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6.4 Mixed-Mode Arithmetic

Mixing integers and floating-point numbers in a basic arithmetic operation converts the integers automatically to floating point. In most cases, this gives exactly the desired results. But sometimes it matters precisely where the conversion occurs.

If i and j are integers, (i + j) * 2.0 adds them as an integer, then converts the sum to floating point for the multiplication. If the addition causes an overflow, that is not equivalent to converting each integer to floating point and then adding the two floating point numbers. You can get the latter result by explicitly converting the integers, as in ((double) i + (double) j) * 2.0. See Explicit Type Conversion.

Adding or multiplying several values, including some integers and some floating point, performs the operations left to right. Thus, 3.0 + i + j converts i to floating point, then adds 3.0, then converts j to floating point and adds that. You can specify a different order using parentheses: 3.0 + (i + j) adds i and j first and then adds that sum (converted to floating point) to 3.0. In this respect, C differs from other languages, such as Fortran.