优化两个简单的嵌套循环

Question

我一直在尝试优化以下两个嵌套循环：

def startbars(query_name, commodity_name):

     global h_list
     nc, s, h_list = [], {}, {}
     query = """ SELECT wbcode, Year, """+query_name+""" 
                 FROM innovotable WHERE commodity='"""+commodity_name+"""' and

                 """+query_name+""" != 'NULL' """
     rows = cursor.execute(query)
     for row in rows:
         n = float(row[2])
         s[str(row[0])+str(row[1])] = n
         nc.append(n)
     for iso in result:
         try:
             for an_year in xrange(1961, 2031, 1):
                 skey = iso+str(an_year)
                 h_list[skey] = 8.0 / max(nc) * s[skey]
         except:
             pass

有什么想法吗？谢谢。

Answer 1

您的代码不完整，这使得很难提供好的建议，但是：

1. 内部循环不依赖于外部循环，因此将其从外部循环中拉出。
2. max(nc) 是第一个循环后的常量，因此将其从循环中拉出。

此外，您还需要知道当前代码有多慢，以及您需要它有多快，否则您的优化可能会错位。

你的数据结构全乱了。也许列出的内容会更快：

def startbars(query_name, commodity_name):

    assert query_name in INNOVOTABLE_FIELD_NAMES

    ## TODO: Replace with proper SQL query
    query = """ SELECT wbcode, Year, """+query_name+""" 
             FROM innovotable WHERE commodity='"""+commodity_name+"""' and

             """+query_name+""" != 'NULL' """
    rows = cursor.execute(query)

    mapYearToWbcodeToField = {}
    nc = []
    global h_list
    h_list = {}

    for row in rows:
        n = float(row[2])
        wbCodeToField = mapYearToWbcodeToField.setdefault(int(row[1]),{})
        wbCodeToField[str(row[0])] = n
        nc.append(n)

    constant = 8.0 / max(nc)


    for (an_year,wbCodeToField) in mapYearToWbcodeToField.iteritems():
        if an_year < 1961 or an_year > 2031:
            continue

        for (wbCode,value) in wbCodeToField.iteritems():
            if wbCode not in result:
                continue

            skey = wbCode+str(an_year)
            h_list[skey] = constant * value

或者将所有检查移到第一个循环中：

def startbars(query_name, commodity_name):

    assert query_name in INNOVOTABLE_FIELD_NAMES

    ## TODO: Replace with proper SQL query
    query = """ SELECT wbcode, Year, """+query_name+""" 
             FROM innovotable WHERE commodity='"""+commodity_name+"""' and

             """+query_name+""" != 'NULL' """
    rows = cursor.execute(query)

    data = []
    maxField = None

    for row in rows:
        an_year = int(row[1])
        if an_year < 1961 or an_year > 2031:
            continue

        wbCode = str(row[0])
        if wbCode not in result:
            continue

        n = float(row[2])

        data.append((wbCode+str(an_year),n))
        if maxField is None or n > maxField:
            maxField = n

    constant = 8.0 / maxField

    global h_list
    h_list = {}

    for (skey,n) in data:
        h_list[skey] = constant * n