Boolean operators and

Question

According to the R language definition  the difference between  amp  and  amp  amp   correspondingly   and     is that the former is vectorized while the latter is not   According to the help text  I read the difference akin to the difference between an  And  and  AndAlso   correspondingly  Or  and  OrElse      Meaning  That not all evaluations if they don t have to be  i e  A or B or C is always true if A is true  so stop evaluating if A is true   Could someone shed light here  Also  is there an AndAlso and OrElse in R

User · Answer

The answer about  short-circuiting  is potentially misleading  but has some truth  see below   In the R S language   amp  amp  and    only evaluate the first element in the first argument  All other elements in a vector or list are ignored regardless of the first ones value  Those operators are designed to work with the if  cond     else   construction and to direct program control rather than construct new vectors   The  amp  and the   operators are designed to work on vectors  so they will be applied  in parallel   so to speak  along the length of the longest argument  Both vectors need to be evaluated before the comparisons are made  If the vectors are not the same length  then recycling of the shorter argument is performed   When the arguments to  amp  amp  or    are evaluated  there is  short-circuiting  in that if any of the values in succession from left to right are determinative  then evaluations cease and the final value is returned    gt  if  print 1     print 2   else  print 3    1  1  1  2  gt  if FALSE  amp  amp  print 1     print 2   else  print 3      print 1   not evaluated  1  3  gt  if TRUE  amp  amp  print 1     print 2   else  print 3    1  1  1  2  gt  if TRUE  amp  amp   print 1     print 2   else  print 3    1  1  1  3  gt  if FALSE  amp  amp   print 1     print 2   else  print 3    1  3   The advantage of short-circuiting will only appear when the arguments take a long time to evaluate  That will typically occur when the arguments are functions that either process larger objects or have mathematical operations that are more complex

User · Answer

The shorter ones are vectorized  meaning they can return a vector  like this     -2 2   gt   0   amp    -2 2   lt   0     1  FALSE FALSE  TRUE FALSE FALSE   The longer form evaluates left to right examining only the first element of each vector  so the above gives     -2 2   gt   0   amp  amp    -2 2   lt   0     1  FALSE   As the help page says  this makes the longer form  appropriate for programming control-flow and  is  typically preferred in if clauses    So you want to use the long forms only when you are certain the vectors are length one     You should be absolutely certain your vectors are only length 1  such as in cases where they are functions that return only length 1 booleans   You want to use the short forms if the vectors are length possibly  1   So if you re not absolutely sure  you should either check first  or use the short form and then use all and any to reduce it to length one for use in control flow statements  like if   The functions all and any are often used on the result of a vectorized comparison to see if all or any of the comparisons are true  respectively  The results from these functions are sure to be length 1 so they are appropriate for use in if clauses  while the results from the vectorized comparison are not    Though those results would be appropriate for use in ifelse   One final difference  the  amp  amp  and    only evaluate as many terms as they need to  which seems to be what is meant by short-circuiting    For example  here s a comparison using an undefined value a  if it didn t short-circuit  as  amp  and   don t  it would give an error   a   Error  object  a  not found TRUE    a    1  TRUE FALSE  amp  amp  a    1  FALSE TRUE   a   Error  object  a  not found FALSE  amp  a   Error  object  a  not found   Finally  see section 8 2 17 in The R Inferno  titled  and and andand

User · Answer

amp  amp  and    are what is called  short circuiting   That means that they will not evaluate the second operand if the first operand is enough to determine the value of the expression    For example if the first operand to  amp  amp  is false then there is no point in evaluating the second operand  since it can t change the value of the expression  false  amp  amp  true and false  amp  amp  false are both false   The same goes for    when the first operand is true   You can read more about this here  http   en wikipedia org wiki Short-circuit evaluation From the table on that page you can see that  amp  amp  is equivalent to AndAlso in VB NET  which I assume you are referring to

[r] Boolean operators && and ||

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