All structured programming languages need constructs which provide a facility for conditional execution. The simplest method of achieving this functionality is by using a combination of IF and GOTO which is exactly what FORTRAN 66 supported. Fortran has progressed since then and now includes a comprehensive basket of useful control constructs. Fortran 90 supports:
These are the basic conditional execution units. They are useful if a section of code needs to be executed depending on a series of logical conditions being satisfied. If the first condition in the IF statement is evaluated to be true then the code between the IF and the next ELSE, ELSEIF or ENDIF is executed. If the predicate is false then the second branch of the construct is entered. This branch could be either null, an ELSE or an ELSEIF corresponding to no action, the default action or another evaluation of a different predicate with execution being dependent upon the result of the current logical expression. Each IF statement has at least one branch and at most one ELSE branch and may contain any number of ELSEIF branches. Very complex control structures can be built up using multiply nested IF constructs.
Before using an IF statement, a programmer should be convinced that a SELECT CASE block or a WHERE assignment block would not be more appropriate.
This is the basic form of iteration mechanism. This structure allows the body of the loop to be executed a number of times. The number of iterations can be a constant, a variable (expression) or can be dependent on a particular condition being satisfied. DO loops can be nested.
A particular branch is selected depending upon the value of the case expression. Due to the nature of this construct it very often works out (computationally) cheaper than an IF block with equivalent functionality. This is because in a SELECT CASE block a single control expression is evaluated once and then its (single) result is compared with each branch. With an IF .. ELSEIF block a different control expression must be evaluated at each branch. Even if all control expressions in an IF construct were the same and were simply compared with different values, the general case would dictate that the SELECT CASE block were more efficient.
Direct jump to a labelled line. This is a very powerful statement, it is very useful and very open to abuse. Unstructured jumps can make a program virtually impossible to follow, the GOTO must be used with care. It is particularly useful for handling exceptions, that is to say, when emergency action is needed to be taken due to the discovery of an unexpected error.
This is a slightly oddball feature of Fortran in the sense that there is currently no other form of exception handling in the language. (The feature originated from FORTRAN 77.) It is possible to add qualifiers to I/O statements to specify a jump in control should there be an unexpected I/O data error, end of record or should the end of a file be encountered.
It is always good practice to use at least the ERR= qualifier in I/O statements.
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