Some confusion with regards to parenthesis in C

Question

Please bear with me, I am trying to learn C as my first programming language and am only 15 minutes in.

Why must parenthesis be used here:

while ((number1 = number2))

...when they do not need to be used here?

while (number1 <= number2)

Thanks in advance.

Possible duplicate of [Why would you use an assignment in a condition?](https://stackoverflow.com/questions/151850/why-would-you-use-an-assignment-in-a-condition) — Ben Harris, Jan 26 '18 at 18:46
@BenHarris what does that link has to do with the question? Anyway that post is not even tagged `C` — machine_1, Jan 26 '18 at 18:48
They should only be used to avoid a *warning*. Treating warnings as errors in C is a great idea, but it's not obligatory. — , Jan 26 '18 at 18:48

score 2 · Accepted Answer · answered Jan 26 '18 at 18:47

2

In:

while (number1 = number2)

number2 is being assigned to number1.

This is optically very similar to comparing number1 and number2, i.e.:

while (number1 == number2)

So a warning is produced in the former case. In order to suppress that warning you need to place the parentheses around the assignment, i.e.:

while ((number1 = number2))

answered Jan 26 '18 at 18:47

JFMR

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Okay, I see now - I was using the assignment operator rather than the equal to operator. Thanks for the help. – CCss1 Jan 26 '18 at 18:51

John Bode · Answer 2 · 2018-01-26T21:27:49.580

It is a very common mistake to write

if ( a = b ) // assign b to a, branch if result is non-zero

when you meant to write

if ( a == b ) // *compare* b to a, branch if equal

leading to all kinds of mayhem.

It's such a common mistake that most compilers will issue a warning if they see an assignment in a conditional like that. To tell the compiler, "no, I really know what I'm doing", you surround the assignment in an additional set of parentheses:

if ( ( a = b ) )

This basically means, "yes, I intend to assign b to a and branch on the result, shut up."

If I could time travel back to Bell Labs in 1970, this is one of several decisions I'd slap Ritchie over. An entire class of bugs would never have existed if the assignment operator had been := or something equally dissimilar to comparison.

Super helpful, nice to learn the difference between assignment and comparison early on - I'll probably never make the same mistake consciously ever again. - Cheers. Upvoted, just cannot be displayed publicly due to my current reputation. — CCss1, Jan 26 '18 at 19:04

user2736738 · Answer 3 · 2018-01-26T19:01:44.770

1

Who said there is a must? You can omit it too.

while (number1 = number2)

yes this would generate compiler warning because you are assigning and checking the assigned value without parentheses. But this is legal and bad practice.

So the reason they did this is to avoid the warning about which the compiler complained. (You shouldn't skip warning to follow a bad practice - rather allow compiler warnings and try to solve them diligently).

Though as you are beginner - aware of the statement, here the while statement is basically

  while(number2)

Point: The most common to use would be to while(number1 == number2).

Also does the same thing as the previous one. Who said in the second one you can't? You can.

while ((number1 <= number2))

edited Jan 26 '18 at 19:01

answered Jan 26 '18 at 18:45

user2736738

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ANSI C does not allow that construction by default. – Ben Harris Jan 26 '18 at 18:46
@BenHarris.: Which one? Both are valid in C99 and C11 – user2736738 Jan 26 '18 at 18:47
Geany states: fdsfs.c:11:9: warning: suggest parentheses around assignment used as truth value [-Wparentheses] – CCss1 Jan 26 '18 at 18:47
@BenHarris.: Yes I edited but yes it's still legal but it;s bad practice. – user2736738 Jan 26 '18 at 18:50
Very very bad. See https://freedom-to-tinker.com/2013/10/09/the-linux-backdoor-attempt-of-2003/ for an example. – nemequ Jan 26 '18 at 18:51
@nemequ.: Bad enough to make it bold and forbid OP to not use it. But stil l it's legal C. – user2736738 Jan 26 '18 at 18:51
That was very insightful - thanks for your help coderredoc. I will "accept answer" both of these solutions in 5 minutes. – CCss1 Jan 26 '18 at 18:53
@BenHarris: are you sure? Both constructions seems legal under C89 – David Ranieri Jan 26 '18 at 18:59
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@KeineLust My mistake. It's a warning and not an error. – Ben Harris Jan 26 '18 at 18:59

score 0 · Answer 4 · answered Jan 26 '18 at 18:56

This is a matter of opinion. Many programmers will still wrap a comparison in parentheses for clarity. That's to make them more legible. For instance:

while(i == 0)   //comparison
while(i = 0) //assignment (and therefore, infinite loop)

but more commonly, because chains of conditions can become very hard to read, and getting into the habit of parenthesizing your comparisons makes complex comparisons easier to understand:

    //whiskey tango foxtrot
    while(i == 0 || j == i && k == 5 || f == "pancakes")
    //oh, now it's clear that we're not checking "i && k". Easier to read! 
    while((i == 0) || (j == i) && (k == 5) || (f == "pancakes"))
    //now I've guaranteed i and j both have the value "0" by using parentheses, 
    //or we get false; but everything is still clearly separated.  
    while(((i == 0) || (j == i)) && (k == 5) || (f == "pancakes"))

score 0 · Answer 5 · answered Jan 26 '18 at 18:57

First off, the first example is extremely bad practice. It's not comparing number1 and number2, it's setting the value of number1 to the value of number2. See https://freedom-to-tinker.com/2013/10/09/the-linux-backdoor-attempt-of-2003/ for some more information.

That said, both forms are allowed; you can always add extra parenthesis. In the case you have above, there is no difference.

That said, odds are good that you're simplifying from an example where multiple conditions are chained together, like

while ((number1 == number2) || (number3 == number4))

As you can see, it makes it a bit easier to see what the code is doing. Technically it's not really necessary here, either (though it's generally considered to be good practice), but it can be necessary for more complicated expressions because different expectations about operator precedence can result in your code doing things you might not expect. To take an example from that page, for something like

e = a < d ? a++ : a = d;

It can take a few moments even for an expect to figure out what is going on, and someone less familiar with C will probably need much longer. On the other hand, if you add parenthesis:

e = ( ((a < d) ? (a++) : a) = d );

Things become much easier to read.

Some confusion with regards to parenthesis in C

5 Answers5