# 3.1节 循坏for,while,map,evaluate和iterate的异同 * 利用 landsat8 的数据，导出 2013-2020 年武汉市年平均 NDVI 值的 GIF 图 * 实现同一个目的，分别展示for,map,evaluate,iterate 之间的不同用法 ## 1 结果显示 | | | | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------: | :---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------: | | ![NDBI](https://357480708-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M4gCiCCbz0Z842iX9K9%2F-M4i_IzhikZeDrk0zW2M%2F-M4i_MktLJHkikVSRevG%2Fgif-ndvi2.gif?generation=1586698226878504\&alt=media) | ![EVI](https://357480708-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M4gCiCCbz0Z842iX9K9%2F-M4i_IzhikZeDrk0zW2M%2F-M4i_MkvWn23Wki5GDUY%2Fgif-true2.gif?generation=1586698226496332\&alt=media) | | NDVI | 真彩色 | ## 2 异同 * for、while：就是基本的循环结构语句，但是在GEE里很少用，基本用 map 替代，因为是顺序执行代码，效率太慢 * map：用的最多，执行效率最快，相当于“并行执行任务”，能用 map,就用 map，但是函数里一般不能用 print,Map,toDrive 等方法 * iterate：“迭代”，顺序执行，每次执行都在前面执行的结果上继续执行，很遗憾，这次的例子没有体现“迭代”的意思 * evaluate：主要用来异步导出和显示，加快速度，一般没有返回值，结合 for 和 map 一起用，这里的 map 里可以有 print,Map,toDrive 方法 ## 3 代码 ```javascript // 去云操作 var maskcloud = function(image) { var qa = image.select('BQA'); var mask = qa.bitwiseAnd(1 << 4) .or(qa.bitwiseAnd(1 << 8)); return image.updateMask(mask.not()); }; // 计算NDVI var ndvi = function(image){ var ndvi = image.normalizedDifference(['B5','B4']).rename('NDVI'); return image.addBands(ndvi) }; var roi = ee.FeatureCollection("users/comingboy1118/China/CH_shi"); var roi = roi.filter(ee.Filter.eq("市","武汉市")).geometry() Map.addLayer(roi,{"color":'red',},"roi") Map.centerObject(roi, 8) // for 很少用，顺序执行，计算慢 var time = ["2013","2014","2015","2016","2017","2018","2019","2020"]; var list = ee.List([])// for(var index in time){ var year = time[index]+"-01"+"-01" var image = ee.ImageCollection("LANDSAT/LC08/C01/T1_RT_TOA") .filterBounds(roi) .filterDate(year,ee.Date(year).advance(1,"year")) .map(maskcloud) .map(ndvi) .median() .select(["B2","B3","B4","NDVI"],['blue','green','red','NDVI']) .clip(roi) .setMulti({"system:index":year,'system:time_start':ee.Date(year)}) list = list.add(image) } var imgMonth = ee.ImageCollection.fromImages(list); print(imgMonth,"imgMonth_for") // map 一般计算最快，用的最多，异步计算,能用map,就用map，但是函数里一般不能用print,Map,toDrive var time = ee.List.sequence(2013,2020) var imgMonth = time.map(function(year){ var year = ee.Number(year).format("%04d").cat("-01-01") var image = ee.ImageCollection("LANDSAT/LC08/C01/T1_RT_TOA") .filterBounds(roi) .filterDate(year,ee.Date(year).advance(1,"year")) .map(maskcloud) .map(ndvi) .median() .select(["B2","B3","B4","NDVI"],['blue','green','red','NDVI']) .clip(roi).setMulti({"system:index":year,'system:time_start':ee.Date(year)}) return image }) var imgMonth = ee.ImageCollection(imgMonth) print(imgMonth,"imgMonth_map") // itetate 迭代的方式和 for 很相似，都是往原list添加元素 var time = ee.List.sequence(2013,2020) var filterImg = function(year,first){ var year = ee.Number(year).format("%04d").cat("-01-01") var image = ee.ImageCollection("LANDSAT/LC08/C01/T1_RT_TOA") .filterBounds(roi) .filterDate(year,ee.Date(year).advance(1,"year")) .map(maskcloud) .map(ndvi) .median() .select(["B2","B3","B4","NDVI"],['blue','green','red','NDVI']) .clip(roi).setMulti({"system:index":year,'system:time_start':ee.Date(year)}) return ee.List(first).add(image); } var imgMonth = time.iterate(filterImg,ee.List([])) var imgMonth= ee.ImageCollection.fromImages(imgMonth) print(imgMonth,"imgMonth_iterate") // evaluate --主要用来异步导出和显示，一般没有返回值，结合for 和map 一起用，这里的map里可以有print,Map,toDrive方法 var time = imgMonth.reduceColumns(ee.Reducer.toList(), ['system:index']) .get('list'); var visParams = {min: -0.8, max: 0.8, palette: ['blue', 'white','green']}; time.evaluate(function(years) { years.map(function(year){ var image = ee.Image(imgMonth.filter(ee.Filter.eq("system:index", year)).first()); Map.addLayer(image.select("NDVI"), visParams, year); }) }) // gif - NDVI var params01 = { crs: 'EPSG:3857', framesPerSecond: 2, region: roi, min: -0.8, max: 0.8, bands: ["NDVI"], dimensions: 512, format:"gif", palette: ['blue', 'white','green'] }; print(ui.Thumbnail({image:imgMonth, params:params01})) print(imgMonth.getVideoThumbURL(params01)); // gif - 真彩色 var params02 = { crs: 'EPSG:3857', framesPerSecond: 2, region: roi, min: 0, max: 0.3, bands: ["red","green","blue"], dimensions: 512, format:"gif", }; print(ui.Thumbnail({image:imgMonth, params:params02})) print(imgMonth.getVideoThumbURL(params02)); ``` ### 激励自己，尽可能每周更新1-2篇，2020加油！！！ ### 需要交流或者有项目合作可以加微信好友 (备注GEE) ### 微信号：comingboy0701 --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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