If x is a list, x#?i tells whether there is an ith element of x. The entries of the list are numbered starting with 0. If i is negative, then the entries are numbered ending with -1.
i1 : L = {a, b, c, b, a};
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i2 : L#?2 o2 = true |
i3 : L#?12 o3 = false |
If x is a hash table or database, x#?i tells whether there is a value associated with the key i.
i4 : T = new HashTable from {a => 103, b => 89.4, c => 92};
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i5 : T#?a o5 = true |
i6 : T#?A o6 = false |
If x is a string, x#?i tells if x has an ith character.
i7 : s = "a perfectly adequate example of a string"; |
i8 : s#?2 o8 = true |
i9 : s#?52 o9 = false |
#? can be very useful in avoiding errors from attempting to access nonexistent elements of lists or hash tables.