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8.3.2 Accessing vector elements

Unlike fortran compilers, C compilers do not usually provide support for range checking of vectors and matrices. Range checking is available in the GNU C Compiler bounds-checking extension, but it is not part of the default installation of GCC. 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 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.

— 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.