| Re: declare variables [message #45939 is a reply to message #45934] |
Tue, 18 October 2005 14:27   |
James Kuyper
Messages: 425
Registered: March 2000
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Senior Member |
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qian wrote:
> Hi,
>
> I am using FORTRAN and IDL at the same time, so sometime I just assume
> some FORTRAN rules when using IDL. I just find out that the variable
> type can change within an IDL program, even you declare it to be some
> specific type. For example:
You don't declare an IDL variable to have a given type. Whenever you
assign a value to an IDL variable, that variable automatically acquires
the data type of whatever it is you assigned into it.
> data=dblarr(2)
This isn't a declaration; it's an assignment statement like any other.
Since dblarr(2) is a 2-element array of doubles, that's the type of
'data'.
> data=[2.3, 3.4]
Since [2.3, 3.4] is a 2-element array of floats, that's the new type of
'data'.
Note: if you type data[0] = 2.3, then 2.3 would have been converted to
a double before storing it in data[0]. A variable's type only changes
when you assign a value to the entire variable, not when you assign to
a single element of an array.
...
> But if I read in data from other files, in which '1' actually means
> '1.0', is it possible I can re-inforce the array to be certain type (or
> let the variables to be certain type throughtout the whole program)? So
> even if I type d=1, it is still a real number?
You don't have to worry about that; it doesn't work that way. When
you're reading data from a text file using commands like:
READF, infile, myarray
Then myarray must already exist, which means it already has a specific
type, size, and array shape. The interpretatation of the text file is
controlled by the data type and size of myarray. If myarray is
currently an array of 26 integers, the above command will try to
interpret the file as containing 26 integers (and it will complain if
that interpretation doesn't work). If myarray is a 4x4 array of double
precision numbers, it will attempt to interpret the file as containing
16 floating point numbers.
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