How an if A B statement is evaluated

Question

if   A   amp  amp   B          do something   else     do something else   The question is  would the statement immediately break to else if A was FALSE   Would B even get evaluated     I ask this in the case that B checking the validity of an array index say array 0  when the array is actually empty and has zero elements   Therefore throwing a segfault because we are trying to access something that is out of bounds of the array   Specifically  if   array GetElements    gt  0   amp  amp   array 0      array 0 - gt doSomething    else     do nothing and return   This may be dangerous if array 0  actually gets evaluated because it segfaults without the first check to the left of the   amp  amp     Precedence tells me that the left side will definitely take precedence but it doesn t tell me that it won t evaluate the right side if the left is FALSE

User · Answer

You are asking about the  amp  amp  operator  not the if statement    amp  amp  short-circuits  meaning that if while working it meets a condition which results in only one answer  it will stop working and use that answer   So  0  amp  amp  x will execute 0  then terminate because there is no way for the expression to evaluate non-zero regardless of what is the second parameter to  amp  amp

User · Answer

for logical  amp  amp  both the parameters must be true   then it ll be entered in if    clock otherwise it ll execute else     for logical    one of parameter or condition is true is sufficient to execute if      if   A   amp  amp   B            if A and B both are true  else    if   A     B            if A or B is true   else

User · Answer

Yes  it is called Short-circuit Evaluation   If the validity of the boolean statement can be assured after part of the statement  the rest is not evaluated   This is very important when some of the statements have side-effects

User · Answer

yes  if   A   amp  amp   B    will fail on the first clause  if  A  evaluates false   this applies to any language btw  not just C derivatives  For threaded and parallel processing this is a different story

User · Answer

In C and C    the  amp  amp  and    operators  short-circuit   That means that they only evaluate a parameter if required  If the first parameter to  amp  amp  is false  or the first to    is true  the rest will not be evaluated   The code you posted is safe  though I question why you d include an empty else block

[c++] How an 'if (A && B)' statement is evaluated?

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