while loop with variable definition in C-CodePudding

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
}

CodePudding user response：

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
}

CodePudding user response：

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 ) {

CodePudding user response：

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'

CodePudding user response：

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;
}

CodePudding user response：

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))