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The usual way to change the value of a variable is with the special
form `setq`

. When you need to compute the choice of variable at
run time, use the function `set`

.

- Special Form:
**setq***[symbol form]…* This special form is the most common method of changing a variable’s value. Each

`symbol`is given a new value, which is the result of evaluating the corresponding`form`. The current binding of the symbol is changed.`setq`

does not evaluate`symbol`; it sets the symbol that you write. We say that this argument is*automatically quoted*. The ‘`q`’ in`setq`

stands for “quoted”.The value of the

`setq`

form is the value of the last`form`.(setq x (1+ 2)) ⇒ 3

`x ;`

`x`

now has a global value. ⇒ 3`(let ((x 5)) (setq x 6) ; The local binding of`

`x`

is set. x) ⇒ 6`x ; The global value is unchanged. ⇒ 3`

Note that the first

`form`is evaluated, then the first`symbol`is set, then the second`form`is evaluated, then the second`symbol`is set, and so on:(setq x 10 ; Notice that

`x`

is set before y (1+ x)) ; the value of`y`

is computed. ⇒ 11

- Function:
**set***symbol value* This function puts

`value`in the value cell of`symbol`. Since it is a function rather than a special form, the expression written for`symbol`is evaluated to obtain the symbol to set. The return value is`value`.When dynamic variable binding is in effect (the default),

`set`

has the same effect as`setq`

, apart from the fact that`set`

evaluates its`symbol`argument whereas`setq`

does not. But when a variable is lexically bound,`set`

affects its*dynamic*value, whereas`setq`

affects its current (lexical) value. See Variable Scoping.(set one 1) error→ Symbol's value as variable is void: one

(set 'one 1) ⇒ 1

(set 'two 'one) ⇒ one

`(set two 2) ;`

`two`

evaluates to symbol`one`

. ⇒ 2one ; So it is

`one`

that was set. ⇒ 2 (let ((one 1)) ; This binding of`one`

is set, (set 'one 3) ; not the global value. one) ⇒ 3one ⇒ 2

If

`symbol`is not actually a symbol, a`wrong-type-argument`

error is signaled.(set '(x y) 'z) error→ Wrong type argument: symbolp, (x y)

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