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自动选题: 20191204 4 ways to control the flow of your awk script
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[#]: collector: (lujun9972)
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[#]: translator: ( )
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (4 ways to control the flow of your awk script)
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[#]: via: (https://opensource.com/article/19/12/control-awk-script)
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[#]: author: (Seth Kenlon https://opensource.com/users/seth)
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4 ways to control the flow of your awk script
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======
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Learn to use switch statements and the break, continue, and next
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commands to control awk scripts.
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![JavaScript in Vim][1]
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There are many ways to control the flow of an awk script, including [loops][2], **switch** statements and the **break**, **continue**, and **next** commands.
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### Sample data
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Create a sample data set called **colours.txt** and copy this content into it:
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```
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name color amount
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apple red 4
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banana yellow 6
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strawberry red 3
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raspberry red 99
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grape purple 10
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apple green 8
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plum purple 2
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kiwi brown 4
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potato brown 9
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pineapple yellow 5
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```
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### Switch statements
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The **switch** statement is a feature specific to GNU awk, so you can only use it with **gawk**. If your system or your target system doesn't have **gawk**, then you should not use a switch statement.
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The **switch** statement in **gawk** is similar to the one in C and many other languages. The syntax is:
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```
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switch (expression) {
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case VALUE:
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<do something here>
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[...]
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default:
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<do something here>
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}
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```
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The **expression** part can be any awk expression that returns a numeric or string result. The **VALUE** part (after the word **case**) is a numeric or string constant or a regular expression.
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When a **switch** statement runs, the _expression_ is evaluated, and the result is matched against each case value. If there's a match, then the code contained within a case definition is executed. If there's no match in any case definition, then the default statement is executed.
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The keyword **break** is at the end of the code in each case definition to break the loop. Without **break**, awk would continue to search for matching case values.
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Here's an example **switch** statement:
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```
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#!/usr/bin/awk -f
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#
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# Example of the use of 'switch' in GNU Awk.
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NR > 1 {
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printf "The %s is classified as: ",$1
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switch ($1) {
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case "apple":
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print "a fruit, pome"
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break
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case "banana":
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case "grape":
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case "kiwi":
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print "a fruit, berry"
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break
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case "raspberry":
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print "a computer, pi"
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break
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case "plum":
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print "a fruit, drupe"
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break
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case "pineapple":
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print "a fruit, fused berries (syncarp)"
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break
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case "potato":
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print "a vegetable, tuber"
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break
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default:
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print "[unclassified]"
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}
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}
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```
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This script notably ignores the first line of the file, which in the case of the sample data is just a header. It does this by operating only on records with an index number greater than 1. On all other records, this script compares the contents of the first field (**$1**, as you know from previous articles) to the value of each **case** definition. If there's a match, the **print** function is used to print the botanical classification of the entry. If there are no matches, then the **default** instance prints **"[unclassified]"**.
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The banana, grape, and kiwi are all botanically classified as a berry, so there are three **case** definitions associated with one **print** result.
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Run the script on the **colours.txt** sample file, and you should get this:
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```
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The apple is classified as: a fruit, pome
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The banana is classified as: a fruit, berry
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The strawberry is classified as: [unclassified]
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The raspberry is classified as: a computer, pi
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The grape is classified as: a fruit, berry
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The apple is classified as: a fruit, pome
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The plum is classified as: a fruit, drupe
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The kiwi is classified as: a fruit, berry
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The potato is classified as: a vegetable, tuber
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The pineapple is classified as: a fruit, fused berries (syncarp)
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```
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### Break
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The **break** statement is mainly used for the early termination of a **for**, **while**, or **do-while** loop or a **switch** statement. In a loop, **break** is often used where it's not possible to determine the number of iterations of the loop beforehand. Invoking **break** terminates the enclosing loop (which is relevant when there are nested loops or loops within loops).
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This example, straight out of the [GNU awk manual][3], shows a method of finding the smallest divisor. Read the additional comments for a clear understanding of how the code works:
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```
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#!/usr/bin/awk -f
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{
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num = $1
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# Make an infinite FOR loop
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for (divisor = 2; ; divisor++) {
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# If num is divisible by divisor, then break
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if (num % divisor == 0) {
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printf "Smallest divisor of %d is %d\n", num, divisor
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break
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}
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# If divisor has gotten too large, the number has no
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# divisor, so is a prime
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if (divisor * divisor > num) {
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printf "%d is prime\n", num
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break
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}
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}
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}
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```
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Try running the script to see its results:
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```
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$ echo 67 | ./divisor.awk
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67 is prime
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$ echo 69 | ./divisor.awk
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Smallest divisor of 69 is 3
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```
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As you can see, even though the script starts out with an explicit _infinite_ loop with no end condition, the **break** function ensures that the script eventually terminates.
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### Continue
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The **continue** function is similar to **break**. It can be used in a **for**, **while**, or **do-while** loop (it's not relevant to a **switch** statements, though). Invoking **continue** skips the rest of the enclosing loop and begins the next cycle.
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Here's another good example from the GNU awk manual to demonstrate a possible use of **continue**:
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```
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#!/usr/bin/awk -f
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# Loop, printing numbers 0-20, except 5
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BEGIN {
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for (x = 0; x <= 20; x++) {
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if (x == 5)
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continue
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printf "%d ", x
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}
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print ""
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}
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```
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This script analyzes the value of **x** before printing anything. If the value is exactly 5, then **continue** is invoked, causing the **printf** line to be skipped, but leaves the loop unbroken. Try the same code but with **break** instead to see the difference.
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### Next
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This statement is not related to loops like **break** and **continue** are. Instead, **next** applies to the main record processing cycle of awk: the functions you place between the BEGIN and END functions. The **next** statement causes awk to stop processing the _current input record_ and to move to the next one.
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As you know from the earlier articles in this series, awk reads records from its input stream and applies rules to them. The **next** statement stops the execution of rules for the current record and moves to the next one.
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Here's an example of **next** being used to "hold" information upon a specific condition:
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```
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#!/usr/bin/awk -f
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# Ignore the header
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NR == 1 { next }
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# If field 2 (colour) is less than 6
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# characters, then save it with its
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# line number and skip it
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length($2) < 6 {
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skip[NR] = $0
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next
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}
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# It's not the header and
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# the colour name is > 6 characters,
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# so print the line
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{
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print
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}
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# At the end, show what was skipped
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END {
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printf "\nSkipped:\n"
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for (n in skip)
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print n": "skip[n]
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}
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```
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This sample uses **next** in the first rule to avoid the first line of the file, which is a header row. The second rule skips lines when the color name is less than six characters long, but it also saves that line in an array called **skip**, using the line number as the key (also known as the _index_).
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The third rule prints anything it sees, but it is not invoked if either rule 1 or rule 2 causes it to be skipped.
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Finally, at the end of all the processing, the **END** rule prints the contents of the array.
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Run the sample script on the **colours.txt** file from above (and previous articles):
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```
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$ ./next.awk colours.txt
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banana yellow 6
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grape purple 10
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plum purple 2
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pineapple yellow 5
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Skipped:
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2: apple red 4
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4: strawberry red 3
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6: apple green 8
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8: kiwi brown 4
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9: potato brown 9
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```
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### Control freak
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In summary, **switch**, **continue**, **next**, and **break** are important preemptive exceptions to awk rules that provide greater control of your script. You don't have to use them directly; often, you can gain the same logic through other means, but they're great convenience functions that make the coder's life a lot easier. The next article in this series covers the **printf** statement.
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* * *
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Would you rather listen to this article? It was adapted from an episode of [Hacker Public Radio][4], a community technology podcast by hackers, for hackers.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/19/12/control-awk-script
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作者：[Seth Kenlon][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/seth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/javascript_vim.jpg?itok=mqkAeakO (JavaScript in Vim)
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[2]: https://opensource.com/article/19/11/loops-awk
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[3]: https://www.gnu.org/software/gawk/manual/
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[4]: http://hackerpublicradio.org/eps.php?id=2438
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