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COMPARE_STRUCT

## Name

COMPARE_STRUCT

## Purpose

Compare all matching tag names and return differences

## Explanation

Compare all matching Tags names (except for "except_Tags")
between two structure arrays (may have different struct.definitions),
and return a structured List of fields found different.
The Exelis contrib library has a faster but less powerful procedure
struct_equal.pro, see
http://www.exelisvis.com/Default.aspx?tabid=1540&id=1175

## Calling Sequence

diff_List = compare_struct( struct_A, struct_B [ EXCEPT=, /BRIEF,
/FULL, /NaN, /RECUR_A, /RECUR_B )

## Inputs

struct_A, struct_B : the two structure arrays to compare.
Struct_Name : for internal recursion use only.

## Optional Input Keywords

EXCEPT = string array of Tag names to ignore (NOT to compare).
/BRIEF = number of differences found for each matching field
of two structures is printed.
/FULL = option to print even if zero differences found.
/NaN = if set, then tag values are considered equal if they
are both set to NaN
/RECUR_A = option to search for Tag names
in sub-structures of struct_A,
and then call compare_struct recursively
for those nested sub-structures.
/RECUR_B = search for sub-structures of struct_B,
and then call compare_struct recursively
for those nested sub-structures.

## Note

compare_struct is automatically called recursively
for those nested sub-structures in both struct_A and struct_B
(otherwise cannot take difference)

## Output

Returns a structure array describing differences found.
which can be examined using print,diff_List or help,/st,diff_List.
The tags are
TAG_NUM_A - the tag number in structure A
TAG_NUM_B - the tag number in structure B
FIELD - the tag name
NDIFF - number of differences (always 1 for a scalar tag).

## Procedure

Match Tag names and then use where function on tags.

## Example

Find the tags in the !X system variable which are changed after a
simple plot.
IDL> x = !X ;Save original values
IDL> plot, indgen(25) ;Make a simple plot
IDL> help,/str,compare_struct(x,!X) ;See how structure has changed
and one will see that the tags !X.crange and !X.S are changed
by the plot.

## Modification History

written 1990 Frank Varosi STX @ NASA/GSFC (using copy_struct)
modif Aug.90 by F.V. to check and compare same # of elements only.
Added /NaN keyword W. Landsman March 2004
Don't test string for NaN values W. Landsman March 2008

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