Best way to render a map in Grails

Question 1

Assuming that you have already validated that the columns and header lists have the same number of elements, you could do something like this...

<html>
    <head>
        <meta name="layout" content="main">
        <title>Parsed Map</title>
    </head>
    <body>
        <table>
            <g:each var="heading" in="${headings}" status="counter">
                <tr>
                    <th>${heading}</th>
                    <td>${values[counter]}</td>
                </tr>
            </g:each>
        </table>
    </body>
</html>

Question 2

I assume your example is a simplified version of your real problem. Depending on what you are really trying to do, you have a number of options. A simple thing might be like this (organize the table however you like, but this should give you the idea...):

// grails-app/controllers/com/demo/DemoController.groovy
package com.demo

class DemoController {

    def index() {
        // presumably you got this Map from somewhere and it isn't hardcoded here
        def someMap = [0:['A', 'B', 'C', 'D'], 1:['a', 'b', 'c', 'd']]

        def headings = someMap[0]
        def values = someMap[1]

        [headings: headings, values: values]
    }
}


// grails-app/views/demo/index.gsp
<html>
<head>
    <meta name="layout" content="main">
    <title>Parsed Map</title>
</head>
<body>
    <table>
        <tr>
            <g:each var="heading" in="${headings}">
                <th>${heading}</th>
            </g:each>
        </tr>
        <tr>
            <g:each var="value" in="${values}">
                <th>${value}</th>
            </g:each>
        </tr>
    </table>
</body>
</html>