现有数据如下：有4个城市，各3年数据，3个变量，分别为humcaptal wage house
. dataex city year humcaptal wage house

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Listed 12 out of 12 observations
还有个距离矩阵distance如下
      1     2    3
1   0      6    7
2   8     0     5
3  7     5      0
.
想要对每个城市分别生成各个年份的新变量pfl



感谢各位大神解答，不胜感激涕零