title: How to add or subtract values from multiple dataframe
body: <p>Hello I have 3 different dataframes whose values I want to add or subtract values.</p>
  <p>I have the following dataframes below and the code I used to add the values all together but I encountered an error. I would appreciate any help, thank you very much.</p>
  <pre><code>df1 = pd.read_csv('/content/drive/MyDrive/노후교량/DZ_Middle_G/20km_hr_lane1_acc_middle_girder.csv')
  df1.head()
  
  output:
  
          TIME    DZ_middle_G_L1_20km
  0       0.001   1.021e-003
  1       0.002   1.597e-003
  2       0.003   1.564e-003
  3       0.004   1.031e-003
  4       0.005   3.022e-004
  
  
  df2 = pd.read_csv('/content/drive/MyDrive/노후교량/DZ_Middle_G/20km_hr_lane2_acc_middle_girder.csv')
  df2.head()
  
  output:
  
      TIME    DZ_middle_G_L2_20km
  0   0.001   -6.168e-005
  1   0.002   -9.240e-005
  2   0.003   -7.781e-005
  3   0.004   -1.962e-005
  4   0.005    6.591e-005
  
  
  df3 = pd.read_csv('/content/drive/MyDrive/노후교량/DZ_Middle_G/25km_hr_lane1_acc_middle_girder.csv')
  df3.head()
  
  output:
  
      TIME    DZ_middle_G_L1_25km
  0   0.001   1.277e-003
  1   0.002   1.996e-003
  2   0.003   1.955e-003
  3   0.004   1.288e-003
  4   0.005   3.777e-004
  </code></pre>
  <p>I used this code to add the values in the dataframes:
  This code is suppose to add the values of df2 to df1 starting from TIME = 3
  and also add the values of df3 to df2 starting from TIME = 6.</p>
  <pre><code>df = df1.merge(df2.assign(TIME=df2['TIME']+3), how='outer') \
      .merge(df3.assign(TIME=df3['TIME']+6), how='outer')
  df['DZ_middle_G_L2_20km'] += df['DZ_middle_G_L1_20km'].fillna(0)
  df['DZ_middle_G_L1_25km'] += df['DZ_middle_G_L2_20km'].fillna(0)
  </code></pre>
  <p>However I got this Error:</p>
  <pre><code>TypeError: can only concatenate str (not &quot;float&quot;) to str    
  </code></pre>
  <p>Here is an example of how I want the result will be:</p>
  <p>Final DataFrame:</p>
  <pre><code>TIME df1              df2              df3
  0    1.021e-003
  1    1.597e-003
  2    1.564e-003
  3    1.031e-003       -6.168e-005
  4    3.022e-004       -9.240e-005
  5                     -7.781e-005
  6                     -1.962e-00       1.277e-003
  7                      6.591e-005      1.996e-003
  8                                      1.955e-003
  9                                      1.288e-003
  10                                     3.777e-004                   
  </code></pre>
  <p>looks like this but the values are added together or subtracted.
  I did not change the values here but the values should be added or subtracted to each other as they are combined.</p>