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No Such Key

rascal-0.41.2

Synopsis​

A map does not contain a requested key.

Types​

data RuntimeException = NoSuchKey(value v);

Usage​

import Exception; (only needed when NoSuchKey is used in catch)

Description​

Rascal provides many operations and functions on maps, see map values and map functions. This error is generated when a function or operation cannot find a requested key value in a map.

Remedies:

Examples​

Import the Map and IO libraries and introduce map M:

rascal>import Map;
ok
rascal>import IO;
ok
rascal>M = ("a" : 1, "b" : 2);
map[str, int]: ("a":1,"b":2)

Indexing M with a non-existing key gives an error:

rascal>M["c"]
|prompt:///|(2,3,<1,2>,<1,5>): NoSuchKey("c")
    at $(|prompt:///|(0,23,<1,0>,<1,23>))

Use the postfix isDefined operator ? to test whether the value is defined:

rascal>if (M["c"]?) {
|1 >>>>  println("defined"); 
|2 >>>>} else {
|3 >>>>  println("not defined");
|4 >>>>}
not defined
ok

Or use the binary ifDefinedElse operator ? to return an alternative value when the value of M["c"] is undefined:

rascal>M["c"] ? 3
int: 3

Yet another solution is to use try/catch. First we import the Rascal exceptions (which are also included in Prelude):

rascal>import Exception;
ok
rascal>try 
|1 >>>>  println(M["c"]);
|2 >>>>catch NoSuchKey(k): 
|3 >>>>  println("Key <k> does not exist");
Key c does not exist
ok