我编写了以下 for 循环代码。主要思想是,在“A_D”列中每次出现“D”时,它都会查找应发生某些特定条件的所有可能情况。当所有条件都得到验证后,一个值将添加到列表中。
a = []
for i in df.index:
if df['A_D'][i] == 'D':
if df['TROUND_ID'][i] == ' ':
vb = df[(df['O_D'] == df['O_D'][i])
& (df['A_D'] == 'A' )
& (df['Terminal'] == df['Terminal'][i])
& (df['Operator'] == df['Operator'][i])]
number = df['number_ac'][i]
try: ## if all the conditions above are verified a value is added to a list
x = df.START[i] - pd.Timedelta(int(number), unit='m')
value = vb.loc[(vb.START-x).abs().idxmin()].FlightID
except: ## if are not verified, several strings are added to the list
value = 'No_link_found'
else:
value = 'Has_link'
else:
value = 'IsArrival'
a.append(value)
我的主要问题是 df 有数百万行,因此这个 for 循环太耗时。有没有不需要使用 for 循环的矢量化解决方案?
最佳答案
一组初始改进:使用apply
而不是循环;在 df["A_D"] == "A"
的行开头创建第二个数据框;并对值x
进行向量化。
arr = df[df["A_D"] == "A"]
# if the next line is slow, apply it only to those rows where x is needed
df["x"] = df.START - pd.Timedelta(int(df["number_ac"]), unit='m')
def link_func(row):
if row["A_D"] != "D":
return "IsArrival"
if row["TROUND_ID"] != " ":
return "Has_link"
vb = arr[arr["O_D"] == row["O_D"]
& arr["Terminal"] == row["Terminal"]
& arr["Operator"] == row["Operator"]]
try:
return vb.loc[(vb.START - row["x"]).abs().idxmin()].FlightID
except:
return "No_link_found"
df["a"] = df.apply(link_func, axis=1)
使用apply
是apparently more efficient但不会自动矢量化计算。但是,无论实现的效率如何,根据 df
的每一行在 arr
中查找值本身就很耗时。考虑原始数据帧的两个部分(其中 df["A_D"] == "A"
和 df["A_D"] == "D"
是否分别)可以以某种方式重新整形为宽格式。
编辑:您可以通过将查询字符串存储在 df
中来加快 arr
的查询速度,如下所示:
df["query_string"] = ('O_D == "' + df["O_D"]
+ '" & Terminal == "' + df["Terminal"]
+ '" & Operator == "' + df["Operator"] + '"')
def link_func(row):
vb = arr.query(row["query_string"])
try:
row["a"] = vb.loc[(vb.START - row["x"]).abs().idxmin()].FlightID
except:
row["a"] = "No_link_found"
df.query('(A_D == "D") & (TROUND_ID == " ")').apply(link_func, axis=1)
关于python - for循环pandas和numpy : Performance,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52225908/