Control Statements in Actions

Control Statements in Actions

Control statements such as if, while, and so on control the flow of execution in awk programs. Most of the control statements in awk are patterned on similar statements in C.

All the control statements start with special keywords such as if and while, to distinguish them from simple expressions.

Many control statements contain other statements; for example, the if statement contains another statement which may or may not be executed. The contained statement is called the body. If you want to include more than one statement in the body, group them into a single compound statement with curly braces, separating them with newlines or semicolons.

The if-else Statement

The if-else statement is awk's decision-making statement. It looks like this:

if (condition) then-body [else else-body]

The condition is an expression that controls what the rest of the statement will do. If condition is true, then-body is executed; otherwise, else-body is executed. The else part of the statement is optional. The condition is considered false if its value is zero or the null string, and true otherwise.

Here is an example:

if (x % 2 == 0)
    print "x is even"
else
    print "x is odd"

In this example, if the expression `x % 2 == 0' is true (that is, the value of x is evenly divisible by two), then the first print statement is executed, otherwise the second print statement is executed.

If the else appears on the same line as then-body, and then-body is not a compound statement (i.e. not surrounded by curly braces), then a semicolon must separate then-body from else. To illustrate this, let's rewrite the previous example:

if (x % 2 == 0) print "x is even"; else
        print "x is odd"

If you forget the `;', awk won't be able to interpret the statement, and you will get a syntax error.

We would not actually write this example this way, because a human reader might fail to see the else if it were not the first thing on its line.

The while Statement

In programming, a loop means a part of a program that can be executed two or more times in succession.

The while statement is the simplest looping statement in awk. It repeatedly executes a statement as long as a condition is true. It looks like this:

while (condition)
  body

Here body is a statement that we call the body of the loop, and condition is an expression that controls how long the loop keeps running.

The first thing the while statement does is test condition. If condition is true, it executes the statement body. After body has been executed, condition is tested again, and if it is still true, body is executed again. This process repeats until condition is no longer true. If condition is initially false, the body of the loop is never executed, and awk continues with the statement following the loop.

This example prints the first three fields of each record, one per line.

awk '{ i = 1
       while (i <= 3) {
           print $i
           i++
       }
}' inventory-shipped

Here the body of the loop is a compound statement enclosed in braces, containing two statements.

The loop works like this: first, the value of i is set to one. Then, the while tests whether i is less than or equal to three. This is true when i equals one, so the i-th field is printed. Then the `i++' increments the value of i and the loop repeats. The loop terminates when i reaches four.

As you can see, a newline is not required between the condition and the body; but using one makes the program clearer unless the body is a compound statement or is very simple. The newline after the open-brace that begins the compound statement is not required either, but the program would be harder to read without it.

The do-while Statement

The do loop is a variation of the while looping statement. The do loop executes the body once, and then repeats body as long as condition is true. It looks like this:

do
  body
while (condition)

Even if condition is false at the start, body is executed at least once (and only once, unless executing body makes condition true). Contrast this with the corresponding while statement:

while (condition)
  body

This statement does not execute body even once if condition is false to begin with.

Here is an example of a do statement:

awk '{ i = 1
       do {
          print $0
          i++
       } while (i <= 10)
}'

This program prints each input record ten times. It isn't a very realistic example, since in this case an ordinary while would do just as well. But this reflects actual experience; there is only occasionally a real use for a do statement.

The for Statement

The for statement makes it more convenient to count iterations of a loop. The general form of the for statement looks like this:

for (initialization; condition; increment)
  body

The initialization, condition and increment parts are arbitrary awk expressions, and body stands for any awk statement.

The for statement starts by executing initialization. Then, as long as condition is true, it repeatedly executes body and then increment. Typically initialization sets a variable to either zero or one, increment adds one to it, and condition compares it against the desired number of iterations.

Here is an example of a for statement:

awk '{ for (i = 1; i <= 3; i++)
          print $i
}' inventory-shipped

This prints the first three fields of each input record, one field per line.

You cannot set more than one variable in the initialization part unless you use a multiple assignment statement such as `x = y = 0', which is possible only if all the initial values are equal. (But you can initialize additional variables by writing their assignments as separate statements preceding the for loop.)

The same is true of the increment part; to increment additional variables, you must write separate statements at the end of the loop. The C compound expression, using C's comma operator, would be useful in this context, but it is not supported in awk.

Most often, increment is an increment expression, as in the example above. But this is not required; it can be any expression whatever. For example, this statement prints all the powers of two between one and 100:

for (i = 1; i <= 100; i *= 2)
  print i

Any of the three expressions in the parentheses following the for may be omitted if there is nothing to be done there. Thus, `for (; x > 0;)' is equivalent to `while (x > 0)'. If the condition is omitted, it is treated as true, effectively yielding an infinite loop (i.e. a loop that will never terminate).

In most cases, a for loop is an abbreviation for a while loop, as shown here:

initialization
while (condition) {
  body
  increment
}

The only exception is when the continue statement (see section The continue Statement) is used inside the loop; changing a for statement to a while statement in this way can change the effect of the continue statement inside the loop.

There is an alternate version of the for loop, for iterating over all the indices of an array:

for (i in array)
    do something with array[i]

See section Scanning All Elements of an Array, for more information on this version of the for loop.

The awk language has a for statement in addition to a while statement because often a for loop is both less work to type and more natural to think of. Counting the number of iterations is very common in loops. It can be easier to think of this counting as part of looping rather than as something to do inside the loop.

The next section has more complicated examples of for loops.

The break Statement

The break statement jumps out of the innermost for, while, or do loop that encloses it. The following example finds the smallest divisor of any integer, and also identifies prime numbers:

awk '# find smallest divisor of num
     { num = $1
       for (div = 2; div*div <= num; div++)
         if (num % div == 0)
           break
       if (num % div == 0)
         printf "Smallest divisor of %d is %d\n", num, div
       else
         printf "%d is prime\n", num
     }'

When the remainder is zero in the first if statement, awk immediately breaks out of the containing for loop. This means that awk proceeds immediately to the statement following the loop and continues processing. (This is very different from the exit statement which stops the entire awk program. See section The exit Statement.)

Here is another program equivalent to the previous one. It illustrates how the condition of a for or while could just as well be replaced with a break inside an if:

awk '# find smallest divisor of num
     { num = $1
       for (div = 2; ; div++) {
         if (num % div == 0) {
           printf "Smallest divisor of %d is %d\n", num, div
           break
         }
         if (div*div > num) {
           printf "%d is prime\n", num
           break
         }
       }
}'

As described above, the break statement has no meaning when used outside the body of a loop. However, although it was never documented, historical implementations of awk have treated the break statement outside of a loop as if it were a next statement (see section The next Statement). Recent versions of Unix awk no longer allow this usage. gawk will support this use of break only if `--traditional' has been specified on the command line (see section Command Line Options). Otherwise, it will be treated as an error, since the POSIX standard specifies that break should only be used inside the body of a loop (d.c.).

The continue Statement

The continue statement, like break, is used only inside for, while, and do loops. It skips over the rest of the loop body, causing the next cycle around the loop to begin immediately. Contrast this with break, which jumps out of the loop altogether.

The continue statement in a for loop directs awk to skip the rest of the body of the loop, and resume execution with the increment-expression of the for statement. The following program illustrates this fact:

awk 'BEGIN {
     for (x = 0; x <= 20; x++) {
         if (x == 5)
             continue
         printf "%d ", x
     }
     print ""
}'

This program prints all the numbers from zero to 20, except for five, for which the printf is skipped. Since the increment `x++' is not skipped, x does not remain stuck at five. Contrast the for loop above with this while loop:

awk 'BEGIN {
     x = 0
     while (x <= 20) {
         if (x == 5)
             continue
         printf "%d ", x
         x++
     }
     print ""
}'

This program loops forever once x gets to five.

As described above, the continue statement has no meaning when used outside the body of a loop. However, although it was never documented, historical implementations of awk have treated the continue statement outside of a loop as if it were a next statement (see section The next Statement). Recent versions of Unix awk no longer allow this usage. gawk will support this use of continue only if `--traditional' has been specified on the command line (see section Command Line Options). Otherwise, it will be treated as an error, since the POSIX standard specifies that continue should only be used inside the body of a loop (d.c.).

The next Statement

The next statement forces awk to immediately stop processing the current record and go on to the next record. This means that no further rules are executed for the current record. The rest of the current rule's action is not executed either.

Contrast this with the effect of the getline function (see section Explicit Input with getline). That too causes awk to read the next record immediately, but it does not alter the flow of control in any way. So the rest of the current action executes with a new input record.

At the highest level, awk program execution is a loop that reads an input record and then tests each rule's pattern against it. If you think of this loop as a for statement whose body contains the rules, then the next statement is analogous to a continue statement: it skips to the end of the body of this implicit loop, and executes the increment (which reads another record).

For example, if your awk program works only on records with four fields, and you don't want it to fail when given bad input, you might use this rule near the beginning of the program:

NF != 4 {
  err = sprintf("%s:%d: skipped: NF != 4\n", FILENAME, FNR)
  print err > "/dev/stderr"
  next
}

so that the following rules will not see the bad record. The error message is redirected to the standard error output stream, as error messages should be. See section Special File Names in gawk.

According to the POSIX standard, the behavior is undefined if the next statement is used in a BEGIN or END rule. gawk will treat it as a syntax error. Although POSIX permits it, some other awk implementations don't allow the next statement inside function bodies (see section User-defined Functions). Just as any other next statement, a next inside a function body reads the next record and starts processing it with the first rule in the program.

If the next statement causes the end of the input to be reached, then the code in any END rules will be executed. See section The BEGIN and END Special Patterns.

The nextfile Statement

gawk provides the nextfile statement, which is similar to the next statement. However, instead of abandoning processing of the current record, the nextfile statement instructs gawk to stop processing the current data file.

Upon execution of the nextfile statement, FILENAME is updated to the name of the next data file listed on the command line, FNR is reset to one, ARGIND is incremented, and processing starts over with the first rule in the progam. See section Built-in Variables.

If the nextfile statement causes the end of the input to be reached, then the code in any END rules will be executed. See section The BEGIN and END Special Patterns.

The nextfile statement is a gawk extension; it is not (currently) available in any other awk implementation. See section Implementing nextfile as a Function, for a user-defined function you can use to simulate the nextfile statement.

The nextfile statement would be useful if you have many data files to process, and you expect that you would not want to process every record in every file. Normally, in order to move on to the next data file, you would have to continue scanning the unwanted records. The nextfile statement accomplishes this much more efficiently.

Caution: Versions of gawk prior to 3.0 used two words (`next file') for the nextfile statement. This was changed in 3.0 to one word, since the treatment of `file' was inconsistent. When it appeared after next, it was a keyword. Otherwise, it was a regular identifier. The old usage is still accepted. However, gawk will generate a warning message, and support for next file will eventually be discontinued in a future version of gawk.

The exit Statement

The exit statement causes awk to immediately stop executing the current rule and to stop processing input; any remaining input is ignored. It looks like this:

exit [return code]

If an exit statement is executed from a BEGIN rule the program stops processing everything immediately. No input records are read. However, if an END rule is present, it is executed (see section The BEGIN and END Special Patterns).

If exit is used as part of an END rule, it causes the program to stop immediately.

An exit statement that is not part of a BEGIN or END rule stops the execution of any further automatic rules for the current record, skips reading any remaining input records, and executes the END rule if there is one.

If you do not want the END rule to do its job in this case, you can set a variable to non-zero before the exit statement, and check that variable in the END rule. See section Assertions, for an example that does this.

If an argument is supplied to exit, its value is used as the exit status code for the awk process. If no argument is supplied, exit returns status zero (success). In the case where an argument is supplied to a first exit statement, and then exit is called a second time with no argument, the previously supplied exit value is used (d.c.).

For example, let's say you've discovered an error condition you really don't know how to handle. Conventionally, programs report this by exiting with a non-zero status. Your awk program can do this using an exit statement with a non-zero argument. Here is an example:

BEGIN {
       if (("date" | getline date_now) < 0) {
         print "Can't get system date" > "/dev/stderr"
         exit 1
       }
       print "current date is", date_now
       close("date")
}