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Apologies if this is an already answered question, I was unable to find anything until now. I believe the answer will be quite simple, I have just been stuck on it for a few days now.
I have a large list with two layers of sublists

vecC = {{{1., 0.217963, 0.156788, 0.0559926, 0.228347, 0.240743, 0.195968, 
   0.286898, 0.00068041, 0.161386, -0.0345246, 0.185455, 0.283423, 
   0.294763, 0.384614, 0.0994098, -0.0896094, 0.261753, 0.240743, 
   0.10724, -0.054486, 0.0710168, 0.190361, 0.0163173, 0.257922, 
   0.294763, 0.161386, 0.185455, 0.25346}, {0.217963, 1., 0.0830679, 
   0.243362, 0.28446, 0.290599, 0.152392, 0.213711, -0.00825647, 
   0.0540897, 0.0751145, 0.19444, 0.189234, 0.185116, 0.367013, 
   0.0779923, 0.0125763, 0.241502, 0.290599, 0.119696, -0.118852, 
   0.410856, 0.0225762, 0.0290714, 0.130713, 0.185116, 0.0540897, 
   0.19444, 0.155425}, {0.156788, 0.0830679, 1., 0.166155, 0.144748, 
   0.109877, 0.146391, 0.143303, 0.0996916, 0.209669, -0.00988949, 
   0.214008, 0.31862, 0.0783173, 0.132977, -0.0413557, -0.0724764, 
   0.185296, 0.109877, 0.195526, -0.00800798, 0.235018, 
   0.0625874, -0.0100889, 0.129499, 0.0783173, 0.209669, 0.214008, 
   0.207037}, {0.0559926, 0.243362, 0.166155, 1., 0.220657, 0.253729, 
   0.22768, 0.0427638, -0.0129918, 0.172457, -0.017279, 0.0382562, 
   0.142621, 0.0576319, 0.121693, -0.0276115, -0.0307733, 0.101315, 
   0.253729, 0.0717787, -0.0426187, 0.226867, 0.0225817, 0.0936401, 
   0.0795733, 0.0576319, 0.172457, 0.0382562, 0.106609}, {0.228347, 
   0.28446, 0.144748, 0.220657, 1., 0.308409, 0.165781, 0.29274, 
   0.0750658, 0.225226, 0.0769847, 0.343604, 0.464982, 0.152072, 
   0.496885, 0.150435, -0.0135166, 0.354067, 0.308409, 
   0.365327, -0.190913, 0.228264, 0.298089, 0.0178117, 0.247559, 
   0.152072, 0.225226, 0.343604, 0.170803}, {0.240743, 0.290599, 
   0.109877, 0.253729, 0.308409, 1., 0.200094, 0.247223, 0.0307646, 
   0.107573, 0.0833291, 0.335692, 0.448098, 0.316107, 
   0.357461, -0.0915362, -0.0277225, 0.487421, 1., 
   0.281485, -0.213768, 0.115793, 0.280729, 0.0422386, 0.206018, 
   0.316107, 0.107573, 0.335692, 0.289007}, {0.195968, 0.152392, 
   0.146391, 0.22768, 0.165781, 0.200094, 1., 0.191784, -0.0774805, 
   0.225797, -0.0460885, 0.132348, 0.200366, 0.294147, 0.267044, 
   0.00671384, 0.0531465, 0.246438, 0.200094, 0.0822022, -0.140849, 
   0.0953459, 0.216244, 0.00796842, 0.192004, 0.294147, 0.225797, 
   0.132348, 0.23228}}, {{0.286898, 0.213711, 0.143303, 0.0427638, 
   0.29274, 0.247223, 0.191784, 1., 0.162053, 0.22473, 0.0986637, 
   0.334587, 0.321459, 0.256726, 0.328958, 0.0415536, 0.0378936, 
   0.279013, 0.247223, 0.229389, -0.089381, 0.143923, 
   0.161668, -0.0451674, 0.207513, 0.256726, 0.22473, 0.334587, 
   0.233309}, {0.00068041, -0.00825647, 0.0996916, -0.0129918, 
   0.0750658, 0.0307646, -0.0774805, 0.162053, 1., -0.0519902, 
   0.15654, 0.174082, 0.136747, -0.0339565, 0.00992398, 
   0.0278212, -0.0382499, 0.0361599, 0.0307646, 0.0606803, 
   0.0650817, -0.00242357, 0.0397274, 0.138839, 
   0.150964, -0.0339565, -0.0519902, 0.174082, 0.186734}, {0.161386, 
   0.0540897, 0.209669, 0.172457, 0.225226, 0.107573, 0.225797, 
   0.22473, -0.0519902, 1., -0.0458312, 0.140914, 0.269852, 0.180871, 
   0.226324, 0.138795, -0.100714, 0.243389, 0.107573, 
   0.189632, -0.202975, 0.209646, 0.202957, 0.0266071, 0.133996, 
   0.180871, 1., 0.140914, 0.213322}, {-0.0345246, 
   0.0751145, -0.00988949, -0.017279, 0.0769847, 
   0.0833291, -0.0460885, 0.0986637, 0.15654, -0.0458312, 1., 
   0.347256, -0.0418872, -0.101305, -0.0335616, -0.00450231, 
   0.00888515, -0.0139867, 0.0833291, 0.154562, 0.0845733, 
   0.101801, -0.0955512, 0.190919, 0.0649846, -0.101305, -0.0458312, 
   0.347256, 0.0329942}, {0.185455, 0.19444, 0.214008, 0.0382562, 
   0.343604, 0.335692, 0.132348, 0.334587, 0.174082, 0.140914, 
   0.347256, 1., 0.361735, 0.209587, 0.290634, -0.0326136, 0.0671199, 
   0.377538, 0.335692, 0.517926, -0.100502, 0.219092, 0.173128, 
   0.0673563, 0.185353, 0.209587, 0.140914, 1., 
   0.417284}}, {{0.0949247, 0.185442, 0.0330525, 0.0650521, 0.298601, 
   0.280741, 0.155844, 0.179755, 0.175704, 0.0461148, 0.271581, 
   0.351555, 0.23109, 0.162415, 0.304256, 0.00909094, 0.135341, 
   0.294497, 0.280741, 0.258659, -0.0634038, 0.142666, 0.138513, 
   0.0401322, 0.196645, 0.162415, 0.0461148, 0.351555, 
   0.17}, {0.185455, 0.19444, 0.214008, 0.0382562, 0.343604, 0.335692,
    0.132348, 0.334587, 0.174082, 0.140914, 0.347256, 1., 0.361735, 
   0.209587, 0.290634, -0.0326136, 0.0671199, 0.377538, 0.335692, 
   0.517926, -0.100502, 0.219092, 0.173128, 0.0673563, 0.185353, 
   0.209587, 0.140914, 1., 0.417284}, {1., 1., 1., 1., 1., 1., 1., 1.,
    1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 
   1., 1., 1., 1., 
   1.}, {-0.054486, -0.118852, -0.00800798, -0.0426187, -0.190913, \
-0.213768, -0.140849, -0.089381, 0.0650817, -0.202975, 
   0.0845733, -0.100502, -0.223521, -0.082249, -0.243128, -0.023069, \
-0.0557377, -0.18627, -0.213768, -0.226736, 
   1., -0.103047, -0.125198, -0.00718238, -0.00909135, -0.082249, \
-0.202975, -0.100502, -0.115141}, {0.10724, 0.119696, 0.195526, 
   0.0717787, 0.365327, 0.281485, 0.0822022, 0.229389, 0.0606803, 
   0.189632, 0.154562, 0.517926, 0.500292, 0.276636, 
   0.296445, -0.0297005, 0.0308297, 0.430148, 0.281485, 1., -0.226736,
    0.328228, 0.310723, 0.0203472, 0.182388, 0.276636, 0.189632, 
   0.517926, 0.272819}, {0.240743, 0.290599, 0.109877, 0.253729, 
   0.308409, 1., 0.200094, 0.247223, 0.0307646, 0.107573, 0.0833291, 
   0.335692, 0.448098, 0.316107, 0.357461, -0.0915362, -0.0277225, 
   0.487421, 1., 0.281485, -0.213768, 0.115793, 0.280729, 0.0422386, 
   0.206018, 0.316107, 0.107573, 0.335692, 0.289007}, {0.0710168, 
   0.410856, 0.235018, 0.226867, 0.228264, 0.115793, 0.0953459, 
   0.143923, -0.00242357, 0.209646, 0.101801, 0.219092, 0.318488, 
   0.170722, 0.207891, 0.0396435, -0.0716237, 0.228591, 0.115793, 
   0.328228, -0.103047, 1., 0.208746, 0.126916, -0.0163755, 0.170722, 
   0.209646, 0.219092, 0.152321}, {0.249798, 0.178621, 0.0778838, 
   0.103019, 0.174959, 0.223817, 0.294007, 0.248723, 0.167965, 
   0.12488, 0.0333116, 0.37261, 0.281042, 0.274465, 0.2301, 
   0.120495, -0.0421227, 0.282859, 0.223817, 0.243693, -0.114316, 
   0.055469, 0.168794, 0.121367, 0.456702, 0.274465, 0.12488, 0.37261,
    0.473632}, {0.181817, 0.154268, 0.0616132, 0.081493, 0.24894, 
   0.263554, 0.259488, 0.325312, 0.0426164, 0.180956, -0.0349385, 
   0.266582, 0.331316, 0.199862, 0.416977, 0.0115553, 0.127469, 
   0.386543, 0.263554, 0.158443, -0.203263, 0.1385, 
   0.318175, -0.0333518, 0.122834, 0.199862, 0.180956, 0.266582, 
   0.161107}, {0.0163173, 0.0290714, -0.0100889, 0.0936401, 0.0178117,
    0.0422386, 0.00796842, -0.0451674, 0.138839, 0.0266071, 0.190919, 
   0.0673563, -0.068033, -0.126736, -0.0313185, 
   0.0977069, -0.103182, -0.0414052, 0.0422386, 
   0.0203472, -0.00718238, 0.126916, -0.0643211, 1., 
   0.0748332, -0.126736, 0.0266071, 0.0673563, 0.0262367}, {0.257922, 
   0.130713, 0.129499, 0.0795733, 0.247559, 0.206018, 0.192004, 
   0.207513, 0.150964, 0.133996, 0.0649846, 0.185353, 0.295123, 
   0.156421, 0.268833, 0.371724, -0.103355, 0.201009, 0.206018, 
   0.182388, -0.00909135, -0.0163755, 0.144191, 0.0748332, 1., 
   0.156421, 0.133996, 0.185353, 0.347716}, {0.294763, 0.185116, 
   0.0783173, 0.0576319, 0.152072, 0.316107, 0.294147, 
   0.256726, -0.0339565, 0.180871, -0.101305, 0.209587, 0.429202, 1., 
   0.30825, -0.0341961, -0.0187276, 0.342091, 0.316107, 
   0.276636, -0.082249, 0.170722, 0.345913, -0.126736, 0.156421, 1., 
   0.180871, 0.209587, 0.311079}, {0.161386, 0.0540897, 0.209669, 
   0.172457, 0.225226, 0.107573, 0.225797, 0.22473, -0.0519902, 
   1., -0.0458312, 0.140914, 0.269852, 0.180871, 0.226324, 
   0.138795, -0.100714, 0.243389, 0.107573, 0.189632, -0.202975, 
   0.209646, 0.202957, 0.0266071, 0.133996, 0.180871, 1., 0.140914, 
   0.213322}, {1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1.,
    1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 1., 
   1.}, {0.185455, 0.19444, 0.214008, 0.0382562, 0.343604, 0.335692, 
   0.132348, 0.334587, 0.174082, 0.140914, 0.347256, 1., 0.361735, 
   0.209587, 0.290634, -0.0326136, 0.0671199, 0.377538, 0.335692, 
   0.517926, -0.100502, 0.219092, 0.173128, 0.0673563, 0.185353, 
   0.209587, 0.140914, 1., 0.417284}}}

I also have specific positions from this list

posC = {{{1}, {2}, {3}, {4}, {5}, {6}, {7}}, {{8}, {9}, {10}, {11}, 
{12}}, {{12}, {12}, {1}, {21}, {20}, {6}, {22}, {29}, {15}, {24}, 
{25}, {14}, {10}, {1}, {12}}}

Now I'm trying to map the positions to the list. But have been unable to find an efficient way. Currently, I'm using:

alignedRes =  Function[{l}, 
   MapThread[
    vecC[[l, #1, #2]] &, {Range[1, Length@vecC[[l]]], 
     Flatten@posC[[l]]}]] /@ Range[1, Length@vecC]

(*{{1., 1., 1., 1., 1., 1., 1.}, {1., 1., 1., 1., 
  1.}, {0.351555, 1., 1, 1., 1., 1., 1., 0.473632, 0.416977, 1., 1., 
  1., 1., 1, 1.}}*)

However, I believe there must be a better way to do this. I've tried Map and MapIndexed, yet to no avail.
Thanks in advance,

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1 Answer 1

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You can use MapThread twice together with Extract:

alignedRes2 = MapThread[MapThread[Extract]@*List, {vecC, posC}];

alignedRes == alignedRes2
(* True *)

Ideally, you could use MapThread[Extract, {vecC, posC}, 2] directly, but MapThread does not support ragged lists at the moment, so we need to manually nest it. For this, we can use the operator form of MapThread for the inner one. We use Composition (@*) together with List to convert the two arguments that the outer MapThread gives us into a single list that the inner MapThread expects. Alternatively, you could write e.g.:

alignedRes2 = MapThread[MapThread[Extract, {##}] &, {vecC, posC}]
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1
  • $\begingroup$ Thank you, that's perfect. I'm gonna look into the syntax more $\endgroup$
    – Tim B
    Commented Oct 23, 2019 at 11:24

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