Arithmetic in Fortran: Real and Integer Variables

Fortran has several different types of variables. The two most commonly used types for computations are integer variables and real variables.

• Integer variables do not have a fractional part. The value of an integer variable is a whole number, like 0, 2, 5, -76, and so on.

• Real variables have a fractional part, which may be zero. These variables have values like 2.0 or 3.1416. Real variables are sometimes called floating-point variables.

Important! For an integer variable, the fractional part is not merely zero. The fractional part does not exist:

• 2 is an integer value

• 2.0 is a real value

The type of each variable should be declared at the beginning of the program, right after the program is named. The type declarations have to appear before the first executable statement. Additionally, your variable names should adhere to the following conventions:

• The variable name may include both letters and digits, but the first character of the variable name has to be a letter. For example, A2B is an allowable name, but 2AB is not.

• In standard Fortran 77 each variable name has to be six characters or less, but almost all modern systems will accept longer variable names.

Following is a program that illustrates some of the differences between real and integer variables:

      PROGRAM MAIN
      INTEGER I, INTPI, INTE
      REAL X, REALPI, REALE
      I = 1
      X = 1
      PRINT *, 'THE VALUE OF 1 AS A REAL IS', X
      PRINT *, 'THE VALUE OF 1 AS AN INTEGER IS', I
      REALPI = 3.1416
      INTPI = REALPI
      PRINT *, 'THE VALUE OF PI AS A REAL IS', REALPI
      PRINT *, 'THE VALUE OF PI AS AN INTEGER IS', INTPI
      REALE = 2.71828
      INTE = REALE
      PRINT *, 'THE VALUE OF E AS A REAL IS', REALE
      PRINT *, 'THE VALUE OF E AS AN INTEGER IS', INTE
      STOP
      END

Notice that the PRINT statement is a little different from the one in the previous exercise. Any text that you want to print out is inside single quotes, but when printing the value of a variable you don't put it in quotes. If you want to print multiple things (whether text or values) separate them with a comma.

Assignment:

(1) Type the program as listed above, compile it and run it.

(2) What happens to the value of pi when it gets converted to an integer?

(3) What happens to the value of e when it gets converted to an integer? Look closely: Is the result rounded to the nearest integer?

(4) Change the values of pi and e to negative numbers (that is, -3.1416 and -2.71828), then re-compile and re-run the program. What happens when the negative values are converted to integers?

Next: Arithmetic operators

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