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The GNU Awk User's Guide

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6.4 Conversion of Strings and Numbers

Strings are converted to numbers and numbers are converted to strings, if the context of the awk program demands it. For example, if the value of either foo or bar in the expression `foo + bar' happens to be a string, it is converted to a number before the addition is performed. If numeric values appear in string concatenation, they are converted to strings. Consider the following:

 
two = 2; three = 3
print (two three) + 4

This prints the (numeric) value 27. The numeric values of the variables two and three are converted to strings and concatenated together. The resulting string is converted back to the number 23, to which 4 is then added.

If, for some reason, you need to force a number to be converted to a string, concatenate the empty string, "", with that number. To force a string to be converted to a number, add zero to that string. A string is converted to a number by interpreting any numeric prefix of the string as numerals: "2.5" converts to 2.5, "1e3" converts to 1000, and "25fix" has a numeric value of 25. Strings that can't be interpreted as valid numbers convert to zero.

The exact manner in which numbers are converted into strings is controlled by the awk built-in variable CONVFMT (see section 7.5 Built-in Variables). Numbers are converted using the sprintf function with CONVFMT as the format specifier (see section String Manipulation Functions).

CONVFMT's default value is "%.6g", which prints a value with at least six significant digits. For some applications, you might want to change it to specify more precision. On most modern machines, 17 digits is enough to capture a floating-point number's value exactly, most of the time.(21)

Strange results can occur if you set CONVFMT to a string that doesn't tell sprintf how to format floating-point numbers in a useful way. For example, if you forget the `%' in the format, awk converts all numbers to the same constant string. As a special case, if a number is an integer, then the result of converting it to a string is always an integer, no matter what the value of CONVFMT may be. Given the following code fragment:

 
CONVFMT = "%2.2f"
a = 12
b = a ""

b has the value "12", not "12.00". (d.c.)

Prior to the POSIX standard, awk used the value of OFMT for converting numbers to strings. OFMT specifies the output format to use when printing numbers with print. CONVFMT was introduced in order to separate the semantics of conversion from the semantics of printing. Both CONVFMT and OFMT have the same default value: "%.6g". In the vast majority of cases, old awk programs do not change their behavior. However, these semantics for OFMT are something to keep in mind if you must port your new style program to older implementations of awk. We recommend that instead of changing your programs, just port gawk itself. See section The print Statement, for more information on the print statement.


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