The LOAN program would have been more complicated if it had not taken advantage of some implicit rules of Fortran concerning data types: this requires a little more explanation.
Computers can store numbers in several different ways: the most common numerical data types are those called integer and real. Integer variables store numbers exactly and are mainly used to count discrete objects. Real variables are useful many other circumstances as they store numbers using a floating-point representation which can handle numbers with a fractional part as well as whole numbers. The disadvantage of the real data type is that floating-point numbers are not stored exactly: typically only the first six or seven decimal digits will be correct. It is important to select the correct type for every data item in the program. In the last example, the number of years was an integer, but all of the other variables were of real type.
The data type of a constant is always evident from its form: character constants, for example, are enclosed in a pair of apostrophes. In numerical constants the presence of a decimal point indicates that they are real and not integer constants: this is why the value one was represented as ``1.0" and not just ``1".
There are several ways to specify the data type of a variable. One is to use explicit type statements at the beginning of the program. For example, the previous program could have begun like this:
PROGRAM LOAN
INTEGER NYEARS
REAL AMOUNT, PCRATE, RATE, REPAY
Although many programming languages require declarations of this sort for every symbolic name used in the program, Fortran does not. Depending on your point of view, this makes Fortran programs easier to write, or allows Fortran programmers to become lazy. The reason that these declarations can often be omitted in Fortran is that, in the absence of an explicit declaration, the data type of any item is determined by the first letter of its name. The general rule is:
In the preceding program, because the period of the loan was called NYEARS (and not simply YEARS) it automatically became an integer, while all the other variables were of real type.