Indexing operations on arrays, slices or strings should use an index at most one less than the length. If the index to be accessed is checked for being less than or equal to the length (<=), instead of less than the length (<), the index could be out of bounds.

Use less than (<) rather than less than or equals (<=) when comparing a potential index against a length. For loops that iterate over every element, a better solution is to use a range loop instead of looping over explicit indexes.

The following example shows a method which checks whether a value appears in a comma-separated list of values:

A loop using an index variable i is used to iterate over the elements in the comma-separated list. However, the terminating condition of the loop is incorrectly specified as i <= len(values). This condition holds when i is equal to len(values), but the access values[i] in the body of the loop will be out of bounds in this case.

One potential solution would be to replace i <= len(values) with i < len(values). A better solution is to use a range loop instead, which avoids the need for explicitly manipulating the index variable:

The Go Programming Language Specification: For statements.

The Go Programming Language Specification: Index expressions.