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Using Numbers to Subscript Arrays

An important aspect of arrays to remember is that array subscripts are always strings. If you use a numeric value as a subscript, it will be converted to a string value before it is used for subscripting (see section Conversion of Strings and Numbers).

This means that the value of the built-in variable CONVFMT can potentially affect how your program accesses elements of an array. For example:

xyz = 12.153
data[xyz] = 1
CONVFMT = "%2.2f"
if (xyz in data)
    printf "%s is in data\n", xyz
else
    printf "%s is not in data\n", xyz

This prints `12.15 is not in data'. The first statement gives xyz a numeric value. Assigning to data[xyz] subscripts data with the string value "12.153" (using the default conversion value of CONVFMT, "%.6g"), and assigns one to data["12.153"]. The program then changes the value of CONVFMT. The test `(xyz in data)' generates a new string value from xyz, this time "12.15", since the value of CONVFMT only allows two significant digits. This test fails, since "12.15" is a different string from "12.153".

According to the rules for conversions (see section Conversion of Strings and Numbers), integer values are always converted to strings as integers, no matter what the value of CONVFMT may happen to be. So the usual case of:

for (i = 1; i <= maxsub; i++)
    do something with array[i]

will work, no matter what the value of CONVFMT.

Like many things in awk, the majority of the time things work as you would expect them to work. But it is useful to have a precise knowledge of the actual rules, since sometimes they can have a subtle effect on your programs.


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