# HOC Keywords¶↑

Note

This is a page about HOC syntax; it is not directly applicable to Python-based simulations. A HOC-based version of the NEURON documentation is also available.

help

invokes the help system

Syntax:

help

help word

Description:
Help word sends a word to the help system. The word is looked up in the nrn/lib/helpdict file and if found Netscape is sent the appropriate URL to display the help text. If the word is not found, the URL for the table of contents is sent. Netscape must be running for the help system to work.

return

Syntax:

return

return expr

return objref

Description:

The return command will immediately exit from a procedure without returning a value.

The return expr command will immediately exit from a function which must return a value. This command must also be the last executable statement of a function. It is possible for a function to contain more than one return command, for instance in a series of if else statements; however, not more than one of the return commands may return a value at any given time.

The return objref command must be used to return from an obfunc.

Example:

func max(){
if ($1 >$2){
return $1 } else { return$2
}
}


returns the maximum of two arguments which are read into the function. Eg. max(3,6), where $1 is the first argument (3) and$2 is the second argument (6). This use of max would return the value 6.

Warning

See restriction of the break statement.

break

Syntax:
break
Description:
Immediately exit from a loop. Control transfers to the next statement after the loop statement.

Warning

This statement, as well as “return”, “continue”, and “stop” cannot occur within the scope of a statement that modifies the section stack such as

section { statement }

or the stack will not be properly popped. Also it should not be placed on a line that contains object syntax but should be placed on a line by itself. eg.

    x.p() break


should be written

    x.p()
break


Example:

while(1) {
x = fscan()
if (x < 0) {
break;
}
print sqrt(x)
}


continue

Syntax:
continue
Description:
Inside a compound statement of a loop, transfers control to the next iteration of the loop statement.

Example:

for i=1,10{
if(i==6){
continue
}
print i
}


prints the numbers: 1,2,3,4,5,7,8,9,10. 6 is left out because when i==6, the control is passed beyond the print statement to the next iteration of the loop.

You can accomplish the same thing with the following syntax:

for i=1,10{
if(i<6 || i>6){
print i
}
}


Warning

See restriction of the break statement.

stop

Syntax:
stop
Description:
Return control to the command level of the interpreter. This is a useful safety device for stopping the current execution of your program. Eg. you may wish to stop the program and print out an error message that lets you know if you have entered unacceptable arguments.

Warning

See restriction of the break statement.

if

Syntax:

if (expr) stmt1

if (expr) stmt1 else stmt2

Description:
Conditional statement. When the expr evaluates to a nonzero number (true) stmt1 is executed. With the else form, if the expression evaluates to zero (false) stm2 is executed.

Example:

i = 0   //initialize i
j = 0   //initialize j
if(vec.x[i] <= 10 && i < vec.size()){   //if the value of the ith element in vec
//is less than or equal to 10, and
//if i is an index within vec
vec1.x[j] = vec.x[i]        //set the jth element of vec1 equal to that
//ith element of vec
i = i+1             //increment i by 1
j = j+1             //increment j by 1
} else{                 //otherwise (This must be on the same line as the closing brace of
//the previous statement in order to indicate that the compound
//statement has not ended.)
i = i+1             //simply go to the next element of vec
}


float_epsilon, ifsec

else

if

while

Syntax:
while (expr) stmt
Description:
Iteration statement. Repeatedly execute the statement as long as the expr evaluates to true.

Example:

numelements = 20
i = 0
while (i < numelements){
print(cos(vec.x[i]))
print(sin(vec.x[i]))
i += 1
}


prints the cosines and the sines of the vec elements up to numelements, which in this case = 20.

for

Syntax:

for(stmt1; expr2; stmt3) stmt

for var=expr1, expr2  stmt

for (var) stmt

for (var, expr) stmt

for iterator (args) stmt

Description:

Iteration statement. The for statement is similar to while in that it iterates over a statement. However, the for statement is more compact and contains within its parentheses the command to advance to the next iteration. Statements 1 and 3 may be empty.

This command also has a short form which always increments the iterations by one.

for *var*=*expr1*, *expr2*  stmt


is equivalent to

for(*var*=*expr1*; *var* <= *expr2*; *var*=*var*+1) stmt


However, expr1 and expr2 are evaluated only once at the beginning of the for.

for (var) stmt

Loops over all segments of the currently accessed section. var begins at 0 and ends at 1. In between var is set to the center position of each segment. Ie. stmt is executed nseg+2 times.

for (var, expr) stmt

If the expression evaluates to a non-zero value, it is exactly equivalent to for (var) stmt If it evaluates to 0 (within float_epsilon ) then the iteration does not include the 0 or 1 points. Thus for(x, 0) { print x } is exactly equivalent to for (x) if (x > 0 && x < 1) { print x }

The iterator form of the for loop executes the statement with a looping construct defined by the user.

Example:

for(i=0; i<=9; i=i+1){
print i*2
}


is equivalent to

for i=0, 9 {
print i*2
}

create axon
access axon
{nseg = 5  L=1000  diam=50  insert hh }
for (x) print x, L*x
for (x) if (x > 0 && x < 1) { print x, gnabar_hh(x) }


iterator, break, continue, while, forall, forsec

print

Syntax:
print expr, string, ...
Description:
Any number of expressions and/or strings may be printed. A newline is printed at the end.

Example:

x=2
y=3
print x, "hello", "good-bye", y, 7


prints

x hello good-bye 3 7


and then moves to the next line.

delete

Syntax:
delete varname
Description:
Deletes the variable name from the global namespace. Allows the varname to be declared as another type. It is up to the user to make sure it is safe to execute this statement since the variable may be used in an existing function.

Syntax:
read(var)
Description:
var is assigned the number input by the user, or the next number in the standard input, or the file opened with ropen. read(var) returns 0 on end of file and 1 otherwise.

Example:

for i=1, 5 {
print x*x
}


will await input from the user or from a file, and will print the square of each value typed in by the user, or read from the file, for the first five values.

debug

A toggle for parser debugging purposes. Prints the stack machine commands resulting from parsing a statement. Not useful to the user.

double

Syntax:

double var1[expr]

double var2[expr1][expr2]

double varn[expr1][expr2]...[exprn]

Description:

Declares a one-dimensional, a two-dimensional or an n-dimensional array of doubles. This is reminiscent of the command which creates an array in C, however, HOC does not demand that you specify whether or not numbers are integers. All numbers in all arrays will be doubles.

The index for each dimension ranges from 0 to expr-1. Arrays may be redeclared at any time, including within procedures. Thus arrays may have different lengths in different objects.

The Vector class for the ivoc interpreter provides convenient and powerful methods for manipulating arrays.

Example:

double vec[40]


declares an array with 40 elements, whereas

objref vec
vec = new Vector(40)


creates a vector (which is an array by a different name) with 40 elements which you can manipulate using the commands of the Vector class.

em

Syntax:
em
Description:

microemacs editor

This is a reasonably complete editor with many commands. These commands are listed in emacs. A tutorial is also available at emacstut.

When called from the interpreter, the command ^C immediately returns to the interpreter and the current buffer is interpreted. Other commands follow:

^X^F
reads a file into a new buffer.
^X^B
changes buffers.
^X^W filename
saves (writes) a file under a specific name.
^X^S
saves a file under the last specified name.

depvar

Syntax:
depvar
Description:
Declare a variable to be a dependent variable for the purpose of solving simultaneous equations.

Example:

depvar x, y, z
proc equations() {
eqn x:: x + 2*y + z =  6
eqn y:: x - y + z   =  2
eqn z:: 2*x + y -z  = -3
}
equations()
solve()
print x,y,z


prints the values of x, y and z.

eqn

Syntax:

eqn var:: expr = expr

eqn var: expr =

eqn var: = expr

Description:
Introduce a simultaneous equation. The single colon forms add the expressions to the indicated sides. This is convenient for breaking long equations down into more manageable parts which can be added together.

Example:

eqinit()
depvar x, y, z
proc equations() {
eqn x:: x + 2*y + z =  6
eqn y:: x - y + z   =  2
eqn z:: 2*x + y -z  = -3
eqn z: = 5 + 4y
}
equations()
solve()
print x,y,z


makes the right hand side of the z equation “2 + 4y” and solves for the values x, y, and z.

local

Syntax:
local var
Description:
Declare a list of local variables within a procedure or function Must be the first statement on the same line as the function declaration.

Example:

func count() {local i, x
x = 0
for i=0,40 {
if (vec.x[i] == 7) {
x = x+1
}
}
return x
}


returns the number of elements which have the value of 7 in the first 40 elements of vec. i and x are local variables, and their usage here will not affect variables of the same name in other functions and procedures of the same program.

localobj

Syntax:
localobj var
Description:
Declare a list, comma separated, of local objrefs within a proc, func, iterator, or obfunc. Must be after the local statement (if that exists) on the same line as the function declaration

Example:

func sum() { local i, j  localobj tobj // sum from $1 to$2
i = $1 j =$2
tobj = new Vector()
tobj.indgen(i, j ,1)
}
sum(5, 10) == 45


strdef

Syntax:
strdef stringname
Description:

Declare a comma separated list of string variables. String variables cannot be arrays.

Strings can be passed as arguments to functions.

Example:

strdef a, b, c
a = "Hello, "
b = "how are you?"
c = "What is your name?"
print a, b
print c


will print to the screen:

Hello, how are you?


setpointer

Syntax:
setpointer pvar, var
Description:

Connects pointer variables in membrane mechanisms to the address of var. eg. If \$NEURONHOME/examples/nmodl/synpre.mod is linked into NEURON, then:

soma1 syn1=new synp(.5)
setpointer syn1.vpre, axon2.v(1)


would enable the synapse in soma1 to observe the axon2 membrane potential.

insert

Syntax:
insert mechanism
Description:
Insert the density mechanism in the currently accessed section. Not used for point processes–they are inserted with a different syntax.

uninsert

Syntax:
uninsert mechanism
Description:
Delete the indicated mechanism from the currently accessed section. Not for point processes.