CODEWITHSUNDEEP
c
Bo.
Bo.

Reputation: 2541

if statement with equal sign "=" and not "==" in expression

I found a bug in code (the if statement should have had "==" instad of "=") and I have few questions.

Example code:

int i = 5;
if (i = MyFunction())  // MyFunction() returns an int; this is where bug was made
{
  // call A()
}
else
{
  // call B()
}

From what I gather it should always call A().
1. Is my assumption correct()?
2. Is this case for all/most compilers (any exceptions)?

Upvotes: 1

Views: 12870

Answers (6)

unwind
unwind

Reputation: 399743

  1. No, it will only call A() if the assignment is considered true, i.e. non-zero.
  2. Yes, this is standard. It's not a very good way to write the code since the risk of confusion is large.

Some people flip comparisons, writing the constant to the left, to avoid the risk:

if( 12 == x )

but of course this wouldn't have worked in your case, since it really is an assignment. Most compilers can give you a warning when this code is detected, since it's so often a cause of bugs.

Upvotes: 8

mathematician1975
mathematician1975

Reputation: 21351

You code will evaluate

  (i = MyFunction())

and if MyFunction() returns a non zero value, the expression will be evaluated as true and call A(). Otherwise B()

Upvotes: 2

Otto Allmendinger
Otto Allmendinger

Reputation: 28268

Your assumption is incorrect.

The expression in the if-condition is evaluated like any other expression, in your case the result of (i = MyFunction()) is the return value of MyFunction().

Upvotes: 3

vaisakh
vaisakh

Reputation: 1031

This statement is an assignment:

i = MyFunction()

The if statement is in effect checking the value of i. If MyFunction() returns 0, i is assigned 0 and it becomes equivalent to:

if(0)

This evaluates to false, and A() will not be called in this case

Upvotes: 4

ecatmur
ecatmur

Reputation: 157314

If MyFunction() returns zero (0) it will call B(); otherwise it will call A(). The value of an assignment expression is the value that is assigned to the left hand side; in an if statement 0 is treated as false and all other values as true.

This is perfectly legitimate code, although many compilers will issue a warning. The reason it is valid is that in C and similar languages, assignment is an expression (rather than a statement).

If you intended to assign to i and test the return value you should write if ((i = MyFunction())); the extra parentheses signal to the compiler (and to the reader) that the assignment is intended.

If instead you intend to test against the value of i you should write if (MyFunction() == i); by putting the function call on the left you ensure that if you miss out the double equals sign the code will fail to compile (MyFunction() = i is not usually a valid expression).

Upvotes: 6

Rup
Rup

Reputation: 34408

It will take the true branch (i.e. call A) if the function return is non-zero. Zero is treated as false so if MyFunction() returns zero it will call B instead.

In practice, yes this is correct for most / all compilers. I think 0=false was just a convention; I wouldn't be surprised if it is now formalised in C99 or later but I don't have a copy handy to refer you to the right section.

Upvotes: 3

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