how to check a field has value ?

[adrianmak]

there is a optional image field of a template.

If I put this code in a template, for page without uploaded an image , it will throw a php error of

"Call to a member function size() on a non-object"

$header_image = $page->img->size(800, 250)->url;

[kongondo]

If field can contain only one value or a string...

if($page->title) echo $page->title;

If field can contain multiple values, e.g. a multiple image field or multiple page field (i.e. contain arrays)

if(count($page->images)) echo "we have images. Let's foreach them";

https://processwire.com/talk/topic/2239-how-to-test-for-empty-fields/

https://processwire.com/talk/topic/546-how-to-detect-an-empty-field/

https://processwire.com/talk/topic/8067-cant-detect-empty-field/

https://processwire.com/talk/topic/957-how-to-find-elements-with-empty-field/

https://processwire.com/talk/topic/8850-check-if-a-datetime-field-is-empty-does-not-work/

https://processwire.com/talk/topic/4177-checking-for-empty-textarea-field/

https://processwire.com/talk/topic/3050-checking-for-a-field-if-else/

etc....

[Juergen]

Why dont you check the image with an if statement?

if($page->img){
$header_image = $page->img->size(800,250)->URL;
}

Or am I misunderstand your question?

[adrianmak]

Why dont you check the image with an if statement?

if($page->img){
$header_image = $page->img->size(800,250)->URL;
}

Or am I misunderstand your question?

I tried to use isset for checking, but it's not working.

No more php error for image field without an image, however it returned no value even though there is an image in the field

[kongondo]

I suggest you carefully study and bookmark/reference this table. It is an invaluable reference that will teach/remind you when to use/not use isset, empty, is_null, etc..

PHP type comparison tables

http://php.net/manual/en/types.comparisons.php

As for you image, I think I already answered you question but I'll repeat it here:

If it is a single image field  [an image field with a maximum files allowed of 1]

if($page-img) {}

if it is a multiple image field  [an image filed with maximum files allowed of 0 or > 1, even if it contains only one image in your page]

if(count($page->img)){}

[adrian]

When you have a field that is returning an array, like an images field that allows more than 1 file, you want this:

if(count($page->img)) {

[Raymond Geerts]

On selector level you can select the pages where the image field is not empty asfollow.

$pages_with_images = $pages->select("your_image_field!='', limit=25");

[tpr]

Is there any performance difference between these?

if(count($page->img)){}

if($page->img->count()){}

Lastly, is it safe to use "count" only and not "count()"? 

if($page->img->count){}

[LostKobrakai]

They are essentially the same. The latter one is just a convenience function which does also just use php's count(). 

[tpr]

Thanks! So I should better use count($page->img) as it skips one - minor - step?