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emulating_20the_20c_20assignment_20function
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emulating_20the_20c_20assignment_20function [2018/03/31 13:19] 127.0.0.1 external edit |
emulating_20the_20c_20assignment_20function [2018/04/17 16:02] (current) tbest3112 Added syntax highlighting |
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| //by Richard Russell, July 2009//\\ \\ In the **C** programming language, an //assignment// operation such as:\\ \\ | //by Richard Russell, July 2009//\\ \\ In the **C** programming language, an //assignment// operation such as:\\ \\ | ||
| + | <code c> | ||
| variable = expression | variable = expression | ||
| + | </code> | ||
| can be used either as a //statement// or as a //function//. When used as a function, it returns the new value of the variable. Whilst at first sight this might not seem very useful, it is particularly convenient in the case of a **while** loop, for example:\\ \\ | can be used either as a //statement// or as a //function//. When used as a function, it returns the new value of the variable. Whilst at first sight this might not seem very useful, it is particularly convenient in the case of a **while** loop, for example:\\ \\ | ||
| + | <code c> | ||
| while (variable = expression) | while (variable = expression) | ||
| { | { | ||
| // Do something useful here | // Do something useful here | ||
| } | } | ||
| + | </code> | ||
| Here **variable** is set equal to the value of **expression** and if its new value is //non-zero// the body of the loop is executed (note that it is //not// testing whether **variable** is equal to **expression**; in **C** you do that using the == operator).\\ \\ Since in BBC BASIC an assignment is a //statement//, you can't straightforwardly do this, and you have to code it as follows:\\ \\ | Here **variable** is set equal to the value of **expression** and if its new value is //non-zero// the body of the loop is executed (note that it is //not// testing whether **variable** is equal to **expression**; in **C** you do that using the == operator).\\ \\ Since in BBC BASIC an assignment is a //statement//, you can't straightforwardly do this, and you have to code it as follows:\\ \\ | ||
| + | <code bb4w> | ||
| variable = expression | variable = expression | ||
| WHILE variable | WHILE variable | ||
| Line 14: | Line 19: | ||
| variable = expression | variable = expression | ||
| ENDWHILE | ENDWHILE | ||
| + | </code> | ||
| As you can see, this involves writing the assignment statement **twice**, once outside the loop and again inside the loop. This is inelegant and potentially error-prone, for example you might make a change to one of the assignments but forget to change the other.\\ \\ To emulate the **C** behaviour you can utilise this simple function:\\ \\ | As you can see, this involves writing the assignment statement **twice**, once outside the loop and again inside the loop. This is inelegant and potentially error-prone, for example you might make a change to one of the assignments but forget to change the other.\\ \\ To emulate the **C** behaviour you can utilise this simple function:\\ \\ | ||
| + | <code bb4w> | ||
| DEF FNassign(RETURN variable, expression) | DEF FNassign(RETURN variable, expression) | ||
| variable = expression | variable = expression | ||
| = variable | = variable | ||
| + | </code> | ||
| Now you can write the loop as follows:\\ \\ | Now you can write the loop as follows:\\ \\ | ||
| + | <code bb4w> | ||
| WHILE FNassign(variable, expression) | WHILE FNassign(variable, expression) | ||
| REM Do something useful here | REM Do something useful here | ||
| ENDWHILE | ENDWHILE | ||
| + | </code> | ||
| Note that since **variant** numeric variables are used in the function (i.e. without a 'type' suffix character) it will work equally well with **integer** variables and values as with **floating-point** variables. | Note that since **variant** numeric variables are used in the function (i.e. without a 'type' suffix character) it will work equally well with **integer** variables and values as with **floating-point** variables. | ||
emulating_20the_20c_20assignment_20function.1522502358.txt.gz ยท Last modified: 2018/03/31 13:19 by 127.0.0.1
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