NAG Library Routine Document

g05tcf (int_geom)


    1  Purpose
    7  Accuracy


g05tcf generates a vector of pseudorandom integers from the discrete geometric distribution with probability p of success at a trial.


Fortran Interface
Subroutine g05tcf ( mode, n, p, r, lr, state, x, ifail)
Integer, Intent (In):: mode, n, lr
Integer, Intent (Inout):: state(*), ifail
Integer, Intent (Out):: x(n)
Real (Kind=nag_wp), Intent (In):: p
Real (Kind=nag_wp), Intent (Inout):: r(lr)
C Header Interface
#include nagmk26.h
void  g05tcf_ (const Integer *mode, const Integer *n, const double *p, double r[], const Integer *lr, Integer state[], Integer x[], Integer *ifail)


g05tcf generates n integers xi from a discrete geometric distribution, where the probability of xi=I (a first success after I+1 trials) is
P xi=I = p × 1-p I ,   I=0,1, .  
The variates can be generated with or without using a search table and index. If a search table is used then it is stored with the index in a reference vector and subsequent calls to g05tcf with the same parameter value can then use this reference vector to generate further variates. If the search table is not used (as recommended for small values of p) then a direct transformation of uniform variates is used.
One of the initialization routines g05kff (for a repeatable sequence if computed sequentially) or g05kgf (for a non-repeatable sequence) must be called prior to the first call to g05tcf.


Knuth D E (1981) The Art of Computer Programming (Volume 2) (2nd Edition) Addison–Wesley


1:     mode – IntegerInput
On entry: a code for selecting the operation to be performed by the routine.
Set up reference vector only.
Generate variates using reference vector set up in a prior call to g05tcf.
Set up reference vector and generate variates.
Generate variates without using the reference vector.
Constraint: mode=0, 1, 2 or 3.
2:     n – IntegerInput
On entry: n, the number of pseudorandom numbers to be generated.
Constraint: n0.
3:     p – Real (Kind=nag_wp)Input
On entry: the parameter p of the geometric distribution representing the probability of success at a single trial.
Constraint: machine precisionp1.0 (see x02ajf).
4:     rlr – Real (Kind=nag_wp) arrayCommunication Array
On entry: if mode=1, the reference vector from the previous call to g05tcf.
If mode=3, r is not referenced.
On exit: if mode3, the reference vector.
5:     lr – IntegerInput
On entry: the dimension of the array r as declared in the (sub)program from which g05tcf is called.
Suggested values:
  • if mode3, lr=8+42/p approximately (see Section 9);
  • otherwise lr=1.
  • if mode=0 or 2, lr30/p+8;
  • if mode=1, lr should remain unchanged from the previous call to g05tcf.
6:     state* – Integer arrayCommunication Array
Note: the actual argument supplied must be the array state supplied to the initialization routines g05kff or g05kgf.
On entry: contains information on the selected base generator and its current state.
On exit: contains updated information on the state of the generator.
7:     xn – Integer arrayOutput
On exit: the n pseudorandom numbers from the specified geometric distribution.
8:     ifail – IntegerInput/Output
On entry: ifail must be set to 0, -1​ or ​1. If you are unfamiliar with this argument you should refer to Section 3.4 in How to Use the NAG Library and its Documentation for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value -1​ or ​1 is recommended. If the output of error messages is undesirable, then the value 1 is recommended. Otherwise, if you are not familiar with this argument, the recommended value is 0. When the value -1​ or ​1 is used it is essential to test the value of ifail on exit.
On exit: ifail=0 unless the routine detects an error or a warning has been flagged (see Section 6).

Error Indicators and Warnings

If on entry ifail=0 or -1, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
On entry, mode=value.
Constraint: mode=0, 1, 2 or 3.
On entry, n=value.
Constraint: n0.
On entry, p=value.
Constraint: machine precisionp1.0.
p is so small that lr would have to be larger than the largest representable integer. Use mode=3 instead. p=value 
On entry, some of the elements of the array r have been corrupted or have not been initialized.
p is not the same as when r was set up in a previous call.
Previous value of p=value and p=value.
On entry, lr is too small when mode=0 or 2: lr=value, minimum length required =value.
On entry, state vector has been corrupted or not initialized.
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 3.9 in How to Use the NAG Library and its Documentation for further information.
Your licence key may have expired or may not have been installed correctly.
See Section 3.8 in How to Use the NAG Library and its Documentation for further information.
Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.


Not applicable.

Parallelism and Performance

g05tcf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

Further Comments

The time taken to set up the reference vector, if used, increases with the length of array r. However, if the reference vector is used, the time taken to generate numbers decreases as the space allotted to the index part of r increases. Nevertheless, there is a point, depending on the distribution, where this improvement becomes very small and the suggested value for the length of array r is designed to approximate this point.
If p is very small then the storage requirements for the reference vector and the time taken to set up the reference vector becomes prohibitive. In this case it is recommended that the reference vector is not used. This is achieved by selecting mode=3.


This example prints 10 pseudorandom integers from a geometric distribution with parameter p=0.001, generated by a single call to g05tcf, after initialization by g05kff.

Program Text

Program Text (g05tcfe.f90)

Program Data

Program Data (g05tcfe.d)

Program Results

Program Results (g05tcfe.r)

© The Numerical Algorithms Group Ltd, Oxford, UK. 2017