Mixed Type Assignment

When the RHS expression of a mixed type assignment statement has been evaluated it has a specific type, this type must then be converted to fit in with the LHS. This conversion could be either a promotion or a relegation. For example,

• INTEGER = REAL (or DOUBLE PRECISION)

  The RHS needs relegating to be an INTEGER value. The right hand side is evaluated and then the value is truncated (all the decimal places lopped off) then assigned to the LHS.

• REAL (or DOUBLE PRECISION) = INTEGER

  The INTEGER needs promoting to become a REAL. The right hand side expression is simply stored (approximately) in the LHS.

For example, as real values are stored approximately,

    REAL :: a = 1.1, b = 0.1
    INTEGER :: i, j, k
    i = 3.9       ! i will be 3
    j = -0.9      ! j will be 0
    k = a - b     ! k will be 1 or 0
    a = j         ! a will be about 1.0 or 0.0

Notes:

• since i is INTEGER, the value 3.9 must be truncated, integers are always formed by truncating towards zero.
• j (an INTEGER,) would be truncated to 0.
• the result of a - b would be close to 1.0 (it could be 1.0000001 or it could be 0.999999999), thus, due to truncation k could contain either 0 or 1.
• a would be set to (about) 0.000 or 1.000 depending on the value of k.

Care must be taken when mixing types!

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