Can a float value be used as the index of an array? What will happen if an expression used as an index resulted to a float value?
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A very bad idea because of rounding errors. However, sometimes this is useful, but then the value should be converted into integer first and bounds-checking (or limiting) be done before accessing. This operation can be useful e.g. for approximating values for a floating-point function using a lookup table.Tronic– Tronic2010-02-23 10:04:23 +00:00Commented Feb 23, 2010 at 10:04
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5 Answers
The float value will be casted to int (it can give warning or error depending on compiler's warning level)
s1 = q[12.2]; // same as q[12]
s2 = q[12.999999]; // same as q[12]
s3 = q[12.1/6.2]; // same as q[1]
Comments
Yes. But it's pointless. The float value will be truncated to an integer.
(You can use std::map<float, T>, however, but most of the time you'll miss the intended values because of inaccuracy.)
2 Comments
Agnel Kurian
As for inaccuracy, I think there should be some way to provide your own compare function where you can include an 'epsilon'... a tolerance of sorts.
akaltar
And of course you could also use unordered_map for more performance if the order doesn't matter. In this case you have to include epsilon in the hash function.
A C++ array is a contiguous sequence of memory locations. a[x] means "the xth memory location after one pointed to by a."
What would it mean to access the 12.4th object in a sequence?
1 Comment
Crashworks
Note to pedants: yes, I know array offsets are not a simple addition of integers to pointers: this sentence assumes that a "memory location" is the size of an array member, so for an int a[], a[5] means the fifth word, not the fifth byte.
It will be casted to int.
Comments
This is an error. In [expr.sub]:
A postfix expression followed by an expression in square brackets is a postfix expression. One of the expressions shall have the type “pointer to T” and the other shall have unscoped enumeration or integral type.
I am not aware of a clause in the standard that specifies that conversion should happen here (admittedly, I would not be surprised if such a clause existed), although testing with ideone.com did produce a compilation error.
However, if you're subscripting a class rather than a pointer — e.g. std::vector or std::array — then the overload of operator[] will have the usual semantics of a function call, and floating-point arguments will get converted to the corresponding size_type.