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copied!<p>This has been the way that Fortran loops work for decades and you can't simply change this with a compiler option. The Fortran standard clearly states:</p> <blockquote> <p>8.1.4.4.1 Loop initiation</p> <p>(2) The DO variable becomes defined with the value of the initial parameter <em>m<sub>1</sub></em>.</p> <p>(3) The <strong>iteration count</strong> is established and is the value of the expression <code>MAX (INT ((m2 – m1 + m3) / m3), 0)</code></p> </blockquote> <p>Here <em>m<sub>1</sub></em>, <em>m<sub>2</sub></em> and <em>m<sub>3</sub></em> are the three parameters in the <em>loop-control</em>: <code>[,] var = m1,m2[,m3]</code>, Given your example of <code>i=1,1</code> (<em>m<sub>3</sub></em> is implicitly <code>1</code> if omitted) the iteration count is <code>MAX(INT((1-1+1)/1),0)</code> which evaluates to <code>1</code>, i.e. the loop should get executed once. <code>i</code> is initialised to <code>1</code> as per (2).</p> <blockquote> <p>8.1.4.4.2 The execution cycle</p> <p>The <strong>execution cycle</strong> of a DO construct consists of the following steps performed in sequence repeatedly until termination:</p> <p>(1) The iteration count, if any, is tested. If the iteration count is zero, the loop terminates and the DO construct becomes inactive. If <em>loop-control</em> is <code>[ , ] WHILE (scalar-logical-expr)</code>, the <em>scalar-logicalexpr</em> is evaluated; if the value of this expression is false, the loop terminates and the DO construct becomes inactive. If, as a result, all of the DO constructs sharing the <em>do-term-shared-stmt</em> are inactive, the execution of all of these constructs is complete. However, if some of the DO constructs sharing the <em>do-term-shared-stmt</em> are active, execution continues with step (3) of the execution cycle of the active DO construct whose DO statement was most recently executed.</p> </blockquote> <p>Fortran tests if the remaining <strong>iteration count</strong> is greater than zero, not if the DO variable is less than (greater than) the end value.</p> <blockquote> <p>(2) If the iteration count is nonzero, the range of the loop is executed.</p> <p>(3) The iteration count, if any, is decremented by one. The DO variable, if any, is incremented by the value of the incrementation parameter <em>m<sub>3</sub></em>.</p> </blockquote> <p>The DO variable is always incremented as an iteration of the loop is being executed. Thus after the first execution <code>i</code> becomes incremented by <code>1</code> which evaluates to <code>2</code>.</p> <blockquote> <p>Except for the incrementation of the DO variable that occurs in step (3), the DO variable must neither be redefined nor become undefined while the DO construct is active.</p> <p>8.1.4.4.4 Loop termination</p> <p>When a DO construct becomes inactive, the DO-variable, if any, of the DO construct retains its last defined value.</p> </blockquote> <p>The last defined value is <code>2</code>. Thus after the DO loop has ended, <code>i</code> is equal to <code>2</code>.</p> <p>I've pulled the text out of ISO/IEC 1539:1991 (a.k.a. Fortran 90) but one can also find pretty much the same text in §11.10.3 of ISO/IEC 1539:1980 (a.k.a. ANSI X3J3/90.4 a.k.a. FORTRAN 77; sans the <code>WHILE</code> stuff which is not present in F77) as well as in §8.1.6.6 of ISO/IEC 1539-1:2010 (a.k.a. Fortran 2008).</p>

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