generate sinusoidal wave using hash table

sme7000 · 12-18-2009, 01:15 AM

how can i generate sinusoidal wave using hash table, or look up table?

cyberfish · 12-18-2009, 01:17 AM

Using the sin() function perhaps?

brewbuck · 12-18-2009, 01:25 AM

I don't think you'd ever use a hash table to generate a sine wave. A lookup table is more likely. Generally you would precompute a single cycle of a sine wave into a table of some fixed size, then draw values from the table as needed, maybe with interpolation.

If it's not homework I'll show code.

sme7000 · 12-18-2009, 02:49 AM

after determine the point of this half cycle of sinusoidal wave, i choose to use 10 point per half cycle. this point are store in a look-up table, how can i use this point in next step..( such as comparing with other wave). is it any example to call this function and create lookup table?

Adak · 12-18-2009, 04:03 AM

Hard to give advice on a wave that I know very little about:

1) how tall do you want it to be?

2) how wide?

3) how many waves do you want to fit on one screen?

You can get a nice sine wave by taking your maxy setting for your window and setting:
ypos = maxy / 2;

then your crest can be:

crest = maxy / 4;

then using a width of maxx, put that into a for loop, with i as the counter,

and use: swing=crest*sin(10*pi*i/width);
with: putpixel(i, ypos+swing);

and work that from 0 to max x.

That doesn't use a lookup table or a hash, but it works nicely - even on a 486 @ 50 MHz, lol.

since · 12-18-2009, 10:05 AM

sme7000, This is how you generate, and use a lookup table for trig functions, in pseudo.

*note; I've given an example of the most basic way to use a lookup table, in practice it can be much more complicated and requires a lot of numerical 'tricks' to get indices 'right'.

Depending on the application I wouldn't even bother with lookup tables. Sometimes though you need to, maybe on highly limited hardware or something... don't bother with this unless you actually have to. Just IMO.

Code:

#define T_RESOLUTION 360
T_SIN[T_RESOLUTION] = { ... };
T_COS[T_RESOLUTION] = { ... };
// Trig tables contain a range of values, [0, 2pi], with given resolution.

// generate ...
for (int i = 0; i < T_RESOLUTION ; ++i)
{
	T_SIN[i] = sin(i * 2.0 * M_PI / (float)T_RESOLUTION);
	T_COS[i] = cos(i * 2.0 * M_PI / (float)T_RESOLUTION);
}

// draw a circle with lookup tables  ...
for (int i = 0; i < T_RESOLUTION ; ++i)
{
	px = T_COS[i] * Radius; // no trig calculations! yay lookup tables!
	py = T_SIN[i] * Radius;
	draw(px, py);
}

sme7000 · 12-18-2009, 01:17 PM

thanks for this advice and knowledge...i try first and see the result

12-18-2009, 01:15 AM	#1
sme7000 Registered User Join Date: Oct 2009 Posts: 8	generate sinusoidal wave using hash table how can i generate sinusoidal wave using hash table, or look up table?
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12-18-2009, 01:17 AM	#2
cyberfish Registered User Join Date: Dec 2006 Location: Canada Posts: 2,322	Using the sin() function perhaps?
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12-18-2009, 01:25 AM	#3
brewbuck Staff software engineer Join Date: Mar 2007 Location: Portland, OR Posts: 6,014	I don't think you'd ever use a hash table to generate a sine wave. A lookup table is more likely. Generally you would precompute a single cycle of a sine wave into a table of some fixed size, then draw values from the table as needed, maybe with interpolation. If it's not homework I'll show code. __________________ "Congratulations on your purchase. To begin using your quantum computer, set the power switch to both off and on simultaneously." -- raftpeople@slashdot
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12-18-2009, 02:49 AM	#4
sme7000 Registered User Join Date: Oct 2009 Posts: 8	after determine the point of this half cycle of sinusoidal wave, i choose to use 10 point per half cycle. this point are store in a look-up table, how can i use this point in next step..( such as comparing with other wave). is it any example to call this function and create lookup table?
sme7000 is offline

12-18-2009, 04:03 AM	#5
Adak Registered User Join Date: Sep 2006 Posts: 3,720	Hard to give advice on a wave that I know very little about: 1) how tall do you want it to be? 2) how wide? 3) how many waves do you want to fit on one screen? You can get a nice sine wave by taking your maxy setting for your window and setting: ypos = maxy / 2; then your crest can be: crest = maxy / 4; then using a width of maxx, put that into a for loop, with i as the counter, and use: swing=crestsin(10pi*i/width); with: putpixel(i, ypos+swing); and work that from 0 to max x. That doesn't use a lookup table or a hash, but it works nicely - even on a 486 @ 50 MHz, lol.
Adak is offline

12-18-2009, 10:05 AM	#6
since Registered User Join Date: Nov 2009 Posts: 82	sme7000, This is how you generate, and use a lookup table for trig functions, in pseudo. note; I've given an example of the most basic way to use a lookup table, in practice it can be much more complicated and requires a lot of numerical 'tricks' to get indices 'right'. Depending on the application I wouldn't even bother with lookup tables. Sometimes though you need to, maybe on highly limited hardware or something... don't bother with this unless you actually have to. Just IMO. Code: #define T_RESOLUTION 360 T_SIN[T_RESOLUTION] = { ... }; T_COS[T_RESOLUTION] = { ... }; // Trig tables contain a range of values, [0, 2pi], with given resolution. // generate ... for (int i = 0; i < T_RESOLUTION ; ++i) { T_SIN[i] = sin(i 2.0 * M_PI / (float)T_RESOLUTION); T_COS[i] = cos(i * 2.0 * M_PI / (float)T_RESOLUTION); } // draw a circle with lookup tables ... for (int i = 0; i < T_RESOLUTION ; ++i) { px = T_COS[i] * Radius; // no trig calculations! yay lookup tables! py = T_SIN[i] * Radius; draw(px, py); } Last edited by since; 12-18-2009 at 10:12 AM.
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12-18-2009, 01:17 PM	#7
sme7000 Registered User Join Date: Oct 2009 Posts: 8	thanks for this advice and knowledge...i try first and see the result
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