如表有a、b和c三列，获取b列的第三行，怎么获取
a b c ----------------- ----------------- ----------------- 0.133979108417407 1.754130915272981 1.42329662386328 0.668288746615872 0.734417136292905 0.732997396495193 0.862356088589877 1.797701210249215 0.393919392488897 0.629761615302414 0.627890981268138 0.500648534856737 0.001629136269912 0.623473873827606 1.101053179707378 0.359894326189533 1.729495922103524 0.568989174440503 0.791510772192851 0.435095694381744 1.597348631359637 0.708041025325656 0.219414299819618 1.387293450068682
通过tb[`col][2]获取b列的第3行
tb=table(rand(1.,50) as a,rand(2.,50) as b,rand(2.,50) as c) tb[`b][2] print(tb[`b][2])
