Association of else block with if block in C

I have a code snippet as shown below. Can someone please explain me why the else block is getting associated with the nested if block? The output of the following snippet is 1. I executed the code but cannot understand why the blocks are getting associated in the following order.

UPDATE: This is a question I found in a company's previous year interview questions. I always use brackets around my if and else blocks. I now understand why the entire block is not getting executed.

Thank You everyone for your responses

int x=1, y=1;
if (y<0)
    if (y>0) x=3;
    else x=5;
printf("%d", x);

With the proper indentation, it becomes clearer:

    int x = 1, y = 1;

    if (y < 0)         // false, so it jumps directly here
        if (y > 0)     //                                |
            x = 3;     //                                |
        else           //                                |
            x = 5;     //                                |
    printf("%d", x);   // <------------------------------+

Adding braces makes it even clearer. The below has exactly the same meaning as your original code:

    int x = 1, y = 1;

    if (y < 0) {
        if (y > 0) {
            x = 3;
        } else {
            x = 5;
        }
    }
    printf("%d", x);

That is, if(condition) <statement> else <statement>. The <statement> can be either a single statement or a {block} statement. Without braces, only the next single statement will be conditionally executed.

The formal C and C++ grammars do not specify how an else is associated. The choice is specified by a rule outside the grammar. C 2018 6.8.4.1 3 says:

> An else is associated with the lexically nearest preceding if that is allowed by the syntax.

C++ 2020 (draft N4849) 8.5.1 [stmt.if] 1 says:

> … In the second form of if statement (the one including else), if the first substatement is also an if statement then that inner if statement shall contain an else part. [In other words, an else has to be associated with the inner if before the outer if can have an else.]

To see the grammar does not specify how an else is associated, we can examine the formal grammar in C 2018 6.8.4:

> selection-statement:<br>
> &nbsp;&nbsp;&nbsp;&nbsp;if ( expression ) statement<br>
> &nbsp;&nbsp;&nbsp;&nbsp;if ( expression ) statement else statement<br>
> &nbsp;&nbsp;&nbsp;&nbsp;switch ( expression ) statement

With this grammar, if(y<0) if(y>0) x=3; else x=5; may be parsed as if(y>0) x=3; else x=5; being one statement that is “if ( expression ) statement else statement”, or it may be parsed with if(y>0) x=3; being one statement that is if ( expression ) statement. In the former case, the parent if(y>0) would have the if form without an else. In the latter case, the parent if(y>0) would have the if … else form.

The if statements is defined the following way (for simplicity I am using the C grammar)

selection-statement:
    if ( expression ) statement
    if ( expression ) statement else statement

As you can see, the else part can not exist as a separate statement without the if part.

So, according to the definition, this if statement:

if(y<0) ...

can be rewritten like:

if(y<0) statement;

The statement can be any statement, including another if statement:

if(y<0) if(y>0) x=3; else x=5;

That is, the else part belongs to the closest if part.

Otherwise, you could write:

if(y<0) 
{ 
    if(y>0) x=3; 
}
else x=5;

In this case, the statement is the compound statement with the enclosed if statement without the else part.

{ 
    if(y>0) x=3; 
}

As if you are asking for logical explanation why things they are let's look into this code:

if( condition1 ) 
    if( condition2 ) x = 1;
    else x = 2;
else x = 3;

if it would work as you suggested or expected then commenting out line else x = 3; would make statement else x = 2; magically jump from if( condition2 ) to if( condition1 ) and where else x = 2 belongs to would depend of existence of else x = 3; which is a very bad practice. But if you are in doubt use {} to express what you need clearly and unambiguous way:

if( condition1 ) { 
    if( condition2 ) x = 1;
}
else x = 2;

The rules of associativity for if..else in C and C++ (and several other languages) are designed to support if..else if chains without excessive (visual) nesting:

if(a) handle_a();
else if(b) handle_b();
else if(c) handle_c();
else handle_other();

which is actually:

if(a) {
    handle_a();
} else {
    if(b) {
        handle_b();
    } else {
        if(c) {
            handle_c();
        } else {
            handle_other();
        }
    }
}

It's not too hard to design a syntax in which the less-indented version works properly, simply by special casing else if (as many other languages do, generally by having a special elseif keyword), but C accomplishes this behavior more elegantly, by introducing a rule that an else becomes the else for the closest preceding if. That leads to a specific behavior of if(a); if (b); else c(); which isn't particularly useful or obvious, but it keeps the rules simple(r).