apex:variable and apex:pageBlockTable example

ShowDataVfPage

<apex:page standardController="obj__c" recordSetVar="records">
    <apex:form>
        <apex:pageblock title="Records">
            <apex:variable var="count" value="{!1}"/>
            <apex:pageblockTable value="{!records}" var="rec">                
                <apex:column headerValue="S.No">
                    {!count}
                    <apex:variable value="{!count+1}" var="count"/>
                </apex:column>
                <apex:column value="{!rec.A__c}"/>
                <apex:column value="{!rec.B__c}"/>
                <apex:column value="{!rec.C__c}"/>
            </apex:pageblockTable>
        </apex:pageblock>
    </apex:form>
</apex:page>