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tmerge tables


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SEE_ALSO

NAME

tmerge -- Merge two tables, or append one to the other.

USAGE

tmerge intable outtable option

DESCRIPTION

This task is used to either merge or append tables, depending on the option selected by the option parameter. The data type of each column is defined by the first table in the input list containing that column. If subsequent tables use the same column name, then data are converted to the type defined by the first table. Problems may occur when numerical data are written to a boolean column or a narrow character column.

The "merge" option is normally used for tables containing few, if any, common columns. When the user selects "merge", an output table is created containing as many columns as there are unique column names in all the input tables. (But see the description of the allcols parameter.) The output table will have as many rows as the largest number of rows in the input tables. The input tables are read in order, with all values being placed into the output table. If different input tables have the same column names then the first values put into the output table will be overwritten by the later table values. For example, if the two input tables both have the column name "X_VAL", then for each row number, the values written to the output table will be taken from the second input table. See below regarding text tables.

On the other hand, if the "append" option is selected, all rows of the first input table are written to the output table, followed by all rows of the second table, and so on, until all input tables are written to the output table. The total number of output rows will be the sum of the numbers of rows in the input tables. Columns with the same name in different input tables will be written into the same output column, but no data will be overwritten because they are put into different rows. The "append" option would normally be used for tables that have all the same columns.

An input table may have no rows. Such a table may be used as the first input table to control the order and definitions of columns in the output table.

Header parameters are appended, and parameters with the same keyword name in different input tables are overwritten in the output file, except for history and comment keywords.

Care must be taken with text tables. It is very likely that you would want allcols = yes if option = merge and allcols = no if option = append. See the description of the allcols parameter for details. If the output table is a text file, the line length may not be longer than 4095 characters, which is a limitation for any text table.

Column units are not checked. If columns with the same name have different units in two different input tables, the merged (or appended) data are likely to have mixed units. In addition, the column definition is taken from the first input table, but some and perhaps all of the data would come from the second input table, so the units in the output column definition would not be correct for those data.

PARAMETERS

intable [file name template]
Names of the tables to be merged or appended. This parameter will take either a file name template describing several input tables, and may include wildcard characters, or it will take the name of a list file preceded by the "@" character; in the latter case the list file contains a list of file names with each file name on a separate line. Wildcard characters should not be used for file name extensions because files other than tables will be processed, causing the program to crash. For example, if the directory contains files "table.tab" and "table.lis", the command "tmerge tab*" would open both files.
outtable [file name]
The name of the output table.
option = "merge" [string]
allowed values: merge | append

Either merge the columns in each row of each input table--overwriting previous values--or append files to each other. See also allcols below. (These options are discussed in greater detail in the DESCRIPTION section.)

(allcols = yes) [boolean]
Define output table columns using columns from all input tables?

If allcols = no, the output table will contain only those columns defined in the first input table. If allcols = yes, the output table will contain all columns from all input tables. If option = merge, then it is likely that allcols should be set to yes.

For input tables that are simple text tables (i.e. that do not contain explicit column definitions), the allcols parameter serves an additional function. When allcols = yes the name of each column in a simple text table is changed to be "c" followed by the column number in the output table. This is intended to make the column names unique in order to allow merging text tables without having the columns overwrite previously written columns. Since the column names in simple text tables are just c1, c2, etc., columns would overwrite previously written columns in the output if the names were not modified. If all input tables are simple text tables, and the output is also a text table, the new names will be discarded, so the net effect of this scheme is just to preserve all input data. If the output is a binary table, however, the modified column names will be retained. If the modified column names turn out not to be unique, a warning message will be printed.

(tbltype = "default") [string, allowed values: default | row |
column | text]

This parameter specifies the table type. Setting tbltype to "row" or "column" results in an stsdas binary table, the contents of which may be either row ordered or column ordered; row order is recommended. You can also specify that the output be a text table. The default (tbltype = "default") means that the type of the output table will be taken from the first input table.

If the extension of the output file name is ".fits" or ".??f", the table to be created will be a BINTABLE extension in a FITS file, regardless of how tbltype is set.

(allrows = 100) [integer, min=1, max=INDEF]
The number of rows to allocate. This parameter is only used for column-ordered tables (specified by the tbltype parameter). The allrows parameter is the minimum number of rows an output table will contain. If the number of rows required by the input tables is greater than allrows then the number of rows in the output table will be greater than allrows. If option = merge, then the total number of rows will be the larger of allrows or the number of rows in the largest table. If option = append, the total rows in the output table will be the larger of allrows or the total number of rows in all input tables.
(extracol = 0) [integer, min=0, max=INDEF]
Extra space to be reserved for columns in the output table.

This parameter is relevant only for a row-ordered table (specified by the tbltype parameter). The default value of zero is normally appropriate, but if you expect to define additional columns in the output table at a later time then you can allocate the necessary space by specifying a value for extracol. One unit of space is taken by each single-precision real value, integer value, or boolean value. A double-precision column requires two units of allocated space, and a character-string column takes one unit of space for each four characters, or fraction thereof.

EXAMPLES

1. Suppose you have the following two tables.

tbl1.tab:
	one	two	three
	---	---	-----
	1	-17	alpha
	2	-19	beta
	3	-23	gamma

tbl2.tab:
	one	three	four
	---	-----	----
	27	beta	3.14
	28	delta	2.72

then the statement

	cl> tmerge tbl1,tbl2 mrg merge

would create the following output table:

mrg.tab:
	one	two	three	four
	---	---	-----	----
	27	-17	beta	3.14
	28	-19	delta	2.72
	3	-23	gamma	INDEF

while the statement

	cl> tmerge tbl1,tbl2 app append

would create the following table:

app.tab:
	one	two	three	four
	---	---	-----	----
	1	-17	alpha	INDEF
	2	-19	beta	INDEF
	3	-23	gamma	INDEF
	27	INDEF	beta	3.14
	28	INDEF	delta	2.72

BUGS

REFERENCES

This task was written by Phil Hodge.

SEE ALSO

tselect, tproject, and proto.joinlines for text files


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