On entry: ifail must be set to , or to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of means that an error message is printed while a value of means that it is not.
If halting is not appropriate, the value or is recommended. If message printing is undesirable, then the value is recommended. Otherwise, the value is recommended. When the value or is used it is essential to test the value of ifail on exit.
On exit: unless the routine detects an error or a warning has been flagged (see Section 6).
6Error Indicators and Warnings
If on entry or , explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
On entry, .
The routine has been called with an argument less than or equal to zero for which is not defined.
An unexpected error has been triggered by this routine. Please
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.
If and are the absolute and relative errors in the result and is the relative error in the argument then in principle these are related by
That is accuracy will be limited by machine precision near the origin and near the zeros of , but near the zeros of only absolute accuracy can be maintained.
The behaviour of this amplification is shown in Figure 1.
For large values of , , therefore, and since is limited by the finite precision of the machine it becomes impossible to return results which have any relative accuracy. That is, when we have that and hence is not significantly different from zero.
Hence is chosen such that for values of , in principle would have values less than the machine precision and so is essentially zero.
8Parallelism and Performance
Background information to multithreading can be found in the Multithreading documentation.
s13acf is not threaded in any implementation.
This example reads values of the argument from a file, evaluates the function at each value of and prints the results.