title: return value of one column following index of other column
body: <p>I have a dataframe</p>
  <pre><code>name country gender
  john  us      1
  john  ca      1
  john  sa      0
  lisa  sa      1
  lisa  da      1
  lisa  us      0
  wang  da      1
  wang  sa      1
  wang  da      0
  </code></pre>
  <p>for same name, I want all the gender value to be the gender value of country US. For example, we have three johns, I want to change all the gender of john to 1 (which is the gender under us)</p>
  <p>the returned frame is</p>
  <pre><code>john  us  1
  john  ca  1
  john  sa  1
  lisa  sa  0
  lisa  da  0
  lisa  us  0
  wang  da  1
  wang  sa  1
  wang  da  0
  </code></pre>
  <p>I used groupby.transform(), but the code after transform does not work. anyone knows how to solve these kind of problems using pandas</p>