In sections~#secmovecircle#56918>, #secmoveshape#56919>,
#secmovefig#56920>, and~#secabsmoving#56921> we studied how to move
pictures across a canvas. A picture is a list of shapes; a shape is one of
several basic geometric shapes: circles, rectangles, etc. Following our
most basic design principle---one function per concept---we first defined
functions for moving basic geometric shapes, then for mixed classes of
shapes, and finally for lists of shapes. Eventually we abstracted over
related functions.
The functions for moving basic shapes create a new shape from an existing
shape. For example, the function for moving a circle consumes a
<#69980#><#56922#>circle<#56922#><#69980#> structure and produces a new <#69981#><#56923#>circle<#56923#><#69981#> structure. If
we think of the <#69982#><#56924#>circle<#56924#><#69982#> as a painting with a round frame and the
canvas as a wall, however, creating a new shape for each move is
inappropriate. Instead, we should change the shape's current position.
<#56927#>Exercise 41.4.1<#56927#>
Adapt the functions <#69983#><#56929#>translate-circle<#56929#><#69983#> and
<#69984#><#56930#>translate-rectangle<#56930#><#69984#> of exercises~#excircdraw#56931>
and~#exrectdraw#56932>, respectively, so that they mutate their inputs
instead of creating new geometric shapes.
Adapt <#69985#><#56933#>move-circle<#56933#><#69985#>, defined in section~#secmovecircle#56934>, and
<#69986#><#56935#>move-rectangle<#56935#><#69986#> from exercise~#exrectmove#56936> so that they use
these new functions.~ 
Solution<#69987#><#69987#>
<#56942#>Exercise 41.4.2<#56942#>
Adapt the function <#69988#><#56944#>move-picture<#56944#><#69988#> from exercise~#exmoving#56945> to
use the structure-mutating functions from exercise~#exmupicture1#56946>.
<#56947#>Hint:<#56947#> \ Timing effects is critical in this exercise.~ Solution<#69989#><#69989#>
<#56953#>Exercise 41.4.3<#56953#>
Use Scheme's <#69990#><#56955#>for-each<#56955#><#69990#> function to abstract where possible in the
functions of exercise~#exmupicture2#56956>.~ Solution<#69991#><#69991#>