while loop with variable definition in C

Question

I'm currently trying to loop over a collection retrieved by an API in C.

The API offers a function getNext() that returns a reference to the next element in the collection, or NULL if the end has been reached.

Now i wanted to declare a variable in a while expression and loop until NULL:

// create collection from API
Collection collection = create();

if (collection == NULL) {
  return -1
}

while (const Element *e = getNext(collection) != NULL) {
  // print data from e
}

When compiling, 'error: unexpected expression' pops up. Why does that statement not work in C? How can I loop over the collection when all I have is a function getNext() that returns either an element or NULL?

I also tried the following, but the same error occured:

while ((const Element *e = getNext(collection)) != NULL) {
  // print data from e
}

Ted Lyngmo · Accepted Answer · 2022-01-14 08:56:42Z

6

It is not possible because while() expects an expression and it is not possible to place a declaration in the expression (though it is possible to introduce a new type with expression!).

The best you can get is:

const Element *e;
while ((e = getNext(collection)) != NULL) {
  // print data from e
}

It is however possible to achieve the desired functionlity with a for loop.

for (const Element *e; (e = getNext(collection)) != NULL; ) {
  // print data from e
}

edited Jan 14, 2022 at 8:56

Ted Lyngmo

124k7 gold badges93 silver badges155 bronze badges

answered Jan 14, 2022 at 8:46

tstanisl

14.3k3 gold badges31 silver badges45 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

tstanisl Over a year ago

@TedLyngmo, good observation, there are even more bizarre tricks with for loop. It is even possible to emulate if (type x = expr; x)

pmg Over a year ago

You can add an hanging block to further limit the scope:

int e = 42; { const Element *e; while (e = getNext(collection)) { /* e is a pointer here */ } /* e is a pointer here */ } /* e is int here */

Andreas Wenzel · Accepted Answer · 2022-01-14 09:27:46Z

2

First of all, in contrast to a for statement, you cannot declare a new variable in a while statement. Therefore, the following line is wrong:

while (const Element *e = getNext(collection) != NULL) {

You could fix this by writing the following:

const Element *e;

while ( e = getNext(collection) != NULL ) {

A more compact way of writing this is the following:

for ( const Element *e; e = getNext(collection) != NULL ; ) {

The second version has the advantage that the scope of the variable e is limited to the for loop.

However, the lines mentioned above are also wrong, for the following reason:

The != operator has higher precedence than =. So the line

while ( e = getNext(collection) != NULL) {

is equivalent to:

while ( e = ( getNext(collection) != NULL ) ) {

This is not want you want. You should write the following instead:

while ( ( e = getNext(collection) ) != NULL ) {

edited Jan 14, 2022 at 9:27

answered Jan 14, 2022 at 8:47

Andreas Wenzel

27.1k4 gold badges33 silver badges52 bronze badges

7 Comments

Ted Lyngmo Over a year ago

"You could fix this by writing the following" - { const Element *e; while(...) {} } would be more similar to what OP wants in terms of context (I think).

Andreas Wenzel Over a year ago

@TedLyngmo: I don't understand what you are suggesting. Are you suggesting that I put it everything into an extra code block by adding { and } around everything? That is the only difference I can find between your suggested code and my code.

Ted Lyngmo Over a year ago

Yes, that's exactly it. It gives the variable the same (limited) context as I think OP was aiming for.

Andreas Wenzel Over a year ago

@TedLyngmo: I don't have the impression that OP is concerned with the scope of the variable e. If they are indeed concerned with the scope, it would probably be best to use for, as shown in my answer, instead of using while and adding an additional code block.

Lundin Over a year ago

The solution isn't to spam parenthesis all over but to get rid of the assignment inside the loop condition.

|

einpoklum · Accepted Answer · 2022-01-14 08:46:40Z

0

In C, you can't declare a variable in the condition expression of a while loop.

GCC will tell you:

Error: Expected expression before 'const'

answered Jan 14, 2022 at 8:46

einpoklum

137k86 gold badges445 silver badges917 bronze badges

Comments

user17553224 · Accepted Answer · 2022-01-14 08:54:57Z

0

I would write something like this, hope it can help :)

void main()
{
    bool running = true;
    while(running)
    {
        printf("Running while loop\n");
        running = getNext() != NULL; //Check if there is data in your collection - otherwise exit
    }
    printf("getNext() returned NULL\n");
    return;
}

answered Jan 14, 2022 at 8:54

user17553224

1

Comments

Lundin · Accepted Answer · 2022-01-14 13:14:54Z

0

Some rules of thumb in C programming are: "don't do really strange things" and "never do assignments inside control or loop statements". Turns out that we can follow these rules and solve all problems by using a for loop:

for(const Element* e = getNext(collection); e != NULL; e = getNext(collection))

answered Jan 14, 2022 at 13:14

Lundin

220k47 gold badges282 silver badges446 bronze badges

Collectives™ on Stack Overflow

while loop with variable definition in C

5 Answers 5

2 Comments

7 Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

7 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related