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Index Out Of Bounds

rascal-0.41.2

Synopsis​

Index is out of bounds.

Types​

data RuntimeException = IndexOutOfBounds(int index)

Usage​

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

Description​

Subscription is possible on various ordered types, including list, tuple, and node. This error is generated when a subscript is out of bounds for the value that is being subscripted.

Remedies:

  • Guard the subscription with a test that the index is within bounds.
  • Make your code less dependent on index values. Suggestions:
    • Use the index to produce all legal indices of a list. Instead of for(int i <- [0..size(L)]) { ... } use for(int i <- index(L)) { ... }.
    • Use a list slice to automate part of the index computation.
  • Catch the IndexOutOfBounds yourself, see try catch.

Examples​

Initialize a list L:

rascal>L = [0, 10, 20, 30, 40];
list[int]: [0,10,20,30,40]

The legal indices are 0, 1, 2, 3, 4, so index 5 gives an error:

rascal>L[5];
|prompt:///|(2,1,<1,2>,<1,3>): IndexOutOfBounds(5)
    at $(|prompt:///|(0,5,<1,0>,<1,5>))

We can catch the IndexOutOfBounds error. First import the Rascal exceptions (which are also included in Prelude) and import IO for Println:

rascal>import Exception;
ok
rascal>import IO;
ok
rascal>try 
|1 >>>>  L[5]; 
|2 >>>>catch IndexOutOfBounds(msg):
|3 >>>>  println("The message is: <msg>");
The message is: 5
ok