GNU Scientific Library Reference Manual - Third Edition (v1.12)by M. Galassi, J. Davies, J. Theiler, B. Gough, G. Jungman, P. Alken, M. Booth, F. Rossi Paperback (6"x9"), 592 pages, 60 figures ISBN 0954612078 RRP £24.95 ($39.95) |

### 8.3.2 Accessing vector elements

Unlike fortran compilers, C compilers do not usually provide
support for range checking of vectors and matrices.^{(6)} The functions `gsl_vector_get`

and
`gsl_vector_set`

can perform portable range checking for you and
report an error if you attempt to access elements outside the allowed
range.

The functions for accessing the elements of a vector or matrix are
defined in `‘gsl_vector.h’` and declared `extern inline`

to
eliminate function-call overhead. You must compile your program with
the preprocessor macro `HAVE_INLINE`

defined to use these
functions.

If necessary you can turn off range checking completely without
modifying any source files by recompiling your program with the
preprocessor definition `GSL_RANGE_CHECK_OFF`

. Provided your
compiler supports inline functions the effect of turning off range
checking is to replace calls to `gsl_vector_get(v,i)`

by
`v->data[i*v->stride]`

and calls to `gsl_vector_set(v,i,x)`

by
`v->data[i*v->stride]=x`

. Thus there should be no performance
penalty for using the range checking functions when range checking is
turned off.

If you use a C99 compiler which requires inline functions in header
files to be declared `inline`

instead of `extern inline`

,
define the macro `GSL_C99_INLINE`

(see 2.5).
With GCC this is selected automatically when compiling in C99 mode
(`-std=c99`

).

If inline functions are not used, calls to the functions
`gsl_vector_get`

and `gsl_vector_set`

will link to the
compiled versions of these functions in the library itself. The range
checking in these functions is controlled by the global integer
variable `gsl_check_range`

. It is enabled by default--to
disable it, set `gsl_check_range`

to zero. Due to function-call
overhead, there is less benefit in disabling range checking here than
for inline functions.

__Function:__double**gsl_vector_get***(const gsl_vector **`v`, size_t`i`)- This function returns the
`i`-th element of a vector`v`. If`i`lies outside the allowed range of 0 to`n`-1 then the error handler is invoked and 0 is returned.

__Function:__void**gsl_vector_set***(gsl_vector **`v`, size_t`i`, double`x`)- This function sets the value of the
`i`-th element of a vector`v`to`x`. If`i`lies outside the allowed range of 0 to`n`-1 then the error handler is invoked.

__Function:__double ***gsl_vector_ptr***(gsl_vector **`v`, size_t`i`)__Function:__const double ***gsl_vector_const_ptr***(const gsl_vector **`v`, size_t`i`)- These functions return a pointer to the
`i`-th element of a vector`v`. If`i`lies outside the allowed range of 0 to`n`-1 then the error handler is invoked and a null pointer is returned.

ISBN 0954612078 | GNU Scientific Library Reference Manual - Third Edition (v1.12) | See the print edition |