理解MapReduce

1. 用Python编写WordCount程序并提交任务

程序	WordCount
输入	一个包含大量单词的文本文件
输出	文件中每个单词及其出现次数（频数），并按照单词字母顺序排序，每个单词和其频数占一行，单词和频数之间有间隔

1. 编写map函数，reduce函数

   import sys
   for i in stdin:
        i = i.strip()
        words = i.split()
        for word in words:
            print '%s	%s' % (word,1)

   #reduce函数
   from operator import itemgetter
   import sys
    
   current_word = None
   current_count = 0
   word = None
    
   for line in sys.stdin:
       line = line.strip()
       word, count = line.split('	', 1)
       try:
           count = int(count)
       except ValueError: 
           continue
       if current_word == word:
           current_count += count
       else:
           if current_word:
               print "%s	%s" % (current_word, current_count)
           current_count = count
           current_word = word
    
   if word == current_word: 
       print "%s	%s" % (current_word, current_count)

2. 将其权限作出相应修改

   chmod a+x /home/hadoop/mapper.py
   chmod a+x /home/hadoop/wc/reducer.py

3. 本机上测试运行代码

   echo "foo foo quux labs foo bar quux" | /home/hadoop/wc/mapper.py
   echo "foo foo quux labs foo bar quux" | /home/hadoop/wc/mapper.py | sort -k1,1 | /home/hadoop/wc/reducer.p

   

4. 放到HDFS上运行
   1. 将之前爬取的文本文件上传到hdfs上
   2. 用Hadoop Streaming命令提交任务
      
5. 查看运行结果

2. 用mapreduce 处理气象数据集

编写程序求每日最高最低气温，区间最高最低气温

1. 气象数据集下载地址为：ftp://ftp.ncdc.noaa.gov/pub/data/noaa
2. 按学号后三位下载不同年份月份的数据（例如201506110136号同学，就下载2013年以6开头的数据，看具体数据情况稍有变通）
3. 解压数据集，并保存在文本文件中
4. 对气象数据格式进行解析
5. 编写map函数，reduce函数
6. 将其权限作出相应修改
7. 本机上测试运行代码
8. 放到HDFS上运行
   1. 将之前爬取的文本文件上传到hdfs上
   2. 用Hadoop Streaming命令提交任务
9. 查看运行结果

cd /usr/hadoop
sodu mkdir qx
cd /usr/hadoop/qx

wget -D --accept-regex=REGEX -P data -r -c ftp://ftp.ncdc.noaa.gov/pub/data/noaa/2009/4*  

zcat data/ftp.ncdc.noaa.gov/pub/data/noaa/2009/4*.gz >qxdata.txt

chmod a+x /home/hadoop/qx/mapper.py
chmod a+x /home/hadoop/qx/reducer.py

import sys

for line in sys.stdin:
    line = line.strip()
    print('%s	%d' % (line[15:23], int(line[87:92])))

from operator import itemgetter
import sys

current_date = None
current_temperature = 0
date = None

for line in sys.stdin:
    line = line.strip()
    date, temperature = line.split('	', 1)
    try:
        temperature = int(temperature)
    except ValueError:
        continue

    if current_date == date:
        if current_temperature < temperature:
            current_temperature = temperature
    else:
        if current_date:
            print('%s	%d' % (current_date, current_temperature))
        current_temperature = temperature
        current_date = date

if current_date == date:
    print('%s	%d' % (current_date, current_temperature))