title: Why can't add a row to pandas dataframe?
body: <p>I have a 370864*493 dataset, I want to add a new row at the tail of the data set. I have tried Dataframe.loc[370864]=.... and Dataframe.append(). Both methods didn't work as my expectation.But the same code can work on a smaller data set, just 20 thousand rows. I hope to know the reason. The size of dataset is 1.6G. I use pandas and My IDE is spyder. The image shows more detail. The data source is UCSC cancer browser, LUAD methylation data.<a href="http://i.stack.imgur.com/1apUH.png" rel="nofollow">The tail of the dataframe</a></p>
  
  <pre><code>import pandas as pd
  import numpy as np
  from sklearn.cross_validation import train_test_split
  from sklearn.preprocessing import MinMaxScaler
  
  """
  get clinical information and count number of M0 and M1
  """
  def get_Metastasis(sampleID_list,df_clinical):
      num_M0=0
      num_M1=0
      list_Metastasis=[]
      list_Metastasis.append('Metastasis')
      for ID in sampleID_list:
          row_number=df_clinical.loc[df_clinical.sampleID==ID].index[0]
          list_Metastasis.append(df_clinical.loc[row_number,'pathologic_M'])
      for i in range(1,len(list_Metastasis)):
          if list_Metastasis[i]!="M0" and isinstance(list_Metastasis[i],str):
              list_Metastasis[i]="M1"
              num_M1+=1
              list_Metastasis[i]=1
          elif list_Metastasis[i]=="M0":
              num_M0+=1
              list_Metastasis[i]=0
  #        else:
  #            list_Metastasis[i]=None
  
      return list_Metastasis, num_M1, num_M0
  
  
  
  
  """
  read Data
  """
  path_for_clinical_data="clinical_data"                            
  path_for_genomicMatrix="genomicMatrix"
  df_clinical = pd.read_table(path_for_clinical_data)
  df_genomicMatrix = pd.read_table(path_for_genomicMatrix)
  df_genomicMatrix=df_genomicMatrix.dropna(axis=0)      ##get rid of row include nan
  """
  Add metastasis information
  """
  sampleID_list=list(df_genomicMatrix.columns.values)
  sampleID_list=sampleID_list[1:]
  list_M=[]
  
  list_M,num_M1,num_M0=get_Metastasis(sampleID_list,df_clinical)
  df_genomicMatrix.loc[len(df_genomicMatrix)]=list_M             ## Here is the problem.
  </code></pre>
  
  <p>Here is the result:</p>
  
  <pre><code>        sample  TCGA-44-4112-01  TCGA-NJ-A4YP-01  TCGA-86-8278-01  \
  485566  cg15678817          0.02110           0.0961          -0.1652   
  485567  cg14483317         -0.41520          -0.4051          -0.4117   
  485573  cg10230711         -0.42750          -0.3067          -0.4182   
  485574  cg16651827          0.22345           0.2358           0.2007   
  485576  cg07883722          0.36660           0.3932           0.4155  
  </code></pre>