title: Create an empty data frame with index from another data frame
body: <p>I've got a data frame <strong>df1</strong> with multiple columns and rows. Simple example:</p>
  
  <pre><code>    TIME T1  T2 
         1 10 100
         2 20 200
         3 30 300
  </code></pre>
  
  <p>I'd like to create an empty data frame <strong>df2</strong> and later on, add new columns with the calculation results.</p>
  
  <p>For this moment my code looks like this:
  </p>
  
  <pre><code>     df1=pd.read_csv("1.txt",index_col="TIME")
  
       df2=df1.copy()[[]] #copy df1 and erase all columns
  </code></pre>
  
  <p>...adding two new columns:
  </p>
  
  <pre><code>     df2["results1"],df2["results2"]=df1["T1"]*df["T2"]*3,df1["T2"]+100
  </code></pre>
  
  <p>Is there any better/safer/faster way to do this ? 
  Is it possible to create an empty data frame df2 and only copy index from df1 ?</p>