12.2.2 Converting Strings To Numbers

The strtonum function (see String Functions) is a gawk extension. The following function provides an implementation for other versions of awk:

     
     # strtonum --- convert string to number
     
     
     function mystrtonum(str,        ret, chars, n, i, k, c)
     {
         if (str ~ /^0[0-7]*$/) {
             # octal
             n = length(str)
             ret = 0
             for (i = 1; i <= n; i++) {
                 c = substr(str, i, 1)
                 if ((k = index("01234567", c)) > 0)
                     k-- # adjust for 1-basing in awk
     
                 ret = ret * 8 + k
             }
         } else if (str ~ /^0[xX][0-9a-fA-f]+/) {
             # hexadecimal
             str = substr(str, 3)    # lop off leading 0x
             n = length(str)
             ret = 0
             for (i = 1; i <= n; i++) {
                 c = substr(str, i, 1)
                 c = tolower(c)
                 if ((k = index("0123456789", c)) > 0)
                     k-- # adjust for 1-basing in awk
                 else if ((k = index("abcdef", c)) > 0)
                     k += 9
     
                 ret = ret * 16 + k
             }
         } else if (str ~ /^[-+]?([0-9]+([.][0-9]*([Ee][0-9]+)?)?|([.][0-9]+([Ee][-+]?[0-9]+)?))$/) {
             # decimal number, possibly floating point
             ret = str + 0
         } else
             ret = "NOT-A-NUMBER"
     
         return ret
     }
     
     # BEGIN {     # gawk test harness
     #     a[1] = "25"
     #     a[2] = ".31"
     #     a[3] = "0123"
     #     a[4] = "0xdeadBEEF"
     #     a[5] = "123.45"
     #     a[6] = "1.e3"
     #     a[7] = "1.32"
     #     a[7] = "1.32E2"
     #
     #     for (i = 1; i in a; i++)
     #         print a[i], strtonum(a[i]), mystrtonum(a[i])
     # }
     

The function first looks for C-style octal numbers (base 8). If the input string matches a regular expression describing octal numbers, then mystrtonum loops through each character in the string. It sets k to the index in "01234567" of the current octal digit. Since the return value is one-based, the `k--' adjusts k so it can be used in computing the return value.

Similar logic applies to the code that checks for and converts a hexadecimal value, which starts with `0x' or `0X'. The use of tolower simplifies the computation for finding the correct numeric value for each hexadecimal digit.

Finally, if the string matches the (rather complicated) regex for a regular decimal integer or floating-point numer, the computation `ret = str + 0' lets awk convert the value to a number.

A commented-out test program is included, so that the function can be tested with gawk and the results compared to the built-in strtonum function.