alternative_20for_20next_20behaviour
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| alternative_20for_20next_20behaviour [2018/04/17 15:03] – Added syntax highlighting tbest3112 | alternative_20for_20next_20behaviour [2024/01/05 00:22] (current) – external edit 127.0.0.1 | ||
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| =====Alternative FOR NEXT behaviour===== | =====Alternative FOR NEXT behaviour===== | ||
| - | //by Richard Russell, July 2007//\\ \\ Although the **FOR**...**NEXT** loop is fundamental to all versions of BASIC, dialects differ in its detailed implementation. In **BBC BASIC** FOR...NEXT loops have the following properties:\\ \\ | + | //by Richard Russell, July 2007//\\ \\ Although the **FOR**...**NEXT** loop is fundamental to all versions of BASIC, dialects differ in its detailed implementation. In **BBC BASIC** FOR...NEXT loops have the following properties: |
| * The body of the loop is always executed at least once, irrespective of the values specified in the FOR statement. | * The body of the loop is always executed at least once, irrespective of the values specified in the FOR statement. | ||
| * On exit from the loop the loop variable has a value //greater than// (for a positive STEP) the TO value specified in the FOR statement, i.e. one more STEP has taken place. | * On exit from the loop the loop variable has a value //greater than// (for a positive STEP) the TO value specified in the FOR statement, i.e. one more STEP has taken place. | ||
| - | \\ | + | |
| + | The following examples illustrate this behaviour: | ||
| <code bb4w> | <code bb4w> | ||
| FOR i% = 1 TO 0 | FOR i% = 1 TO 0 | ||
| Line 16: | Line 17: | ||
| PRINT i% | PRINT i% | ||
| </ | </ | ||
| - | The first loop prints **1**, even though the ' | + | The first loop prints **1**, even though the ' |
| + | |||
| + | The second loop prints **11**, which is one more than the ' | ||
| + | |||
| + | On occasion you may wish to emulate the behaviour of a different BASIC dialect, for example when porting a program to BBC BASIC. One convenient way of doing that is to use the [[http:// | ||
| + | |||
| + | Some BASIC dialects //do not execute the FOR loop at all// if the initial conditions aren't met, i.e. the ' | ||
| <code bb4w> | <code bb4w> | ||
| FOR loopvar = start TO finish | FOR loopvar = start TO finish | ||
| Line 23: | Line 30: | ||
| NEXT loopvar | NEXT loopvar | ||
| </ | </ | ||
| - | Some BASIC dialects exit the loop //with the loop variable equal to the ' | + | Some BASIC dialects exit the loop //with the loop variable equal to the ' |
| <code bb4w> | <code bb4w> | ||
| FOR loopvar = start TO finish | FOR loopvar = start TO finish | ||
| Line 30: | Line 37: | ||
| NEXT loopvar | NEXT loopvar | ||
| </ | </ | ||
| - | Of course you can combine both behaviours if you wish:\\ \\ | + | Of course you can combine both behaviours if you wish: |
| <code bb4w> | <code bb4w> | ||
| FOR loopvar = start TO finish | FOR loopvar = start TO finish | ||
| Line 38: | Line 45: | ||
| NEXT loopvar | NEXT loopvar | ||
| </ | </ | ||
| - | In all cases if the STEP is negative you need to reverse the comparisons: | + | In all cases if the STEP is negative you need to reverse the comparisons: |
| <code bb4w> | <code bb4w> | ||
| FOR loopvar = start TO finish STEP -delta | FOR loopvar = start TO finish STEP -delta | ||
alternative_20for_20next_20behaviour.1523977405.txt.gz · Last modified: 2024/01/05 00:18 (external edit)