# Transform map keys > Supported in: Batch, Streaming Transforms keys of a map by applying an expression to every key-value pair. **Expression categories:** Map ## Declared arguments * **Expression to apply:** The expression to apply once per key-value pair of the map.
*Expression\* * **Map:** Map expression.
*Expression\>* **Type variable bounds:** *K accepts AnyType\*\*V accepts AnyType* **Output type:** *Map\* ## Examples ### Example 1: Base case **Argument values:** * **Expression to apply:**
stringBeforeDelimiter(
delimiter: -,
expression: `key`,
ignoreCase: false,
) * **Map:** `flight_number` | flight\_number | **Output** | | ----- | ----- | | {
MT-111 -> 2,
XB-134 -> 1,
} | {
MT -> 2,
XB -> 1,
} | *** ### Example 2: Base case **Argument values:** * **Expression to apply:**
cast(
expression: `key`,
type: Integer,
) * **Map:** `flight_number` | flight\_number | **Output** | | ----- | ----- | | {
11 -> 1,
22 -> 2,
} | {
11 -> 1,
22 -> 2,
} | *** ### Example 3: Base case **Argument values:** * **Expression to apply:**
cast(
expression: `value`,
type: String,
) * **Map:** `flight_number` | flight\_number | **Output** | | ----- | ----- | | {
11 -> 1,
22 -> 2,
} | {
1 -> 1,
2 -> 2,
} | *** ### Example 4: Base case **Argument values:** * **Expression to apply:**
concatStrings(
expressions: \[
stringBeforeDelimiter(
delimiter: -,
expression: `key`,
ignoreCase: false,
), `value`],
separator: -,
) * **Map:** `flight_number` | flight\_number | **Output** | | ----- | ----- | | {
MT-111 -> BB,
XB-134 -> AA,
} | {
MT-BB -> BB,
XB-AA -> AA,
} | ***