Looping through the listing of blog posts
<b:loop values='data:posts' var='post'>
Inside the loop data:posts changes to data.post (determined by the value of var, set here as var='post')
We can reference the id of the current post in the loop as <data:post.id />
</b:loop>
Looping through the listing of blog posts with an additional index
<b:loop values='data:posts' var='post' index='i'>
The id of the current post in the look is
<data:post.id />
The position of this post in the loop index is
<data:i />
We can run a IF statement against this value like so:
<b:if cond='data:i == 2'>
This is the post contained in the loop at position 2, which in an array is Post #3 (arrays start counting at 0)
</b:if>
</b:loop>
To do a greater-than and less-than you need to use the HTML code for greater/less, otherwise it will throw a Blogger template error.
<b:if cond='data:i <=2'> for i is less than or equal to
<b:if cond='data:i >=2'> for i is greater than or equal to
"is first" variables
There are a number of "is first" variables in Blogger that are unfortunately not declared on the
official variable list.
data:post.isFirstPost
This variable can be used to show a different behavior on an index page for the first post, by placing it within the data:posts loop, like so:
<b:loop values='data:posts' var='post'>
<b:if cond='data:post.isFirstPost == "true"'>
first psot!
</b:if>
</b:loop>
data:post.isDateStart
This variable is used to group posts by date, often found on a Blogger site's index pages, where it shows all posts under one day/date, then moves to the next day/date without repeating that value.
data:post.firstImageUrl
This variable shows the URL of the first image used in the blog post. This is often handy for creating an index page of image thumbnails. But while the "thumbnail" variable (data:post.thumbnailUrl) only shows a 72 pixel square and
only works for Google hosted images, this firstImageUrl variable shows the full image url and can be used for non-Google-hosted images.
data:post.sharePostUrl
This variable displays the absolute url for a blog post in question. Used in the social media share buttons, but can certainly be used in other cases.
data:post.snippet
This variable displays the first 140 characters of a blog post, or what comes before a jump break, whichever is first.
Example, albeit not for this usage:
<b:if cond='data:blog.metaDescription != ""'>
<meta expr:content='data:blog.metaDescription' name='og:description'/>
<b:else/>
<meta expr:content='data:post.snippet' property='og:description'/>
</b:if>
<meta expr:content='data:post.thumbnailUrl' itemprop='image_url'/>
The above code will show the first image URL as the snippet's thumbnail, along with either the post's meta description (search description in the blog post) or, if none, the 140 character snippet.
Social sharing thumbnail variable that you need
We all wanted to add a larger thumbnail image for social sharing, for example for Facebook and now Google+, but had no way to do so since the other thumbnail was only 72 pixels. Yes, there is now a new variable...
data:blog.postImageUrl
This variable, available to you in the HEAD of your template, if used on item page types, will display the first image from your blog post in all of its glory.
This article is quoted form Loops, loop positions, and isFirst variables in Blogger
