In the previous few sections, we’ve learned how to declare and initialize dynamic arrays. However, what if we need to assign values to individual array elements, one by one, in a loop? This will be the case as we load the data from CSV files into arrays–we’ll iterate over records in files, and assign values to array elements one at a time. However, we don’t really have a way to initialize the data arrays like we did with stock_symbols in listing 5.3. Note that implicitly allocating by assigning an empty array [integer ::] or [real ::] won’t work here because we may need to index elements of an array in some order other than just appending values. This calls for a more explicit mechanism to allocate the array without assigning known values to it:
real, allocatable :: a(:) ❶
integer :: im = 5
allocate(a(im)) ❷❶ Declares a real, dynamic array a
❷ Allocates the array a with size im
This code tells the program to reserve memory for the array a of size im, in this case 5. When invoked like this, a will, by default, have a lower bound of 1, and an upper bound of im. The lower bound of 1 is the default, similar to what you’ll find in Julia, R, or MATLAB. This is unlike C, C++, Python, or JavaScript, where array or list indices begin with 0.
However, Fortran doesn’t impose a constraint to the start index being 1, unlike Python, where the first index is always 0. You can specify the lower and upper bounds in the allocation statement:
integer :: is = -5, ie = 10
allocate(a(is:ie)) ❶❶ Allocates the array a with a range from is to ie
Notice that I used a colon (:) between is and ie to specify the range. This range is inclusive (unlike in Python!), so the size of a is now ie - is + 1–in this case 16.
You can use the built-in functions lbound and ubound to get the lower and upper bound, respectively, of any array.