Subject:
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Re: circular pointer
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Newsgroups:
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lugnet.robotics
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Date:
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Sun, 13 May 2007 00:08:45 GMT
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Viewed:
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5002 times
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I've added my comments to the code you posted (they start with **). I've also
added some indenting.:
#define SIZE 3
int v[SIZE],i,sum,ave;
// initialize the array and some other variables
sum = 0;
for (i=0;i<SIZE-1;i++)
{
//I think this means set 'i' to zero, as long as 'i' is smaller than
SIZE add one to 'i' after every loop
//** It means "as long as 'i' is smaller than SIZE MINUS ONE"
v[i] = SENSOR_1;
sum += v[i];
}
//what would happen is this, using arbitrary numbers to represent sensor
reading:
//v[0] = 50
//sum = 0+50 =50
//v[1] =52
//sum = 50+52 = 102
//v[2] =55
//sum = 102+55 = 157
//** But the loop stops at v[1] because one is less than SIZE-1 (which is two)
i=SIZE-1;
v[i]=0;
//so 'i' becomes 2 and I set v[2] to zero. Why would I want to eliminate
//my last reading?
//** Yes, 'i' NOW becomes two and you initialize v[2] so it's not undefined
// compute moving average (comment from book)
while (true) //** Do it forever
{
sum -= v[i];
//since 'i' now equals 2, and v[2] equals zero, this operations doesn't
//appear to make sense
//** This statement appears here because it makes sense INSIDE the while
//** loop. Basically, this some and the following sensor reading are
//** saying "replace the oldest reading with a new one".
//** At first (before the 'while'), v[2] is the "oldest" reading.
v[i] = SENSOR_1;
//I read a new value into v[2], e.g. 60
sum += v[i];
//since I haven't subtracted anything but 0 from the SUM, the sum now
//holds the total value of 4 readings?!
//** There's not really four readings, because (a) you can interpret the
//** zero as if it meant 'empty' (but only during the initialization,
//** of course), (b) the array only has three elements (SIZE), and
//** (c) you're averaging by dividing by SIZE (three)
ave = sum / SIZE;
//average value, but for the last comment it makes perfect sense.
i = (i+1) % SIZE;
//you lost me there...
// other instructions... (comment from book: what other instructions
//might I want to include here?)
//** As someone else stated, it's the modulus operator, which gives the
//** remainder after integer division. When 'i' is 3, this statement
//** evaluates to '(3 + 1) % 3' so three divided by three is one with
//** a remainder of zero. So the new value of 'i' is zero. As the while
//** loop loops, 'i' goes 2, 3, 0, 1, 2, 3, 0...
}
>
> It looks like only the last value is recycled, when what should happen
> is that the first value is eliminated and the second becomes first, the
> third second and the new one the last.
>
> If I'm not making myself clear, please help me improve my question.
> Thanks in advance for your efforts.
>
> linmix
You don't have to move the values around, because 'i' changes and points to the
oldest value, which then becomes the newest value, hence "circular pointer". A
fun thing to do with them is the "snake" program. The elements of the array hold
the x, y coordinates of each segment of the snake's body. The program clears a
spot on the screen at the oldest position (the end of the snake's tail) and
paints a spot at the new position (the head). This causes the snake to move
around the screen. Some directional control selects which direction it moves
(increment or decrement x or y or both).
Dennis
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Message has 1 Reply:
 | | Re: circular pointer
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| I've inserted some comments, not all are questions, some are simply 'thinking aloud'. (...) So by defining SIZE here I can easily change the size of the macro without having to alter anything else in the code. Nice! (...) Right you are, I should (...) (18 years ago, 13-May-07, to lugnet.robotics)
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Message is in Reply To:
| | circular pointer
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| Some time ago some of you offered to help out if I (and I suppose anyone else) had any questions about programming.... well, here goes I'm reading "Building Robots with Lego Mindstorms" and in Chapter 12, there is some code to create a circular (...) (18 years ago, 12-May-07, to lugnet.robotics)
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