IF statement which allows a
block of statements to be executed conditionally, or allows a choice to be made
between different courses of action.
One obvious defect of the function AREA3 is that has no protection against
incorrect input. Many sets of three real numbers could not possibly form the sides
of a triangle, for example 1.0, 2.0, and 7.0. A little analysis shows that in
all such impossible cases the argument of the square root function will be
negative, which is illegal. Fortran systems should detect errors like this at
run-time but will vary in their response. Even so, a message like "negative
argument for square-root" may not be enough to suggest to the user what
is wrong. The next version of the function is slightly more user-friendly:
REAL FUNCTION AREA3(A, B, C) *Computes the area of a triangle from lengths of its sides. *If arguments are invalid issues error message and returns zero. REAL A, B, C S = (A + B + C)/2.0 FACTOR = S * (S-A) * (S-B) * (S-C) IF(FACTOR .LE. 0.0) THEN WRITE(UNIT=*, FMT=*)'Impossible triangle', A, B, C AREA3 = 0.0 ELSE AREA3 = SQRT(FACTOR) END IF END |
IF statement works with the ELSE and END IF statements to enclose two blocks
of code. The statements in the first block are only executed if the expression in the
IF statement is true, those in the second block only if it is false. The statements in
each block are indented for visibility, but this is, again, just a sensible programming
practice.
With this modification, the value of FACTOR is tested and if it is negative or zero
then an error message is produced; AREA3 is also set to an impossible value (zero) to
flag the mistake. Note that the form ".LE." is used because the less-than-or-equals
character, "<", is not present in the Fortran character set. If S is positive the
calculation proceeds as before.
