CODEWITHSUNDEEP
cif-statementconditional-statementsboolean-logic
Blue7
Blue7

Reputation: 2044

If statement where all the conditions equal each other

I need the if statement to only execute if the 5 value in the statement are the same. e.g if a[0] = a[1] = a[2] = [a3] = [a4] = 1

This is just a matter of boolean algebra but I cant get it right.

I've tried:

if( (A[0]) && (A[1]) && (A[2]) && (A[3]) == (A[4]))
        {
            Flag =1;
        }

and

if( (A[0]),(A[1]),(A[2]),(A[3]) == (A[4]))
        {
            Flag =1;
        }

and

if( (A[0]) == (A[1]) == (A[2]) == (A[3]) == (A[4]))
        {
            Flag =1;
        }

But none of these work.

Can someone please help?

Thanks!

Upvotes: 2

Views: 764

Answers (9)

pepo
pepo

Reputation: 699

As was stated, this is the usual way

if ( A[0] == A[1] && A[1] == A[2] && A[2] == A[3] && A[3] == A[4] )
{
    Flag = 1;
}

Your first attempt however made me think of an alternative way, using bitwise logical operations, since & and | on equal numbers gives the very same number.

But as soon as some of the numbers are different, i.e. there is some bit set to 0 in some number and 1 in another, | will yield 1 and & will yield 0 on this position in the resulting numbers, making them different.

if ( (A[0] & A[1] & A[2] & A[3] & A[4]) == (A[0] | A[1] | A[2] | A[3] | A[4]) )
{
    Flag = 1;
}

This seems to run about twice as fast (under -O2 optimisation)

Upvotes: 0

litaoshen
litaoshen

Reputation: 1862

I just want to add something, you may study the operator priority first; (1) "==" and "&&" which one is first (2) May cause some compilation error (3) Notice that "==" OPERATOR will return true or false you cannot connect them together If you want to explore, you can try to use xor to solve this problem

Upvotes: 0

John Bode
John Bode

Reputation: 123448

Several people have given you the right answer; here's an explanation of why your attempts didn't work.

Why A[0] && A[1] && A[2] && A[3] == A[4] didn't work:

The expression A && B will evaluate to 0 if either A or B are zero, or to 1 if both A and B are non-zero. The && operator is left-associative, meaning expressions like A && B && C are parsed as (A && B) && C; IOW, C is AND-ed with the result of A && B (0 or 1). Thus, A[0] && A[1] && A[2] && A[3] == A[4] will only evaluate to true if all of A[0] through A[3] are non-zero and A[4] == 1, or any one of A[0] through A[3] are 0 and A[4] == 0. Also note that the && operator won't evaluate the right-hand operand if the left-hand operand is 0.

Why (A[0]),(A[1]),(A[2]),(A[3]) == (A[4]) didn't work:

The comma operator evaluates each expression in sequence, and the result is the value of the last expression, so only the value of A[3] is being compared to A[4]. Each of A[0] through A[2] is evaluated, but the result is discarded.

Why (A[0]) == (A[1]) == (A[2]) == (A[3]) == (A[4]) didn't work:

Similar to the && operator, the result of A == B will be 0 or 1. Also similar to the above, the == operator is left-associative, so A == B == C is parsed as (A == B) == C, so C is being compared to the result of A == B, which will be either 0 or 1. So the above will only evaluate to true if A[0] and A[1] are the same, and A[2] through A[4] all equal 1.

Therefore, if you want to check that multiple expressions are all equivalent, you must write something like

A == B && B == C && C == D

Upvotes: 3

Devolus
Devolus

Reputation: 22074

From boolean logic we have if A == B AND A == C then B == C

In C/C++ you can not combine the evaluation of an expression, so each condition has to be done individually.

So you can do it like this:

 if( (A[0] == A[1] && A[0] == A[2]  && A[0] == A[3]  && A[0]== A[4])
     flag = 1;

If this is an array you can do it in a loop as well like this (pseudocode):

 v = A[0]
 flag = 1; // Asume that all are equal.
 for(i = 0; i < maxindex; i++)
 {
      // If one entry doesn't match the whole expression is false.
      if(v != A[i])
      {
           flag = 0;
           break;
      }
}

Upvotes: 3

Lundin
Lundin

Reputation: 213458

Since this happens to be an array, you could take advantage of that:

const int N = 4;
bool everything_is_equal = true;

for(int i=0; i<(N-1); i++)
{
  if(A[i] != A[i+1])
  {
    everything_is_equal = false;
    break;
  }
}

Depending on the number of elements, this may or may not be more readable than a long if statement using the && operator.

Upvotes: 1

Koryu
Koryu

Reputation: 1381

that would check if all are equal A4 which would mean all 5 are equal:

if( A[0] == A[4] && A[1] == A[4] && A[2] == A[4] && A[3] == A[4] )
{
         Flag =1;
}

Upvotes: 0

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272467

Other than for booleans, there is no general way to write this shorthand. You'll need to do something like:

if (A[0] == A[1] && A[1] == A[2] && ...)

or something equivalent, or encapsulate this behaviour in a helper function (something like bool areAllTheThingsEqual(T *things, int num)).

Upvotes: 0

user3067395
user3067395

Reputation: 580

I think the easiest way to do this is:

if( A[0] == A[1] && A[1] == A[2] && A[3] == A[4] && A[4] == A[5])

Upvotes: 0

Dr.Tower
Dr.Tower

Reputation: 995

Each comparison needs to be done individually.

if (A[0] == A[1] && 
    A[1] == A[2] &&
    A[2] == A[3] &&
    A[3] == A[4])
{
    Flag = 1;
}

Upvotes: 2

Related Questions