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Introduction to Ruby Page 2/5


2. BEGIN and END
BEGIN block

Code in BEGIN blocks is executed before all code is executed, and Ruby allows multiple BEGIN blocks to be set up and the code in the blocks is executed in the order in which they appear. C# programmers pay attention to the following code

BEGIN
{
print "OnInit(object sender, EventArgs args)\n"
}
BEGIN
{
print "OnLoad(object sender, EventArgs args)\n"
}
print "Running"
The above code looks beautiful, but unfortunately, the above code segment will have parse error. The correct code should be

BEGIN{
print "OnInit(object sender, EventArgs args)\n"
}
BEGIN{
print "OnLoad(object sender, EventArgs args)\n"
}
print "Running"
As presented in the above code snippet, code within the block can be executed correctly only when the starting braces and the BEGIN identifier are on the same line. At the same time, the BEGIN block is not affected by any control structure, because as long as the BEGIN block appears, it will be executed and only once.

 1i = 0
2while i < 10
3 # Although the loop structure is processed, the code in the BEGIN block is still executed only once
 4 BEGIN{
 5 print "OnInit(object sender, EventArgs args)\n"
 6 }
 7 i += 1
8end
 9
 10if false
 11 # BEGIN is completely unaffected by if, and will be executed as long as the BEGIN block appears.
 12 BEGIN{
 13 print "OnLoad(object sender, EventArgs args)\n"
 14 }
 15end
 16
 17print "Running"
Based on the principle that BEGIN will be executed and BEGIN will be executed before all code execution, even if the code appears before the BEGIN block, the code will still wait for the BEGIN block to be executed before execution. For example, the output result of the following code segment is still OnInit - OnLoad - Running.

 print "OnLoad(object sender, EventArgs args)\n"
BEGIN{
print "OnInit(object sender, EventArgs args)\n"
}
print "Running"
END block

The END block is contrary to the BEGIN block. It is executed after all code is executed, and the END block that appears first when multiple END blocks are executed last. In addition, although the END block is not affected by while, it may use if to control whether the END block is executed. For example, the output result of the following code is Start - Load - Unload.

if false
 END{
 # Never output
 print "Init"
 }
 end
 END{
 # Last output
 print "Unload\n"
 }
 END{
 # Output before Unload
 print "Load\n"
 }
 # First output
 print "Start\n"
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  • Introduction to Ruby

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