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### B.1.9 Arrays

Functions which operate on array-valued cells. The array parameters of these functions can only be used on values which are already arrays (usually, numeric arrays). In most cases that means on values in table columns which are declared as array-valued. FITS and VOTable tables can have columns which contain array values, but other formats such as CSV cannot.

The functions fall into a number of categories:

- Aggregating operations, which map an array value to a scalar, including `size`, `count`, `maximum`, `minimum`, `sum`, `mean`, `median`, `quantile`, `stdev`, `variance`, `join`.
- Operations on one or more arrays which produce array results, including `add`, `subtract`, `multiply`, `divide`, `reciprocal`, `condition`.
- A set of functions named `array` with various numbers of arguments, which let you assemble an array value from a list of scalar numbers. This can be used for instance to get the mean of a set of three magnitudes by using an expression like "`mean(array(jmag, hmag, kmag))`".

sum( array )
  Returns the sum of all the non-blank elements in the array. If `array` is not a numeric array, `null` is returned.
mean( array )
  Returns the mean of all the non-blank elements in the array. If `array` is not a numeric array, `null` is returned.
variance( array )
  Returns the population variance of all the non-blank elements in the array. If `array` is not a numeric array, `null` is returned.
stdev( array )
  Returns the population standard deviation of all the non-blank elements in the array. If `array` is not a numeric array, `null` is returned.
minimum( array )
  Returns the smallest of the non-blank elements in the array. If `array` is not a numeric array, `null` is returned.
maximum( array )
  Returns the largest of the non-blank elements in the array. If `array` is not a numeric array, `null` is returned.
median( array )
  Returns the median of the non-blank elements in the array. If `array` is not a numeric array, `null` is returned.
quantile( array, quant )
  Returns a quantile value of the non-blank elements in the array. Which quantile is determined by the `quant` value; values of 0, 0.5 and 1 give the minimum, median and maximum respectively. A value of 0.99 would give the 99th percentile.
size( array )
  Returns the number of elements in the array. If `array` is not an array, zero is returned.
count( array )
  Returns the number of non-blank elements in the array. If `array` is not an array, zero is returned.
join( array, joiner )
  Returns a string composed of concatenating all the elements of an array, separated by a joiner string. If `array` is not an array, null is returned.
add( array1, array2 )
  Returns the result of adding two numeric arrays element by element. Both arrays must be numeric, and the arrays must have the same length. If either of those conditions is not true, `null` is returned. The types of the arrays do not need to be the same, so for example it is permitted to add an integer array to a floating point array.
add( array, constant )
  Returns the result of adding a constant value to every element of a numeric array. If the supplied `array` argument is not a numeric array, `null` is returned.
subtract( array1, array2 )
  Returns the result of subtracting one numeric array from the other element by element. Both arrays must be numeric, and the arrays must have the same length. If either of those conditions is not true, `null` is returned. The types of the arrays do not need to be the same, so for example it is permitted to subtract an integer array from a floating point array.
multiply( array1, array2 )
  Returns the result of multiplying two numeric arrays element by element. Both arrays must be numeric, and the arrays must have the same length. If either of those conditions is not true, `null` is returned. The types of the arrays do not need to be the same, so for example it is permitted to multiply an integer array by a floating point array.
multiply( array, constant )
  Returns the result of multiplying every element of a numeric array by a constant value. If the supplied `array` argument is not a numeric array, `null` is returned.
divide( array1, array2 )
  Returns the result of dividing two numeric arrays element by element. Both arrays must be numeric, and the arrays must have the same length. If either of those conditions is not true, `null` is returned. The types of the arrays do not need to be the same, so for example it is permitted to divide an integer array by a floating point array.
reciprocal( array )
  Returns the result of taking the reciprocal of every element of a numeric array. If the supplied `array` argument is not a numeric array, `null` is returned.
condition( flagArray, trueValue, falseValue )
  Maps a boolean array to a numeric array by using supplied numeric values to represent true and false values from the input array.

  This has the same effect as applying the expression `outArray[i] = flagArray[i] ? trueValue : falseValue`.

array( x1 )
  Returns a numeric array built from a given element.
array( x1, x2 )
  Returns a numeric array built from given elements.
array( x1, x2, x3 )
  Returns a numeric array built from given elements.
array( x1, x2, x3, x4 )
  Returns a numeric array built from given elements.
array( x1, x2, x3, x4, x5 )
  Returns a numeric array built from given elements.
array( x1, x2, x3, x4, x5, x6 )
  Returns a numeric array built from given elements.
array( x1, x2, x3, x4, x5, x6, x7 )
  Returns a numeric array built from given elements.
array( x1, x2, x3, x4, x5, x6, x7, x8 )
  Returns a numeric array built from given elements.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@bristol.ac.uk