execution of parforlop in matlab - matlab

Question

I have sample code emulating my actual code. Where I have cell arrays outside the parfor loop. I have to perform computations on strings and numerical outputs will be stored in arrays which I can write to a csv file after each parfor loop. So I made dummy code. But I couldn't get it to execute. The error message is: "subscription mismatch at line 6".
ftemp=fopen('temp.csv','w');
march=cell(1,20);tc=0;
march={'ab' 'cd' 'ef' 'gh' 'ij' 'kl' 'mn' 'op' 'qr' 'st' 'uv' 'AB' 'CD' 'EF' 'GH' 'IJ' 'KL' 'MN' 'OP' 'QR'};
for i=1:10
matlabpool open 4;
parfor j=1:1:20
a(j,1)=randi(200,1,1);
b(j,2)=j+tc;
c(j,3)=march{1,j};
d(j,4)=(randi(200,1,1)/200);
end
fprintf(ftemp,'%d\t%d\t%s\t%f',a,b,c,d);
matlabpool close
clear a b c d;
tc=tc+20;
end
fclose(ftemp);
quit

Answer 1

The cause of the Error is that you are trying to assign a cell into an array in line 9.
I made some changes into your code, they are described in comments.
ftemp=fopen('temp.csv','w');
march=cell(1,20);tc=0;
march={'ab' 'cd' 'ef' 'gh' 'ij' 'kl' 'mn' 'op' 'qr' 'st' 'uv' 'AB' 'CD' 'EF' 'GH' 'IJ' 'KL' 'MN' 'OP' 'QR'};
for i=1:10
parpool local; % here in my version of Matlab 2015 there are no more "matlabpool" if it doesen't work change it back
parfor j=1:20
%i changed variable a(j,1) into a(j) and b(j,2) into b(j): may
%contain empty arrays , idem: for c and d
a(j)=randi(200,1,1);
b(j)=j+tc;
c{j}=march{1,j}; % changed c(j)= march{1,j}; : cause of error
d(j)=randi(200,1,1)/200;
end
fprintf(ftemp,'%d\t%d\t%c\t%d',a,b,char(c),d); % char(c) in order to convert cell array to array of strings
delete(gcp) % there isn't such thing "matlabpool close" , the right expression is "delete(gcp)"
clear a b c d;
tc=tc+20;
end
fclose(ftemp);
%quit : i remove because it closes matlab , please put it back if u really
%want to close matlab after operation
Note: if parpool doesent work for your version of matlab , please change it back to the expression you were using matlabpool open 4;

Answer 2

Well for the unexpected outputs and number of rows in the output , it's because the wrong use of fprintf . We need to print element by element , which means the fprintf has to be inside the parfor loop, so your code should look like this:
ftemp=fopen('temp.csv','w');
march=cell(1,20);tc=0;
march={'ab';'cd';'ef';'gh';'ij';'klm';'mn';'op';'qr';'st';'uv';'ABls';'CD';'E3F';'GH';'IJ';'dynaKL';'MN';'OP';'QR'};
matlabpool open 4;
for k=1:10
%parpool local; % here in my version of Matlab 2015 there are no more "matlabpool" if it doesen't work change it back
parfor j=1:20
%i changed variable a(j,1) into a(j) and b(j,2) into b(j): may
%contain empty arrays , idem: for c and d
a(j)=randi(200,1,1);
b(j)=j+tc;% indexing purposes to identify the order when parallel processing going on
if length(march{j})>2 % this kind of conditions and computations are there in my actual code
c{j}='skip';
else
c{j}=march{j};
end
%d(j)=randi(200,1,1)/200;
fprintf(ftemp,'%d\t%d\t%s\t\n',a(j),b(j),c{j}); % char(c) in order to convert cell array to array of strings
end
clear a b c;
tc=tc+20;
end
fclose(ftemp);
matlabpool close;
the output is:
9 1 ab
171 2 cd
7 3 ef
98 4 gh
102 5 ij
20 6 skip
53 7 mn
174 8 op
36 9 qr
30 10 st
130 11 uv
127 12 skip
133 13 CD
65 14 skip
118 15 GH
130 16 IJ
139 17 skip
168 18 MN
57 19 OP
25 20 QR
22 21 ab
39 22 cd
83 23 ef
22 24 gh
159 25 ij
40 26 skip
164 27 mn
78 28 op
194 29 qr
88 30 st
125 31 uv
1 32 skip
2 33 CD
112 34 skip
161 35 GH
170 36 IJ
55 37 skip
59 38 MN
18 39 OP
134 40 QR
80 41 ab
118 42 cd
108 43 ef
174 44 gh
97 45 ij
157 46 skip
85 47 mn
98 48 op
40 49 qr
11 50 st
171 51 uv
139 52 skip
90 53 CD
70 54 skip
173 55 GH
150 56 IJ
186 57 skip
155 58 MN
136 59 OP
96 60 QR
158 61 ab
118 62 cd
124 63 ef
127 64 gh
26 65 ij
124 66 skip
91 67 mn
186 68 op
63 69 qr
137 70 st
170 71 uv
98 72 skip
132 73 CD
80 74 skip
160 75 GH
20 76 IJ
156 77 skip
142 78 MN
110 79 OP
51 80 QR
18 81 ab
29 82 cd
40 83 ef
49 84 gh
102 85 ij
113 86 skip
96 87 mn
44 88 op
166 89 qr
90 90 st
21 91 uv
60 92 skip
44 93 CD
166 94 skip
103 95 GH
123 96 IJ
77 97 skip
163 98 MN
138 99 OP
111 100 QR
94 101 ab
133 102 cd
158 103 ef
13 104 gh
26 105 ij
117 106 skip
90 107 mn
58 108 op
156 109 qr
79 110 st
196 111 uv
168 112 skip
192 113 CD
160 114 skip
56 115 GH
129 116 IJ
191 117 skip
157 118 MN
170 119 OP
30 120 QR
137 121 ab
128 122 cd
12 123 ef
38 124 gh
38 125 ij
122 126 skip
97 127 mn
129 128 op
142 129 qr
154 130 st
99 131 uv
85 132 skip
129 133 CD
111 134 skip
108 135 GH
78 136 IJ
102 137 skip
48 138 MN
100 139 OP
89 140 QR
114 141 ab
12 142 cd
184 143 ef
145 144 gh
5 145 ij
48 146 skip
92 147 mn
64 148 op
87 149 qr
32 150 st
136 151 uv
103 152 skip
90 153 CD
73 154 skip
28 155 GH
191 156 IJ
63 157 skip
120 158 MN
104 159 OP
178 160 QR
148 161 ab
36 162 cd
129 163 ef
11 164 gh
172 165 ij
186 166 skip
145 167 mn
142 168 op
150 169 qr
185 170 st
14 171 uv
141 172 skip
22 173 CD
163 174 skip
48 175 GH
164 176 IJ
117 177 skip
25 178 MN
110 179 OP
111 180 QR
175 181 ab
60 182 cd
195 183 ef
44 184 gh
163 185 ij
4 186 skip
103 187 mn
95 188 op
127 189 qr
10 190 st
11 191 uv
182 192 skip
162 193 CD
179 194 skip
76 195 GH
104 196 IJ
153 197 skip
103 198 MN
4 199 OP
154 200 QR
Also the output has 200 rows.

Answer 3

ftemp=fopen('temp.csv','w');
march=cell(1,20);tc=0;
march={'ab';'cd';'ef';'gh';'ij';'klm';'mn';'op';'qr';'st';'uv';'ABls';'CD';'E3F';'GH';'IJ';'dynaKL';'MN';'OP';'QR'};
matlabpool open 4;
for k=1:10
%parpool local; % here in my version of Matlab 2015 there are no more "matlabpool" if it doesen't work change it back
parfor j=1:20
%i changed variable a(j,1) into a(j) and b(j,2) into b(j): may
%contain empty arrays , idem: for c and d
a(j)=randi(200,1,1);
b(j)=j+tc;% indexing purposes to identify the order when parallel processing going on
if length(march{j})>2 % this kind of conditions and computations are there in my actual code
c{j}='skip';
else
c{j}=march{j};
end
%d(j)=randi(200,1,1)/200;
end
fprintf(ftemp,'%d\t%d\t%s\n',a,b,c{:}); % char(c) in order to convert cell array to array of strings
clear a b c;
tc=tc+20;
end
fclose(ftemp);
matlabpool close;
but the output of the program i coudlnt get it
130 127 Abf5ȱ3³]À#¾
1 2

97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82 168 57 ®$ «E�,vp|}¤
21 22 !"#$%&'(
97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82 88 125 S([g 0'0Â"
41 42 +,-./0123456789:;<
97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82 161 170 7Na�U;6´q¶nl®v¤
61 62 ?#ABCDEFGHIJKLMNOP
97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82 171 139 ZF­ºPµ~R(`b[
81 82 STUVWXYZ[\]^_`abcd
97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82 132 80 ||£)Á1[ªavb-
101 102 ghijklmnopqrstuvwx
97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82 186 63 ª`neepw»fq°r1
121 122 {|}~�
97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82 51 160 M(1´uOAF¦{¬©
141 142 ��� 
97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82 90 21 <,uZgh�lu¨B,¦
0
161 162 £¤¥¦§¨©ª«¬­®¯°±²³´
97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82 192 160 o£¿^hyµÄ¨�Ä�O
181 182 ·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈ
97 98 cd
101 102 gh
105 106 skip
109 110 op
113 114 st
117 118 skip
67 68 skip
71 72 IJ
115 107 ip
77 78 OP
81 82
i mean to say why only 124 iterations took place instead of 200. and why these arbitrary outputs are there in the 3rd column