Select entries in one column based on a condition in another column

2.23 Select entries in one column based on a condition in another column

Given

A=[4  3 
   6  4  ----> 
   7  6  ----> 
   2  1 
   1  3 
   9  2 
   2  5  ----> 
   1  2 
   ]

Select elements in the ﬁrst column only which has corresponding element in the second column greater than 3, hence the result will be

     [6 7 2]

Mathematica

mat = {{4, 3}, 
       {6, 4}, 
       {7, 6}, 
       {2, 1}, 
       {1, 3}, 
       {9, 2}, 
       {2, 5}, 
       {1, 2}}; 
 
r = Select[mat, #[[2]] > 3 &]; 
r[[All, 1]]

      {6, 7, 2}

another way is to ﬁnd the index using Position and then use Extract

loc = Position[mat, {x_, y_} /; y > 3] 
r = Extract[mat, loc]; 
r[[All, 1]]

    {6, 7, 2}

another way is to use Cases[]. This is the shortest way

Cases[mat, {x_, y_} /; y > 3 :> x]

    {6, 7, 2}

Matlab

A=[4, 3; 
   6, 4; 
   7, 6; 
   2, 1; 
   1, 3; 
   9, 2; 
   2, 5; 
   1, 2]; 
 
A(A(:,2)>3,1)

     6 
     7 
     2

Maple

In Maple, we must convert the Matrix to list of lists to work on each row at a time. I could not ﬁnd a way to avoid this.

restart; 
A:=<<4|3>, 
    <6|4>, 
    <7|6>, 
    <2|1>, 
    <1|3>, 
    <9|2>, 
    <2|5>, 
    <1|2>>; 
select(z->evalb(z[2]>3),convert(A,listlist))[..,1]

\([6, 7, 2]\)

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