CODEWITHSUNDEEP
c
The Artist
The Artist

Reputation: 213

What does "Comparing constant with boolean expression is always true" warning mean?

What does this warning mean (i and j are not constants):

I have been trying to Google this but it does not give me any results.

warning: comparison of constant 10 with boolean expression is always true [-Wtautological-constant-out-of-range-compare]

 if ((0<=i<=10)&&(0<=j<=10)){

In my program, i and j are not constant values and they do change.

Upvotes: 5

Views: 25080

Answers (4)

dagelf
dagelf

Reputation: 1739

It means that the expression can be evaluated at compile time, and that the evaluation will not be compiled because it's true (ie. non-zero) at compile time.

Usually this happened to me when I accidentally used && instead of &. Which is not the case here of course, but it gives the same error message.

Also note that gcc makes a distinction between logical and bitwise:

Logical means bool which is either 0 meaning false or non-zero meaning true and the operator that yields these is && with keyword and.

Bitwise means boolean logic and the operator is & with keyword bitand.

&& Always yields a bool, which is true unless it's 0. If operating on bools they're equivalent, but on anything else & and && test different things ie. "Is it nonzero" vs "return only the matching bits".

Upvotes: 0

Stephen
Stephen

Reputation: 1617

I believe you need to understand what's going on in your statement at a deeper level.

0<=i is a boolean expression, it will become true or false. The result of that expression is then compared with 10.

So you end up with true <= 10 or false <= 10.

I think you meant to write 

if ( ( 0 <= i ) && ( i <= 10 ) )

You cannot connect clauses together the way you did.

Upvotes: 3

R Sahu
R Sahu

Reputation: 206627

The other answers have already explained the core problem. You can use:

if ( ( ( 0 <= i) && (i <= 10)) && ( ( 0 <= i) && (i <= 10)) )

to resolve your problem.

My recommendation will be to wrap that logic in a function.

int isInRange(int x, int lower, int upper)
{
   return (lower <= x && x <= upper);
}

and use

if ( isInRange(i, 0, 10) && isInRange(j, 0, 10) )

Upvotes: 3

Sourav Ghosh
Sourav Ghosh

Reputation: 134346

In C, chaining of relational operators like this are not valid design. Thus,

 (0<=i<=10)

is not doing what you think it should be doing. it is getting evaluated as

((0<=i) <= 10 )

which is basically either

  • 0 < = 10, producing 1 (considered TRUE value)
  • 1 < = 10, also producing 1 (considered TRUE value)

sadly, both of which are way out than the expected path.

Solution: you need to break down your condtion check like

 (0 <= i) && ( i<=10)

Upvotes: 16

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