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##指定 list 元素

In[2]:= {{1.2, 3.4}, {5.6, 7.8}}[[2, 2]]

Out[2]= 7.8

In[1]:= {{3, 4}, {5, 6}}[[2]][[2]]

Out[1]= 6

In[9]:= a = {9, 8, 7, 6, 5};
a[[{3, 4}]]

Out[10]= {7, 6}

In[12]:= Range[11, 20][[Range[4, 5]]]

Out[12]= {14, 15}

Module

to define a module by

Module[{list of local variables}, things to do]

In[75]:= F[n_] := 
  Module[{tempsum}, tempsum = 0; Do[tempsum = tempsum + i, {i, 1, n}];
    tempsum];
Table[F[n], {n, 1, 10}]

Out[76]= {1, 3, 6, 10, 15, 21, 28, 36, 45, 55}

Do loop

s=0;
Do[s = s + 1; Print[i, " ", s], {i, 1, 10}]

Or simply，to repeat 10 times,

s=0;
Do[s=s+1;Print[s], 10]

Or loop through a list:

s=0;
Do[s=s+i;Print[i," ",s], {i,{1,5,6,2}}]

如何批量去掉表格如
{yy -> 8.90323265887200425958769930845,
yy -> 8.90323265943529241591021289811,
yy -> 8.90323265800195651367281533850}中的右箭头？谢谢！
追答
“->”是Mathematica里最重要指令之一Rule的简写……要“去掉”的话，可以：
yy /. List/@{yy -> 8.90323265887200425958769930845,
yy -> 8.90323265943529241591021289811,
yy -> 8.90323265800195651367281533850}

道理：

In[111]:= x /. x -> 3
Out[111]= 3

In[112]:= x /. {x -> 3}
Out[112]= 3

In[81]:= x /. {{x -> 3}}

Out[81]= {3}

In[113]:= x /. {{{x -> 3}}}
Out[113]= {{3}}


In[115]:= res = N[Solve[x^7 + x + 1 == 0, x]]
Out[115]= {{x -> -0.796544}, {x -> -0.705298 - 
    0.637624 I}, {x -> -0.705298 + 0.637624 I}, {x -> 
   0.123762 - 1.05665 I}, {x -> 0.123762 + 1.05665 I}, {x -> 
   0.979808 - 0.516677 I}, {x -> 0.979808 + 0.516677 I}}

In[116]:= x /. res
Out[116]= {-0.796544, -0.705298 - 0.637624 I, -0.705298 + 0.637624 I, 
 0.123762 - 1.05665 I, 0.123762 + 1.05665 I, 0.979808 - 0.516677 I, 
 0.979808 + 0.516677 I}

In[117]:= x /. Flatten[res]
Out[117]= -0.796544

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