Integer Division

Fortran distinguishes between floating point and integer arithmetic. It is important to note that division for integers is always using integer arithmetic. Furthermore, while Fortran uses the standard order-of-operations (e.g. multiplication and division preceed addition and subtraction, in the absence of parenthesis), operations of the same precedence are evaluated from left to right. Consider the following example for integer division of an odd number:

integer :: n
n = 3
print *, n / 2  ! prints 1
print *, n*(n + 1)/2  ! prints 6
print *, n/2*(n + 1)  ! prints 4 (left-to-right evaluation order)
n = -3
print *, n / 2  ! prints -1

Be careful about whether you want to actually use integer arithmetic in this context. If you want to use floating point arithmetic instead make sure to cast to reals before using the division operator, or separate the integers by multiplying by 1.0_dp:

integer :: n
n = 3
print *, real(n, dp) / 2  ! prints 1.5
print *, n * 1.0_dp / 2  ! prints 1.5
n = -3
print *, real(n, dp) / 2  ! prints -1.5
print *, n * 1.0_dp / 2  ! prints -1.5