Python, For loop: How to append the TRUE value[i] in the next cells[i+1,i+2,..] until the condition become FALSE

问题

Could someone help with a logical scheme in for loop statement in Python? I try to be more clear: i want to build a for loop cycle where if a condition is TRUE i want to append the same value for the next cells until another condition become TRUE.

For example: i have a dataframe called df with 3 columns. The first column is the closing price of a stock (close_price), the second column contains the fast exponential moving average (fast_ema) and the third column contains the slow exponential moving average (ema_slow). At this point I run the code below:

list = []

for i in range(1,len(df)):
  
  # FIRST CONDITION:
  if (df.ema_fast[i-1] < df.ema_slow[i-1]  and df.ema_fast[i] > df.ema_slow[i]:
  
  list.append(df.close_price[i]) # i want to append close.close[i] for al the next cells i (i+1, i+2,...)
  #until the SECOND condition become TRUE. 
  
  # SECOND CONDITION:
  elif if (df.fast_ema[i-1] > df.ema_slow[i-1] and df.ema_fast[i] > df.ema_slow[i]:
  list.append(df.close_price[i])

  else: 

  list.append(0)

when the short ema cross over the slow ema this code append the close_price in the list and then, if the ema_fast cross down the ema_slow the code appends the close_price when the crossdown is occured. Otherwise the code appends 0.

A this point, if I assume that cross over occured in data 2019-08-19 and the cross down occured in data 2019-08-27, I get:

data         Price
2019-08-19   df.close_price[i=2019-08-19] # The closing price in data 2019-08-19
xxxx-xx-xx   0
xxxx-xx-xx   0
xxxx-xx-xx   0
xxxx-xx-xx   0
xxxx-xx-xx   0
xxxx-xx-xx   0
xxxx-xx-xx   0
2019-08-27   df.close_i[i=2019-08-27] # The closing price in data 2019-08-27
xxxx-xx-xx   0
xxxx-xx-xx   0

Now i want:

2019-08-19   df.close_price[i=2019-08-19] # The close in data 2019-08-19
xxxx-xx-xx   df.close_price[i=2019-08-19] # The close in data 2019-08-19
xxxx-xx-xx   df.close_price[i=2019-08-19] # The close in data 2019-08-19
xxxx-xx-xx   df.close_price[i=2019-08-19] # The close in data 2019-08-19
xxxx-xx-xx   df.close_price[i=2019-08-19] # The close in data 2019-08-19
xxxx-xx-xx   df.close_price[i=2019-08-19] # The close in data 2019-08-19
xxxx-xx-xx   df.close_price[i=2019-08-19] # The close in data 2019-08-19
xxxx-xx-xx   df.close_price[i=2019-08-19] # The close in data 2019-08-19
2019-08-27   *df.close_price[i=2019-08-27]* # The close in data 2019-08-27
xxxx-xx-xx   0
xxxx-xx-xx   0

I'm not a python expert and I hope I was clear enough. Thank you in advance, and if someone decide to help me I will be very grateful.

回答1:

You can use a variable, here adding, to both note that you have encountered condition 1, remember the value you got then, and also to see that you have not encountered condition 2 yet, so can keep adding it:

adding = None

for i in range(1,len(close)):  
  if first_condition():  
    adding = close.close[i]
    list.append(adding) # i want to append close.close[i] for al the next cells i (i+1, i+2,...)

  #until the SECOND condition become TRUE.   
  # SECOND CONDITION:
  elif if (close.fast_ema[i-1] > close.int_ema[i-1] and close.fast_ema[i] > close.slow_ema[i]:
    list.append(close.close)
    adding = None

  else: 
    if adding is not None:
      list.append(adding)
    else:
      list.append(0)

来源：https://stackoverflow.com/questions/63919473/python-for-loop-how-to-append-the-true-valuei-in-the-next-cellsi1-i2

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