48 lines
1.4 KiB
QBasic
48 lines
1.4 KiB
QBasic
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10 REM A structure parameter must name its type: @ says "a structure" without
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20 REM saying which, so B@ does not state a contract the way B$ does.
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30 TYPE CRATE
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40 W#
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50 H#
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60 END TYPE
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70 DIM A@ AS CRATE
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80 A@.W# = 5
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90 A@.H# = 3
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100 DEF AREA(B@ AS CRATE) = B@.W# * B@.H#
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110 PRINT AREA(A@)
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120 REM Passing is BY VALUE, because a parameter is bound by assignment and
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130 REM assignment copies. A function cannot change its caller's record by
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140 REM accident.
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150 DEF WIDEN(B@ AS CRATE)
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160 B@.W# = 99
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170 RETURN B@.W#
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180 PRINT WIDEN(A@)
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190 PRINT A@.W#
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200 REM To change one on purpose, pass a pointer. Assignment copies a pointer's
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210 REM reference, so the callee is looking at the caller's own record.
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220 DIM Q@ AS PTR TO CRATE
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230 POINT Q@ AT A@
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240 DEF POKEIT(P@ AS PTR TO CRATE)
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250 P@->W# = 42
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260 RETURN P@->W#
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270 PRINT POKEIT(Q@)
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280 PRINT A@.W#
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290 REM And a recursive function can carry a record down with it.
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300 TYPE NODE
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310 COUNT#
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320 TAIL@ AS PTR TO NODE
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330 END TYPE
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340 DIM N1@ AS NODE
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350 DIM N2@ AS NODE
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360 N1@.COUNT# = 10
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370 N2@.COUNT# = 20
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380 POINT N1@.TAIL@ AT N2@
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390 DEF TOTAL(P@ AS PTR TO NODE)
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395 REM A pointer is true when it points at something, which is how a walk knows
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396 REM where the list ends. NOT is the bitwise operator here, so the test is
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397 REM written the positive way round.
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400 IF P@->TAIL@ THEN RETURN P@->COUNT# + TOTAL(P@->TAIL@)
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410 RETURN P@->COUNT#
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420 DIM W@ AS PTR TO NODE
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430 POINT W@ AT N1@
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440 PRINT TOTAL(W@)
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