how to use a variable as a field name?

[admin]

I made this function:


function alertvalue(number){
alert(document.formname.number.value);
}


when i call the function: alertvalue("field6")
and i have <input name="field6" value=http://www.webdeveloper.com/forum/archive/index.php/"right"> somewhere in my document, it SHOULD alert "right"

BUT that doesnt work, it tries to alert the value of the field named "number" instead of "field6"

so how can i use the variable "number" in the field name?
something needs to be changed in "alert(document.formname.number.value);", but I dont know how to do it, could anyone help me plz?