selectInput
selectInput(inputId, label, choices, selected = NULL, multiple = FALSE,
  selectize = TRUE, width = NULL, size = NULL)
selectizeInput(inputId, ..., options = NULL, width = NULL)Arguments
| inputId | The inputslot that will be used to access the value. | 
|---|---|
| label | Display label for the control, or NULLfor no label. | 
| choices | List of values to select from. If elements of the list are named then that name rather than the value is displayed to the user. | 
| selected | The initially selected value (or multiple values if multiple = TRUE). If not specified then defaults to the first value
for single-select lists and no values for multiple select lists. | 
| multiple | Is selection of multiple items allowed? | 
| selectize | Whether to use selectize.js or not. | 
| width | The width of the input, e.g. '400px', or'100%';
seevalidateCssUnit. | 
| size | Number of items to show in the selection box; a larger number
will result in a taller box. Not compatible with selectize=TRUE.
Normally, whenmultiple=FALSE, a select input will be a drop-down
list, but whensizeis set, it will be a box instead. | 
| ... | Arguments passed to selectInput(). | 
| options | A list of options. See the documentation of selectize.js
for possible options (character option values inside I()will
be treated as literal JavaScript code; seerenderDataTable()for details). | 
Value
- 
A select list control that can be added to a UI definition.
Description
Create a select list that can be used to choose a single or multiple items from a list of values.
Details
By default, selectInput() and selectizeInput() use the
JavaScript library selectize.js
(https://github.com/brianreavis/selectize.js) to instead of the basic
select input element. To use the standard HTML select input element, use
selectInput() with selectize=FALSE.
Note
The selectize input created from selectizeInput() allows
  deletion of the selected option even in a single select input, which will
  return an empty string as its value. This is the default behavior of
  selectize.js. However, the selectize input created from
  selectInput(..., selectize = TRUE) will ignore the empty string
  value when it is a single choice input and the empty string is not in the
  choices argument. This is to keep compatibility with
  selectInput(..., selectize = FALSE).
Examples
selectInput("variable", "Variable:",
            c("Cylinders" = "cyl",
              "Transmission" = "am",
              "Gears" = "gear"))
<div class="form-group shiny-input-container">
  <label class="control-label" for="variable">Variable:</label>
  <div>
    <select id="variable"><option value="cyl" selected>Cylinders</option>
<option value="am">Transmission</option>
<option value="gear">Gears</option></select>
    <script type="application/json" data-for="variable" data-nonempty="">{}</script>
  </div>
</div>
See also
Other input.elements: actionButton,
  actionLink; animationOptions,
  sliderInput;
  checkboxGroupInput;
  checkboxInput; dateInput;
  dateRangeInput; fileInput;
  numericInput; passwordInput;
  radioButtons; submitButton;
  textInput