6.16. Ruby judgment

Grammar #

ifconditional[then]code...[elsifconditional[then]code...]...[elsecode...]end

if expressions are used for conditional execution. Value false and nil if false, all other values are true. Note that Ruby uses the elsif , not using the else if and elif .

If conditional is true, execute code . If conditional is not true, then execute else clause specified in the code .

Usually we omit the reserved words. then . If you want to write a complete if type, you must use the then separate conditional and program blocks as follows:

ifa==4thena=7end

Example #

#!/usr/bin/ruby#-*- coding: UTF-8 -*-x=1ifx>2puts"x >
2"elsifx<=2andx!=0puts"X is 1 "elseputs" and cannot determine the value of x "end

The output result of the above example:

x is 1

Grammar #

codeifcondition

if modifier phrase indicates that when if will not be implemented until the condition on the right is established. if , the formula on the left. That is, if conditional is true, execute code .

Example #

#!/usr/bin/ruby$debug=1print"debug\\n"if$debug

The output result of the above example:

debug

Grammar #

unlessconditional[then]code[elsecode]end

unless formula sum if has the opposite effect, that is, if conditional is false, execute code . If conditional is true, execute else clause specified in the code .

Example #

#!/usr/bin/ruby#-*- coding: UTF-8 -*-x=1unlessx>2puts"x >
2"elseputs"x ＜ 2"end

The output result of the above example is:

x ＜ 2

Grammar #

codeunlessconditional

If conditional is false, execute code .

Example #

#!/usr/bin/ruby#-*- coding: UTF-8 -*-$var=1print"1 --
This line outputs\\n"if$varprint"2 --
This line does not output\\n"unless$var$var=falseprint"3
-- This line outputs\\n"unless$var

The output result of the above example:

1 -- This line outputs
3 -- This line outputs

Grammar #

caseexpression[whenexpression[,expression...][then]code]...[elsecode]end

case to one first expression the matching judgment is made, and then the branches are selected according to the matching results.

It uses the === operator comparison when specified expression .If it is consistent, then execute it when part of the content.

Usually we omit the reserved words then . If you want to write a complete when type, you must use the then separate conditional andprogram blocks. As follows:

whena==4thena=7end

Therefore:

caseexpr0whenexpr1,expr2stmt1whenexpr3,expr4stmt2elsestmt3end

Basically similar to:

\_tmp=expr0ifexpr1===\_tmp\|\|expr2===\_tmpstmt1elsifexpr3===\_tmp\|\|expr4===\_tmpstmt2elsestmt3end

Example #

#!/usr/bin/ruby#-*- coding: UTF-8
-*-$age=5case$agewhen0..2puts"baby"when3..
6puts"child"when7..12puts"child"when13..18puts"
youth"elseputs"Other age groups"end

The output result of the above example is:

child

When case the expression part of the is omitted, the first when expression whose conditional part is true.

foo=falsebar=truequu=falsecasewhenfoothenputs'foo is
true'whenbarthenputs'bar is true'whenquuthenputs'quu is true'end#display
"bar is true"