

Actions can use conventional arithmetic expressions to compute numeric values. As a simple example, suppose we want to print the population density for each country in the file countries. Because the second field is the area in thousands of square miles, and the third field is the population in millions, the expression 1000 $3 / $2 gives the population density in people per square mile. Use the following program to print the name of each country and its population density:
{ printf "%10s %6.1f\n", $1, 1000 * $3 / $2 }The output looks like this:
CIS 30.3 Canada 6.2 China 234.6 USA 60.6 Brazil 35.3 Australia 4.7 India 502.0 Argentina 24.3 Sudan 19.6 Algeria 19.6Arithmetic is done internally in floating point. The arithmetic operators are +, , , /, % (remainder), and ^ (exponentiation; is a synonym). Arithmetic expressions can be created by applying these operators to constants, variables, field names, array elements, functions, and other expressions, all of which are discussed later. Note that awk recognizes and produces scientific (exponential) notation: 1e6, 1E6, 10e5, and 1000000 are numerically equal.
awk has assignment statements like those found in the
C programming language. The simplest form is the
assignment statement:
v = e
where v is a variable or field name, and e is an expression. For example, to compute the number of Asian countries and their total populations, use this program:
$4 == "Asia" { pop = pop + $3; n = n + 1 } END { print "population of", n, "Asian countries in millions is", pop }Applied to countries, this program produces the following:
population of 3 Asian countries in millions is 1765The action associated with the pattern $4 == "Asia" contains two assignment statements, one to accumulate population and the other to count countries. The variables are not explicitly initialized, yet everything works properly because awk initializes each variable with the string value "" and the numeric value 0.
The assignments in the previous program can be written more concisely using the operators += and ++ as follows:
$4 == "Asia" { pop += $3; ++n }The += operator is borrowed from the C programming language:
pop += $3It has the same effect as the following:
pop = pop + $3The += operator is shorter and runs faster. The same is true of the ++ operator, which increments a variable by one.
The abbreviated assignment operators are +=, =, =, /=, %=, and ^=. These are shorthand versions of traditional operations: a operator = b has the same effect as a = a operator b.
The increment and decrement operators are ++ and . As in C, you can use them as prefix (++x) or postfix (x++) operators. If x is 1, then i=++x increments x, then sets i to 2. On the other hand, i=x++ sets i to 1, then increments x. An analogous interpretation applies to prefix  and postfix . Assignment, increment, and decrement operators can all be used in arithmetic expressions.
We use default initialization to advantage in the following program, which finds the country with the largest population:
maxpop < $3 { maxpop = $3; country = $1 } END { print country, maxpop }Note that this program is not correct if all values of $3 are negative.