# HOC Syntax¶↑

HOC was the original programming language supported by NEURON. NEURON may also be programmed in Python.

Syntax:

/*...*/

//...

Description:
Comments are similar in style to C++. Enclose any text in // (but do not nest them). The rest of a line after // is a comment.

## Expressions¶↑

Description:
An expression has a double precision value. It usually appears as the right hand side of an assignment statement. An expression may be a number, variable, function call, or combination of simpler expressions.
Options:

The ways in which expressions can be combined are listed below in order of precedence. e stands for any expression, v stands for any variable and operators are left associative except for assignment operators which are right associative.

(e)
grouping
e^e
exponentiation
-e
negation
e*e  e/e  e%e
multiplication, division, modulus
e+e  e-e
e==e  e!=e  e<e  e<=e  e>e  e>=e

logical equal, unequal, less than, less than or equal, greater than, greater than or equal. These expressions have the numerical value 1 (true) or 0 (false). The expression is considered true if it is within float_epsilon of being mathematically exact.

Special logical expressions of the form objref1 == objref2 (and obj != obj) are also allowed and return 1 (0) if the object references label the same object. This makes the former comparison idiom using object_id() obsolete. Logical expressions of the strdef1 == strdef2 cannot be directly compared because of parser consistency reasons. However obj1.string1 == obj2.string2 will return true if the strings are identical in the sense of strcmp() .

e&&e
Logical and. Both expressions are always evaluated. A subexpression is considered false if it is within float_epsilon of 0 and true otherwise. If the entire expression is true its value is 1.
e||e
Logical or. Both expressions are always evaluated. A subexpression is considered false if it is within float_epsilon of 0 and true otherwise. If the entire expression is true its value is 1.
v=e  v+=e  v-=e  v*=e  v/=e
assignment. others are equivalent to v = (v + e), v = (v - e), v = (v * e), v = (v / e), respectively.

## Statements¶↑

Syntax:

stmt

{stmt}

{stmt stmt ...stmt}

Description:
A statement is something executable that does not have a value, eg. for loops, procedure calls, or a compound statement between braces. An expression may be used anywhere a statement is required.

Example:

i = 0   //initialize i
j = 0   //initialize j
if(vec.x[i] <= 10 && i < vec.size()){   //In the parentheses is an expression:
//if the value of the ith element in vec
//is less than or equal to 10, and
//if i is an index within vec
//
//Between the braces is/are statement(s):
vec1.x[j] = vec.x[i]
i = i+1                         //increment i by 1
j = j+1                         //increment j by 1
} else{
//Here is also a statement
i = i+1                         //simply go to the next element of vec
}


Statements exist between the braces following the if and else commands. The parentheses after the if command contain an expression.

## proc¶↑

Syntax:
proc name() stmt
Description:
Introduce the definition of a procedure. A procedure does not return a value. You should always try to distill your programs into small, manageable procedures and functions.

Example:

proc printsquare() {local x
x = $1 print x*x } printsquare(5)  prints the square of 5. Procedures can also be called within other procedures. The code which produces the interactive examples for the Random class contains procedures for both creating the buttons which allow you to select parameters as well as for creating the histograms which appear on the screen. ## func¶↑ Syntax: func name() {stmt1, stmt2, stmt3…} Description: Introduce the definition of a function. A function returns a double precision value. Example:  func tan() { return sin($1)/cos($1) } tan(PI/8)  creates a function tan() which takes one argument (floating point or whole number), and contains one statement. ## obfunc¶↑ Syntax: obfunc name() { statements } Description: Introduce the definition of a function returning an objref Example: obfunc last() { // arg is List return$o1.object($o1.count - 1) }  See also ## iterator¶↑ Syntax: iterator name() stmt Description: Define a looping construct to be used subsequently in looping over a statement. Example: iterator case() {local i for i = 2, numarg() { //must begin at 2 because the first argument is //in reference to the address$&1 = $i //what is at the address will be changed *iterator_statement* //This is where the iterator statement will //be executed. } }  In this case x=0 for case (&x, 1,2,4,7,-25) { print x //the iterator statement }  will print the values 1, 2, 4, 7, -25 The body of the for name(..) statement is executed in the same context as a normal for statement. The name is executed in the same context as a normal procedure but should use only variables local to the iterator. ## Arguments¶↑ Arguments to functions and procedures are retrieved positionally. $1, $2,$3 refer to the first, second, and third scalar arguments respectively.

If “i” is declared as a local variable, $i refers to the scalar argument in the position given by the value of i. The value of i must be in the range {1…numarg()}. The normal idiom is for i=1, numarg() {print$i}

Scalar arguments use call by value so the variable in the calling statement cannot be changed.

If the calling statement has a ‘&’ prepended to the variable then it is passed by reference and must be retrieved with the syntax $&1,$&2, ..., $&i. If the variable passed by reference is a one dimensional array then $&1 refers to the first (0th) element and index i is denoted $&1[i]. Warning, NO array bounds checking is done and the array is treated as being one-dimensional. A scalar or array reference may be passed to another procedure with &$&1. To save a scalar reference use the Pointer class.

Retrieval of strdef arguments uses the syntax: $s1,$s2, ..., $si. Retrieval of objref arguments uses the syntax: $o1, $o2, ...,$oi. Arguments of type strdef and objref use call by reference so the calling value may be changed.

Example:

func mult(){
return $1*$2
}


defines a function which multiplies two arguments. Therefore mult(4,5) will return the value 20.

proc pr(){
print $s3 print$1*$2 print$o4
}


defines a procedure which first prints the string defined in position 3, then prints the product of the two numbers in positions 1 and 2, and finally prints the pointer reference to an object in position 4.

For a string ‘s’ which is defined as s = "hello", and an objref ‘r’, pr(3,5,s,r) will return

hello
15
Graph[0]


assuming r refers to the first graph.

numarg()
Syntax:
n = numarg()
Description:
Number of arguments passed to a user written hoc function.

argtype()
itype = argtype(iarg)