title: nested for loop to append data per unique value onto list
body: <p>I have a dataframe with a column of multiple stores and the sales per date from the month of march to present. </p>
  
  <pre><code>         Date                Sales          Store   
  0         20/05/2020         581             A      
  1         19/05/2020         408             A      
  2         18/05/2020         262             A      
  3         17/05/2020         0               A  
  4         16/05/2020         1063            A  
  ... ... ... ... ... ... ... ... ... ... ... ...
  
  595       12/05/2020         245             Z  
  596       11/05/2020         13              Z  
  597       10/05/2020         165             Z  
  598       09/05/2020         240             Z  
  599       08/05/2020         163             Z  
  600   rows × 3 columns
  </code></pre>
  
  <p>I am trying to sum up the total number of sales per individual date e.g total sales for all stores on 12/05/2020 = x amount. The problem is the way the data is stored in the dataframe which makes it difficult to simply use sum(). Store A is listed first with the dates from March to present then comes store B with the dates from March per individual day till today in the present. </p>
  
  <p>I extracted the unique dates from the dataframe and converted them to an array. I don't work with python, pandas, numpy very often and thus am rubbish at using the syntax correctly. I want to create an array of the the "total sales per individual date" i.e all sales from all stores on the 01/03/2020 till today 25/05/2020. This is my code and I would appreciate if readers could help me with the syntax.</p>
  
  <pre><code>total_sales_per_date = []
  
  for i in unique_dates:
  
      for i in csv_list:
          int a
          int temp
  
          if csv_list.date[i] == unique_dates[j]:
  
          temp = list.sales[i]
  
          a = a + temp
  
          if i == rows.Length
  
          a.append(total_sales_per_date)
  </code></pre>
  
  <p>My goal is that I create two arrays of equal size and shape e.g:</p>
  
  <pre><code> unique_dates.shape = (142, 1)
     total_sales_per_date = (142, 1)
  </code></pre>
  
  <p>All suggestions, tips,example and advice will be much appreciated</p>