File: gawk.info,  Node: Comparison Operators,  Next: POSIX String Comparison,  Prev: Variable Typing,  Up: Typing and Comparison

6.3.2.2 Comparison Operators
............................

"Comparison expressions" compare strings or numbers for relationships
such as equality.  They are written using "relational operators", which
are a superset of those in C. *note Table 6.3: table-relational-ops.
describes them.


Expression         Result
--------------------------------------------------------------------------
X '<' Y            True if X is less than Y
X '<=' Y           True if X is less than or equal to Y
X '>' Y            True if X is greater than Y
X '>=' Y           True if X is greater than or equal to Y
X '==' Y           True if X is equal to Y
X '!=' Y           True if X is not equal to Y
X '~' Y            True if the string X matches the regexp denoted by Y
X '!~' Y           True if the string X does not match the regexp
                   denoted by Y
SUBSCRIPT 'in'     True if the array ARRAY has an element with the
ARRAY              subscript SUBSCRIPT

Table 6.3: Relational operators

   Comparison expressions have the value one if true and zero if false.
When comparing operands of mixed types, numeric operands are converted
to strings using the value of 'CONVFMT' (*note Conversion::).

   Strings are compared by comparing the first character of each, then
the second character of each, and so on.  Thus, '"10"' is less than
'"9"'.  If there are two strings where one is a prefix of the other, the
shorter string is less than the longer one.  Thus, '"abc"' is less than
'"abcd"'.

   It is very easy to accidentally mistype the '==' operator and leave
off one of the '=' characters.  The result is still valid 'awk' code,
but the program does not do what is intended:

     if (a = b)   # oops! should be a == b
        ...
     else
        ...

Unless 'b' happens to be zero or the null string, the 'if' part of the
test always succeeds.  Because the operators are so similar, this kind
of error is very difficult to spot when scanning the source code.

   The following list of expressions illustrates the kinds of
comparisons 'awk' performs, as well as what the result of each
comparison is:

'1.5 <= 2.0'
     Numeric comparison (true)

'"abc" >= "xyz"'
     String comparison (false)

'1.5 != " +2"'
     String comparison (true)

'"1e2" < "3"'
     String comparison (true)

'a = 2; b = "2"'
'a == b'
     String comparison (true)

'a = 2; b = " +2"'
'a == b'
     String comparison (false)

   In this example:

     $ echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }'
     -| false

the result is 'false' because both '$1' and '$2' are user input.  They
are numeric strings--therefore both have the strnum attribute, dictating
a numeric comparison.  The purpose of the comparison rules and the use
of numeric strings is to attempt to produce the behavior that is "least
surprising," while still "doing the right thing."

   String comparisons and regular expression comparisons are very
different.  For example:

     x == "foo"

has the value one, or is true if the variable 'x' is precisely 'foo'.
By contrast:

     x ~ /foo/

has the value one if 'x' contains 'foo', such as '"Oh, what a fool am
I!"'.

   The righthand operand of the '~' and '!~' operators may be either a
regexp constant ('/'...'/') or an ordinary expression.  In the latter
case, the value of the expression as a string is used as a dynamic
regexp (*note Regexp Usage::; also *note Computed Regexps::).

   A constant regular expression in slashes by itself is also an
expression.  '/REGEXP/' is an abbreviation for the following comparison
expression:

     $0 ~ /REGEXP/

   One special place where '/foo/' is _not_ an abbreviation for '$0 ~
/foo/' is when it is the righthand operand of '~' or '!~'.  *Note Using
Constant Regexps::, where this is discussed in more detail.