示例代码：
val todayCount = keyValue.value.filter {
val after = it.expectedArrivalDate?.after(today)
val before = it.expectedArrivalDate?.before(tomorrow)
after != null && before != null && after && before
}.size
如果it.expectedArrivalDate不是nullable，我会写这样的东西：
val todayCount = keyValue.value.filter {
it.expectedArrivalDate.after(today) && it.expectedArrivalDate.before(tomorrow)
}.size
是否可以简化我的代码？
答案 0 :(得分：4)
扩展功能或Elvis运算符的替代方法只是== true。
在Java中，==比较引用，这很危险，因为对于任何新的布尔实例而言，它都会失败。但是在Kotlin中，==使用.equals()比较值（在进行了必要的空检查之后），因此可以很好地处理这种情况。
所以您的代码可能是：
val todayCount = keyValue.value.filter {
it.expectedArrivalDate?.after(today) == true
&& it.expectedArrivalDate?.before(tomorrow) == true
}.size
类似地，如果您希望null算作true，则可以使用!= false。
可以说这不是最优雅的解决方案，但它可能是最简单的！
答案 1 :(得分：2)
您可以创建extension functions来简化该检查。
假设expectedArrivalDate的类型为Date，则可以为可空的Date类型after添加before和Date?函数，如果日期类型为{实例为null，或者如果不为null，则调用原始的after / before函数：
fun Date?.after(anotherDate: Date): Boolean = this?.after(anotherDate) ?: false
fun Date?.before(anotherDate: Date): Boolean = this?.before(anotherDate) ?: false
并保持您的代码不变：
val todayCount = keyValue.value.filter {
it.expectedArrivalDate.after(today) && it.expectedArrivalDate.before(tomorrow)
}.size
或者您可以直接在代码中直接使用实现：
val todayCount = keyValue.value.filter {
(it.expectedArrivalDate?.after(today) ?: false) && (it.expectedArrivalDate?.before(tomorrow) ?: false)
}.size
答案 2 :(得分：0)
考虑使用?:使用variable re-declaration
变量重新声明（和阴影）。
fun leftPad(value: String, length: Int? = null, char: Char? = null): String { val length = length ?: 4 val char = char ?: ' '
答案 3 :(得分：0)
不确定这是否可以简化，但是作为另一种选择，您可以：
val todayCount = keyValue.value.filter {
it.expectedArrivalDate?.takeIf{d -> d.after(today)}?.takeIf{d -> d.before(tomorrow)}?.let{true}?:false
}.size
答案 4 :(得分：0)
不带扩展功能的替代方法：由于所有比较都基于同一个字段，因此您可以在删除所有空值的同时实际映射它。
下面是一个带有伪旅行类的示例代码，因此您可以看到它进行了编译：
import sys
sys.path.insert(1, 'path')
import pandas as pandas
import matplotlib.pyplot as plt
import dash
import dash_core_components as dcc
import dash_html_components as html
import plotly.graph_objs as go
from dash.dependencies import Input, Output
import forex_factory_calendar_scraper as ffcs
import economic_calendar_reader as ecr
import time
# Fuctions for RangeSlider. Dash does not support DateTime objects
def unixTimeMillis(dt):
''' Convert datetime to unix timestamp '''
return int(time.mktime(dt.timetuple()))
def unixToDatetime(unix):
''' Convert unix timestamp to datetime. '''
return pd.to_datetime(unix,unit='s')
def getMarks(start, end, daterange, Nth=50000):
''' Returns the marks for labeling.
Every Nth value will be used.
'''
result = {}
for i, date in enumerate(daterange):
if(i%Nth == 1):
# Append value to dict
result[unixTimeMillis(date)] = str(date.strftime('%Y-%m-%d'))
return result
app = dash.Dash(__name__)
# Importing data
ffcs.get_economic_calendar("calendar.php?month=dec.2019","calendar.php?month=dec.2019")
calendar = ecr.read_economic_calendar()
#calendar = calendar.loc[calendar.Currency == 'EUR']
available_currencies = calendar.Currency.unique()
app.layout = html.Div([
html.Div([
html.Div([
dcc.Dropdown(
id='currency1',
options=[{'label': i, 'value': i} for i in available_currencies],
value='EUR'
)
],
style={'width': '48%', 'display': 'inline-block'}),
html.Div([
dcc.Dropdown(
id='currency2',
options=[{'label': i, 'value': i} for i in available_currencies],
value='USD'
)
],style={'width': '48%', 'float': 'right', 'display': 'inline-block'}),
html.Div([
dcc.Dropdown(
id='events1',
)
],
style={'width': '48%', 'display': 'inline-block'}),
html.Div([
dcc.Dropdown(
id='events2',
)
],style={'width': '48%', 'float': 'right', 'display': 'inline-block'}),
html.Div([
dcc.RadioItems(
id='disp_col1',
options=[
{'label': 'Actual', 'value':'Actual'},
{'label': 'Surprise %', 'value':'Surprise_Pct'}
],
value='Actual'
)
],
style={'width': '48%', 'display': 'inline-block'}),
html.Div([
dcc.RadioItems(
id='disp_col2',
options=[
{'label': 'Actual', 'value':'Actual'},
{'label': 'Surprise %', 'value':'Surprise_Pct'}
],
value='Actual'
)
],style={'width': '48%', 'float': 'right', 'display': 'inline-block'}),
]),
html.Div([
dcc.Graph(id='surprice_pct-ts1'),
],style={'width': '48%', 'display': 'inline-block'}),
html.Div([
dcc.Graph(id='surprice_pct-ts2'),
],style={'width': '48%', 'float': 'right', 'display': 'inline-block'}),
html.Div([
html.Div([
dcc.RangeSlider(
id='year_slider',
min = unixTimeMillis(calendar.Date.min()),
max = unixTimeMillis(calendar.Date.max()),
value = [unixTimeMillis(calendar.Date.min()),
unixTimeMillis(calendar.Date.max())]
#marks=getMarks(calendar.Date.min(),
# calendar.Date.max(), calendar.Date)
)
],style={'width': '70%', 'display': 'inline-block'})
],style={'width': '100%', 'text-align': 'center'})
])
# Left Graph ------------------------------------------------------------->>
@app.callback(
dash.dependencies.Output('surprice_pct-ts1', 'figure'),
[dash.dependencies.Input('currency1', 'value'),
dash.dependencies.Input('events1', 'value'),
dash.dependencies.Input('disp_col1', 'value'),
dash.dependencies.Input('year_slider', 'value')])
def update_ts1(currency1,events1, disp_col1, year_slider):
output = calendar[calendar.Currency == currency1]
output = output[output.Event == events1]
data = []
trace_data = go.Bar(x=list(output.Date),
y=list(output[disp_col1]))
data.append(trace_data)
layout = {'title':'Economic Indicators - {} - {}'.format(currency1,disp_col1)}
return {
'data':data,
'layout': layout
}
@app.callback(
dash.dependencies.Output('events1', 'options'),
[dash.dependencies.Input('currency1', 'value')])
def update_evets1(currency1):
output = calendar[calendar.Currency == currency1]
available_events1 = output.Event.unique()
return [{'label': i, 'value': i} for i in available_events1]
@app.callback(
dash.dependencies.Output('events1', 'value'),
[dash.dependencies.Input('currency1', 'value')])
def update_evets1(currency1):
output = calendar[calendar.Currency == currency1]
available_events1 = output.Event.unique()
return available_events1[0]
# Right Graph ------------------------------------------------------------->>
@app.callback(
dash.dependencies.Output('surprice_pct-ts2', 'figure'),
[dash.dependencies.Input('currency2', 'value'),
dash.dependencies.Input('events2', 'value'),
dash.dependencies.Input('disp_col2', 'value'),
dash.dependencies.Input('year_slider', 'value')])
def update_ts1(currency2,events2, disp_col2, year_slider):
output = calendar[calendar.Currency == currency2]
output = output[output.Event == events2]
data = []
trace_data = go.Bar(x=list(output.Date),
y=list(output[disp_col2]))
data.append(trace_data)
layout = {'title':'Economic Indicators - {} - {}'.format(currency2,disp_col2)}
return {
'data':data,
'layout': layout
}
@app.callback(
dash.dependencies.Output('events2', 'options'),
[dash.dependencies.Input('currency2', 'value')])
def update_evets1(currency2):
output = calendar[calendar.Currency == currency2]
available_events2 = output.Event.unique()
return [{'label': i, 'value': i} for i in available_events2]
@app.callback(
dash.dependencies.Output('events2', 'value'),
[dash.dependencies.Input('currency2', 'value')])
def update_evets1(currency2):
output = calendar[calendar.Currency == currency2]
available_events2 = output.Event.unique()
return available_events2[0]
if __name__ == '__main__':
app.run_server(debug=True)
将解决方案缩减到125个字符，并接近您所想到的代码。