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I.e., e and ee look identical when printed, but one is a call that generates an expression object and the other is the object itself.

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It is possible for a function to find out how it has been called by looking at the result of as in the following example of a function that simply returns its own call:

However, this is not really useful except for debugging because it requires the function to keep track of argument matching in order to interpret the call. For instance, it must be able to see that the 2nd actual argument gets matched to the first formal one ( x in the above example).

More often one requires the call with all actual arguments bound to the corresponding formals. To this end, the function is used. Here’s a variant of the preceding example, a function that returns its own call with arguments matched

Notice that the second argument now gets matched to x and appears in the corresponding position in the result.

The primary use of this technique is to call another function with the same arguments, possibly deleting some and adding others. A typical application is seen at the start of the lm function:

Notice that the resulting call is evaluated in the parent frame, in which one can be certain that the involved expressions make sense. The call can be treated as a list object where the first element is the name of the function and the remaining elements are the actual argument expressions, with the corresponding formal argument names as tags. Thus, the technique to eliminate undesired arguments is to assign NULL , as seen in lines 2 and 3, and to add an argument one uses tagged list assignment (here to pass drop.unused.levels = TRUE ) as in line 4. To change the name of the function called, assign to the first element of the list and make sure that the value is a name, either using the"model.frame") construction here or quote(model.frame) .

The function has an expand.dots argument which is a switch which if set to FALSE lets all ‘ ... ’ arguments be collected as a single argument with the tag ‘ ... ’.

The ‘ ... ’ argument is a list (a pairlist to be precise), not a call to list like it is in S:

One reason for using this form of is simply to get rid of any ‘ ... ’ arguments in order not to be passing unspecified arguments on to functions that may not know them. Here’s an example paraphrased from plot.formula :


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study of 18 leading mobile sites, and our most recent large-scale eye-tracking study of checkouts, have all confirmed the same thing: checkout processes and long sign-up forms need to mark both the required and optional fields explicitly.

On the sites that didn’t explicitly denote both field types (i.e. optional and required) the test subjects spent longer time filling out the fields and more frequently ran into entirely preventable “Field is required” validation errors. In fact, when testing mobile checkouts, 75% of the test subjects experienced severe form usability issues on sites that failed to mark both required and optional fields clearly.

explicitly denote

Which fields are required and which are optional? Is ‘Apt. or Suite #’ required? How about ‘Contact Phone’? Some sites require a phone number while other sites don’t – alas, the user has no way of knowing without proper indicators for required and optional fields.

The most common mistake – made by 63% of the top 100 e-commerce checkouts – is to only denote one of the types – i.e. either only denoting required fields or the optional ones (the latter has the most severe usability consequences). Another common issue was observed on checkout steps where all the fields were required (this is often the case at the ‘Payment’ step), where some sites neglect to explicitly mark any of the fields as being required – leading to unnecessary form errors and confusion.

only denoting

The fix is easy: explicitly mark each and every field as optional / required (denoting both types throughout the form). Yet, when benchmarking the top 100 US checkout processes, only 9% of the sites explicitly marked both field types. In this article we’ll therefore share our research findings on why:

only 9%

We’ll end the article with a couple of “best practice” examples from sites that do denote both field types to remove any uncertainty and improve form completion times.

“best practice” examples

Note: the findings shared in this article relate to checkout forms and long account creation forms. Other forms, in particular short sign-up and contact forms, don’t seem to be affected anywhere nearly as much by the issues described and may therefore employ alternative designs.

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