Why is this recursive code so slow? The 2019 Stack Overflow Developer Survey Results Are In ...

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Why is this recursive code so slow?



The 2019 Stack Overflow Developer Survey Results Are In
Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)What are the hidden specifications for FindRootWhy does this function inside FindRoot fail to evaluate?Very slow mathematica finite differencesUsing Mathematica to solve a recursive system of differential equationsImproving the speed on an iterated differential systemForward iterations of coupled recursion equationsManipulate+FindRoot+Plot3D very slow/crashAttacking a “Mathematica can't solve” problemErrors using FindRoot on slow numerical functionAvoiding a for loop to create a list












5












$begingroup$


This code for the first five iterations the speed is okay, but after that the speed is very slow, I cannot understand what is wrong with this? Would you please help me fix it?



Clear[A, r, x, s, e]
s := 0.3405
e := 1.6539*10^-21
u[0] := 0.
u[1] := 0.1

A[r_] := A[r] =
Piecewise[{{r - 2.5 s - 48*e *s^12*r^-13 + 24*e*s^6*r^-7,
r > 2.5 s}, {-48*e*s^12*r^-13 + 24*e*s^6*r^-7,
s <= r <= 2.5 s}, {r - s -
24*e*s^-1, r < s}}]
For[i = 2, i < 101,
i++, { u[i_] :=
x /. FindRoot[
u[i - 1] +
1/(i^2 (u[i - 1] - u[i - 2])^2) (u[i - 1] - u[i - 2]) -
0.9 A[x] == x , {x, 1.}]; Print[u[i]]}]









share|improve this question











$endgroup$












  • $begingroup$
    How slow? How many minutes/seconds?
    $endgroup$
    – JonyD
    yesterday
















5












$begingroup$


This code for the first five iterations the speed is okay, but after that the speed is very slow, I cannot understand what is wrong with this? Would you please help me fix it?



Clear[A, r, x, s, e]
s := 0.3405
e := 1.6539*10^-21
u[0] := 0.
u[1] := 0.1

A[r_] := A[r] =
Piecewise[{{r - 2.5 s - 48*e *s^12*r^-13 + 24*e*s^6*r^-7,
r > 2.5 s}, {-48*e*s^12*r^-13 + 24*e*s^6*r^-7,
s <= r <= 2.5 s}, {r - s -
24*e*s^-1, r < s}}]
For[i = 2, i < 101,
i++, { u[i_] :=
x /. FindRoot[
u[i - 1] +
1/(i^2 (u[i - 1] - u[i - 2])^2) (u[i - 1] - u[i - 2]) -
0.9 A[x] == x , {x, 1.}]; Print[u[i]]}]









share|improve this question











$endgroup$












  • $begingroup$
    How slow? How many minutes/seconds?
    $endgroup$
    – JonyD
    yesterday














5












5








5





$begingroup$


This code for the first five iterations the speed is okay, but after that the speed is very slow, I cannot understand what is wrong with this? Would you please help me fix it?



Clear[A, r, x, s, e]
s := 0.3405
e := 1.6539*10^-21
u[0] := 0.
u[1] := 0.1

A[r_] := A[r] =
Piecewise[{{r - 2.5 s - 48*e *s^12*r^-13 + 24*e*s^6*r^-7,
r > 2.5 s}, {-48*e*s^12*r^-13 + 24*e*s^6*r^-7,
s <= r <= 2.5 s}, {r - s -
24*e*s^-1, r < s}}]
For[i = 2, i < 101,
i++, { u[i_] :=
x /. FindRoot[
u[i - 1] +
1/(i^2 (u[i - 1] - u[i - 2])^2) (u[i - 1] - u[i - 2]) -
0.9 A[x] == x , {x, 1.}]; Print[u[i]]}]









share|improve this question











$endgroup$




This code for the first five iterations the speed is okay, but after that the speed is very slow, I cannot understand what is wrong with this? Would you please help me fix it?



Clear[A, r, x, s, e]
s := 0.3405
e := 1.6539*10^-21
u[0] := 0.
u[1] := 0.1

A[r_] := A[r] =
Piecewise[{{r - 2.5 s - 48*e *s^12*r^-13 + 24*e*s^6*r^-7,
r > 2.5 s}, {-48*e*s^12*r^-13 + 24*e*s^6*r^-7,
s <= r <= 2.5 s}, {r - s -
24*e*s^-1, r < s}}]
For[i = 2, i < 101,
i++, { u[i_] :=
x /. FindRoot[
u[i - 1] +
1/(i^2 (u[i - 1] - u[i - 2])^2) (u[i - 1] - u[i - 2]) -
0.9 A[x] == x , {x, 1.}]; Print[u[i]]}]






equation-solving recursion






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited yesterday









Roman

5,24511131




5,24511131










asked 2 days ago









morapimorapi

355




355












  • $begingroup$
    How slow? How many minutes/seconds?
    $endgroup$
    – JonyD
    yesterday


















  • $begingroup$
    How slow? How many minutes/seconds?
    $endgroup$
    – JonyD
    yesterday
















$begingroup$
How slow? How many minutes/seconds?
$endgroup$
– JonyD
yesterday




$begingroup$
How slow? How many minutes/seconds?
$endgroup$
– JonyD
yesterday










1 Answer
1






active

oldest

votes


















12












$begingroup$

I recommend you learn the distinction between immediate (=) and delayed (:=) assignments. They make the difference between slow and fast code here. Start with this tutorial or this book chapter, then look at memoization.



s = 0.3405;
e = 1.6539*10^-21;
u[0] = 0.;
u[1] = 0.1;

A[r_] = Piecewise[{{r - 2.5 s - 48*e*s^12*r^-13 + 24*e*s^6*r^-7, r > 2.5 s},
{-48*e*s^12*r^-13 + 24*e*s^6*r^-7, s <= r <= 2.5 s},
{r - s - 24*e*s^-1, r < s}}];

u[i_] := u[i] = x /. FindRoot[
u[i - 1] + 1/(i^2 (u[i - 1] - u[i - 2])^2) (u[i - 1] - u[i - 2]) - 0.9 A[x] == x, {x, 1.}]

Array[u, 100]



{0.1, 1.77164, 1.37065, 1.04259, 0.887781, 0.708344, 0.59461,
0.457228, 0.367364, 0.296071, 0.256104, 0.20463, 0.208487, 1.20917,
1.04197, 0.939331, 0.879865, 0.827963, 0.774591, 0.72775, 0.67934,
0.63666, 0.592369, 0.553172, 0.512352, 0.476112, 0.438261, 0.404563,
0.369277, 0.339073, 0.321616, 0.301118, 0.296195, 0.224688, 0.273538,
0.31357, 0.33593, 0.366902, 0.38813, 0.417572, 0.437777, 0.465834,
0.48511, 0.511907, 0.530336, 0.55598, 0.573633, 0.598219, 0.615159,
0.638772, 0.655054, 0.677768, 0.693441, 0.715321, 0.73043, 0.751535,
0.766118, 0.786503, 0.800596, 0.820306, 0.833941, 0.852182, 0.85901,
0.874152, 0.871531, 0.78396, 0.781416, 0.696402, 0.693931, 0.611329,
0.608927, 0.528603, 0.526267, 0.448099, 0.445825, 0.369701, 0.367485,
0.315658, 0.325798, 0.341207, 0.351098, 0.366134, 0.375788, 0.390468,
0.399897, 0.414237, 0.42345, 0.437466, 0.446473, 0.46018, 0.46899,
0.4824, 0.491022, 0.504149, 0.51259, 0.525444, 0.533712, 0.546306,
0.554408, 0.56675}




(takes about 1.3 seconds)



Alternatively, use



Table[u[i], {i, 1, 100}]


(same result). Your combination of For and Print shows the results but doesn't let you keep using them for more calculations.






share|improve this answer











$endgroup$













  • $begingroup$
    thank you very much. I really appreciate it.
    $endgroup$
    – morapi
    yesterday










  • $begingroup$
    delayed assignments definitely sound slower than immediate, even if I have never worked with Mathematica
    $endgroup$
    – Roland
    yesterday








  • 2




    $begingroup$
    @Roland it's not just that one is necessarily faster or slower than the other, it's more that they are completely different things with very different applications. For some reason this point is often overlooked by beginners in Mathematica.
    $endgroup$
    – Roman
    yesterday












Your Answer








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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









12












$begingroup$

I recommend you learn the distinction between immediate (=) and delayed (:=) assignments. They make the difference between slow and fast code here. Start with this tutorial or this book chapter, then look at memoization.



s = 0.3405;
e = 1.6539*10^-21;
u[0] = 0.;
u[1] = 0.1;

A[r_] = Piecewise[{{r - 2.5 s - 48*e*s^12*r^-13 + 24*e*s^6*r^-7, r > 2.5 s},
{-48*e*s^12*r^-13 + 24*e*s^6*r^-7, s <= r <= 2.5 s},
{r - s - 24*e*s^-1, r < s}}];

u[i_] := u[i] = x /. FindRoot[
u[i - 1] + 1/(i^2 (u[i - 1] - u[i - 2])^2) (u[i - 1] - u[i - 2]) - 0.9 A[x] == x, {x, 1.}]

Array[u, 100]



{0.1, 1.77164, 1.37065, 1.04259, 0.887781, 0.708344, 0.59461,
0.457228, 0.367364, 0.296071, 0.256104, 0.20463, 0.208487, 1.20917,
1.04197, 0.939331, 0.879865, 0.827963, 0.774591, 0.72775, 0.67934,
0.63666, 0.592369, 0.553172, 0.512352, 0.476112, 0.438261, 0.404563,
0.369277, 0.339073, 0.321616, 0.301118, 0.296195, 0.224688, 0.273538,
0.31357, 0.33593, 0.366902, 0.38813, 0.417572, 0.437777, 0.465834,
0.48511, 0.511907, 0.530336, 0.55598, 0.573633, 0.598219, 0.615159,
0.638772, 0.655054, 0.677768, 0.693441, 0.715321, 0.73043, 0.751535,
0.766118, 0.786503, 0.800596, 0.820306, 0.833941, 0.852182, 0.85901,
0.874152, 0.871531, 0.78396, 0.781416, 0.696402, 0.693931, 0.611329,
0.608927, 0.528603, 0.526267, 0.448099, 0.445825, 0.369701, 0.367485,
0.315658, 0.325798, 0.341207, 0.351098, 0.366134, 0.375788, 0.390468,
0.399897, 0.414237, 0.42345, 0.437466, 0.446473, 0.46018, 0.46899,
0.4824, 0.491022, 0.504149, 0.51259, 0.525444, 0.533712, 0.546306,
0.554408, 0.56675}




(takes about 1.3 seconds)



Alternatively, use



Table[u[i], {i, 1, 100}]


(same result). Your combination of For and Print shows the results but doesn't let you keep using them for more calculations.






share|improve this answer











$endgroup$













  • $begingroup$
    thank you very much. I really appreciate it.
    $endgroup$
    – morapi
    yesterday










  • $begingroup$
    delayed assignments definitely sound slower than immediate, even if I have never worked with Mathematica
    $endgroup$
    – Roland
    yesterday








  • 2




    $begingroup$
    @Roland it's not just that one is necessarily faster or slower than the other, it's more that they are completely different things with very different applications. For some reason this point is often overlooked by beginners in Mathematica.
    $endgroup$
    – Roman
    yesterday
















12












$begingroup$

I recommend you learn the distinction between immediate (=) and delayed (:=) assignments. They make the difference between slow and fast code here. Start with this tutorial or this book chapter, then look at memoization.



s = 0.3405;
e = 1.6539*10^-21;
u[0] = 0.;
u[1] = 0.1;

A[r_] = Piecewise[{{r - 2.5 s - 48*e*s^12*r^-13 + 24*e*s^6*r^-7, r > 2.5 s},
{-48*e*s^12*r^-13 + 24*e*s^6*r^-7, s <= r <= 2.5 s},
{r - s - 24*e*s^-1, r < s}}];

u[i_] := u[i] = x /. FindRoot[
u[i - 1] + 1/(i^2 (u[i - 1] - u[i - 2])^2) (u[i - 1] - u[i - 2]) - 0.9 A[x] == x, {x, 1.}]

Array[u, 100]



{0.1, 1.77164, 1.37065, 1.04259, 0.887781, 0.708344, 0.59461,
0.457228, 0.367364, 0.296071, 0.256104, 0.20463, 0.208487, 1.20917,
1.04197, 0.939331, 0.879865, 0.827963, 0.774591, 0.72775, 0.67934,
0.63666, 0.592369, 0.553172, 0.512352, 0.476112, 0.438261, 0.404563,
0.369277, 0.339073, 0.321616, 0.301118, 0.296195, 0.224688, 0.273538,
0.31357, 0.33593, 0.366902, 0.38813, 0.417572, 0.437777, 0.465834,
0.48511, 0.511907, 0.530336, 0.55598, 0.573633, 0.598219, 0.615159,
0.638772, 0.655054, 0.677768, 0.693441, 0.715321, 0.73043, 0.751535,
0.766118, 0.786503, 0.800596, 0.820306, 0.833941, 0.852182, 0.85901,
0.874152, 0.871531, 0.78396, 0.781416, 0.696402, 0.693931, 0.611329,
0.608927, 0.528603, 0.526267, 0.448099, 0.445825, 0.369701, 0.367485,
0.315658, 0.325798, 0.341207, 0.351098, 0.366134, 0.375788, 0.390468,
0.399897, 0.414237, 0.42345, 0.437466, 0.446473, 0.46018, 0.46899,
0.4824, 0.491022, 0.504149, 0.51259, 0.525444, 0.533712, 0.546306,
0.554408, 0.56675}




(takes about 1.3 seconds)



Alternatively, use



Table[u[i], {i, 1, 100}]


(same result). Your combination of For and Print shows the results but doesn't let you keep using them for more calculations.






share|improve this answer











$endgroup$













  • $begingroup$
    thank you very much. I really appreciate it.
    $endgroup$
    – morapi
    yesterday










  • $begingroup$
    delayed assignments definitely sound slower than immediate, even if I have never worked with Mathematica
    $endgroup$
    – Roland
    yesterday








  • 2




    $begingroup$
    @Roland it's not just that one is necessarily faster or slower than the other, it's more that they are completely different things with very different applications. For some reason this point is often overlooked by beginners in Mathematica.
    $endgroup$
    – Roman
    yesterday














12












12








12





$begingroup$

I recommend you learn the distinction between immediate (=) and delayed (:=) assignments. They make the difference between slow and fast code here. Start with this tutorial or this book chapter, then look at memoization.



s = 0.3405;
e = 1.6539*10^-21;
u[0] = 0.;
u[1] = 0.1;

A[r_] = Piecewise[{{r - 2.5 s - 48*e*s^12*r^-13 + 24*e*s^6*r^-7, r > 2.5 s},
{-48*e*s^12*r^-13 + 24*e*s^6*r^-7, s <= r <= 2.5 s},
{r - s - 24*e*s^-1, r < s}}];

u[i_] := u[i] = x /. FindRoot[
u[i - 1] + 1/(i^2 (u[i - 1] - u[i - 2])^2) (u[i - 1] - u[i - 2]) - 0.9 A[x] == x, {x, 1.}]

Array[u, 100]



{0.1, 1.77164, 1.37065, 1.04259, 0.887781, 0.708344, 0.59461,
0.457228, 0.367364, 0.296071, 0.256104, 0.20463, 0.208487, 1.20917,
1.04197, 0.939331, 0.879865, 0.827963, 0.774591, 0.72775, 0.67934,
0.63666, 0.592369, 0.553172, 0.512352, 0.476112, 0.438261, 0.404563,
0.369277, 0.339073, 0.321616, 0.301118, 0.296195, 0.224688, 0.273538,
0.31357, 0.33593, 0.366902, 0.38813, 0.417572, 0.437777, 0.465834,
0.48511, 0.511907, 0.530336, 0.55598, 0.573633, 0.598219, 0.615159,
0.638772, 0.655054, 0.677768, 0.693441, 0.715321, 0.73043, 0.751535,
0.766118, 0.786503, 0.800596, 0.820306, 0.833941, 0.852182, 0.85901,
0.874152, 0.871531, 0.78396, 0.781416, 0.696402, 0.693931, 0.611329,
0.608927, 0.528603, 0.526267, 0.448099, 0.445825, 0.369701, 0.367485,
0.315658, 0.325798, 0.341207, 0.351098, 0.366134, 0.375788, 0.390468,
0.399897, 0.414237, 0.42345, 0.437466, 0.446473, 0.46018, 0.46899,
0.4824, 0.491022, 0.504149, 0.51259, 0.525444, 0.533712, 0.546306,
0.554408, 0.56675}




(takes about 1.3 seconds)



Alternatively, use



Table[u[i], {i, 1, 100}]


(same result). Your combination of For and Print shows the results but doesn't let you keep using them for more calculations.






share|improve this answer











$endgroup$



I recommend you learn the distinction between immediate (=) and delayed (:=) assignments. They make the difference between slow and fast code here. Start with this tutorial or this book chapter, then look at memoization.



s = 0.3405;
e = 1.6539*10^-21;
u[0] = 0.;
u[1] = 0.1;

A[r_] = Piecewise[{{r - 2.5 s - 48*e*s^12*r^-13 + 24*e*s^6*r^-7, r > 2.5 s},
{-48*e*s^12*r^-13 + 24*e*s^6*r^-7, s <= r <= 2.5 s},
{r - s - 24*e*s^-1, r < s}}];

u[i_] := u[i] = x /. FindRoot[
u[i - 1] + 1/(i^2 (u[i - 1] - u[i - 2])^2) (u[i - 1] - u[i - 2]) - 0.9 A[x] == x, {x, 1.}]

Array[u, 100]



{0.1, 1.77164, 1.37065, 1.04259, 0.887781, 0.708344, 0.59461,
0.457228, 0.367364, 0.296071, 0.256104, 0.20463, 0.208487, 1.20917,
1.04197, 0.939331, 0.879865, 0.827963, 0.774591, 0.72775, 0.67934,
0.63666, 0.592369, 0.553172, 0.512352, 0.476112, 0.438261, 0.404563,
0.369277, 0.339073, 0.321616, 0.301118, 0.296195, 0.224688, 0.273538,
0.31357, 0.33593, 0.366902, 0.38813, 0.417572, 0.437777, 0.465834,
0.48511, 0.511907, 0.530336, 0.55598, 0.573633, 0.598219, 0.615159,
0.638772, 0.655054, 0.677768, 0.693441, 0.715321, 0.73043, 0.751535,
0.766118, 0.786503, 0.800596, 0.820306, 0.833941, 0.852182, 0.85901,
0.874152, 0.871531, 0.78396, 0.781416, 0.696402, 0.693931, 0.611329,
0.608927, 0.528603, 0.526267, 0.448099, 0.445825, 0.369701, 0.367485,
0.315658, 0.325798, 0.341207, 0.351098, 0.366134, 0.375788, 0.390468,
0.399897, 0.414237, 0.42345, 0.437466, 0.446473, 0.46018, 0.46899,
0.4824, 0.491022, 0.504149, 0.51259, 0.525444, 0.533712, 0.546306,
0.554408, 0.56675}




(takes about 1.3 seconds)



Alternatively, use



Table[u[i], {i, 1, 100}]


(same result). Your combination of For and Print shows the results but doesn't let you keep using them for more calculations.







share|improve this answer














share|improve this answer



share|improve this answer








edited yesterday

























answered yesterday









RomanRoman

5,24511131




5,24511131












  • $begingroup$
    thank you very much. I really appreciate it.
    $endgroup$
    – morapi
    yesterday










  • $begingroup$
    delayed assignments definitely sound slower than immediate, even if I have never worked with Mathematica
    $endgroup$
    – Roland
    yesterday








  • 2




    $begingroup$
    @Roland it's not just that one is necessarily faster or slower than the other, it's more that they are completely different things with very different applications. For some reason this point is often overlooked by beginners in Mathematica.
    $endgroup$
    – Roman
    yesterday


















  • $begingroup$
    thank you very much. I really appreciate it.
    $endgroup$
    – morapi
    yesterday










  • $begingroup$
    delayed assignments definitely sound slower than immediate, even if I have never worked with Mathematica
    $endgroup$
    – Roland
    yesterday








  • 2




    $begingroup$
    @Roland it's not just that one is necessarily faster or slower than the other, it's more that they are completely different things with very different applications. For some reason this point is often overlooked by beginners in Mathematica.
    $endgroup$
    – Roman
    yesterday
















$begingroup$
thank you very much. I really appreciate it.
$endgroup$
– morapi
yesterday




$begingroup$
thank you very much. I really appreciate it.
$endgroup$
– morapi
yesterday












$begingroup$
delayed assignments definitely sound slower than immediate, even if I have never worked with Mathematica
$endgroup$
– Roland
yesterday






$begingroup$
delayed assignments definitely sound slower than immediate, even if I have never worked with Mathematica
$endgroup$
– Roland
yesterday






2




2




$begingroup$
@Roland it's not just that one is necessarily faster or slower than the other, it's more that they are completely different things with very different applications. For some reason this point is often overlooked by beginners in Mathematica.
$endgroup$
– Roman
yesterday




$begingroup$
@Roland it's not just that one is necessarily faster or slower than the other, it's more that they are completely different things with very different applications. For some reason this point is often overlooked by beginners in Mathematica.
$endgroup$
– Roman
yesterday


















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