## Scorpio News |
## January–March 1987 – Volume 1. Issue 1. |

Page 19 of 63 |
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Two types of IF UF statement are used in FORTRAN – the arithmetic form and the logical form. The arithmetic IF takes the form:

IF(expression) L1,L2,L3 (L1,L2 and L3 are statement numbers)

If the value of the expression is less than 0, control passes to statement no. 1, if it is equal to 0 then statement no. 2 to and if it is greater than 0, control passes to the last named statement –

IE(N-1) 3,5,7 | – | if the value of N-1 is 0 then control passes to the statement label no. 5 |

The Logical IF statement has an almost exact parallel in BASIC and it takes the form:

IF(logical expr.) statement

The logical expression is evaluated as .TRUE. or .FALSE.; if true, control passes to the statement immediately following otherwise control passes to the next program line. The logical expression can contain both logical and relational operators, which are defined as follows:

.NOT. .AND. .OR, .XOR.

.LT. | less than |

.EQ. | equal to |

.GT. | greater than |

.LE. | less than or equal to |

.NE. | not equal to |

.GE. | greater than or equal to |

Relational operators are self explanatory but Logical operators may need a little clarification.

If A and B are logical expressions, then:

.NOT.A | is the logical opposite of A (0 bits become 1 and vice versa) |

A.AND.B | the value of this expression is the product (logical) of A and B. This implies that the value is .TRUE. if A and B are .TRUE. and .FALSE. otherwise |

A.OR.B | this produces the logical sum of A and B. In this case the value is .FALSE. if A and B are .FALSE. and .TRUE. otherwise. |

A.XOR.B | the value of this expression is the exclusive OR of A and B. Here, the value is .TRUE. if only one of A and B is .TRUE. otherwise it is .FALSE.. |

This is self explanatory. It causes program execution to cease at that point; if the form:

STOP (string of 1 – 6 characters)

is used, then the characters in the string will be printed out on the monitor and program execution then finishes.

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