为循环的条件，在红宝石

为循环的条件，在红宝石

for variable_name[, variable...] in expression [do]

   # code to be executed

end
for: A special Ruby keyword which indicates the beginning of the loop.
variable_name: This is a variable name that serves as the reference to the current iteration of the loop. 
in: This is a special Ruby keyword that is primarily used in for loop.
expression: It executes code once for each element in expression. Here expression can be range or array variable.
do: This indicates the beginning of the block of code to be repeatedly executed. do is optional.
end: This keyword represents the ending of ‘for‘ loop block which started from ‘do‘ keyword.