson, APL's inventor, watching some hapless Fortran programmer increment a loop counter by typing a = a + 1. “But that’s false,” Iverson said.

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