How Google earth engine realizes the arrangement and selection of our time list

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The main purpose of this tutorial is to solve our very simple and practical function,The original problem was that this should be simple,But I didn't find a good solution.I want to create a variable that contains the earlier of two dates,But I haven't found a way to compare two dates and extract the earlier date,因为 .min() 或 .first() 不适用于列表. 

This article needs to make use of several functions：

ee.Date.fromYMD(year, month, day, timeZone)
Returns a given year、月、日的Date.

参数.
年 (整数)
月 (整数)
day (整数)
timeZone (String, default: null).
时区（例如 "America/Los_Angeles"）;默认为UTC.

返回.日期

get(unit, timeZone)
Returns the specified unit for this date.

参数.
this:date (Date)
单位（字符串）.
年"、"月"（返回1-12）、"周"（1-53）、"日"（1-31）、"小时"（0-23）、"分钟"（0-59）或 "秒"（0-59）之一.

timeZone（字符串,默认为空）.
时区（例如：'America/Los_Angeles'）;默认为UTC.

返回.Long

sort(keys)
Sort the list in ascending order.如果提供了'keys'参数,then it will be sorted first,'list'The elements in will be in the same order.

参数.
this:list (列表).
要排序的列表.

keys (List, default: null):
Optional key value,用于排序.如果提供'keys',它的长度必须与'list'相同.

返回.列表

先看代码：

var date = ee.Date.fromYMD(2022, 9, 30);var date1 = ee.Date.fromYMD(2021, 6, 30);var date2 = ee.Date.fromYMD(2020, 5, 30);var date3 = ee.Date.fromYMD(2019, 4, 30);var date3 = ee.Date(Date.now());var list = ee.List([date, date1, date2, date3]).sort();var get0 = list.get(0)print(list); print(get0); print("get3", list.get(3));

The overall solution is to merge the lists,Follow after mergingsort进行筛选,You can get a list of time series,And the list is sorted from low to high,So we just have the list merged,You can sort and choose the first one（或第 n 个）索引.If you want to keep the list separate,Then compare and find the relative position between the two lists（小于或大于）,就可以了.

Our default list：

Our sorted list：

 我们用get获取index,That is the position of the list,get0 It means to get the first parameter in the list：

 Here time is expressed in milliseconds.

 We can also acquire images in the same way,But you have to get the systemidvalue to load,Or we can use cloud images to filter for loading and so on.

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​这个公开的github可以查看：GitHub - jdbcode/Snazzy-EE-TS-GIF: Apps for creating Landsat time series animations

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(1307条消息) Google earth engine——Display animations onlineui.Thumbnail_此星光明的博客-CSDN博客

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