[TOC] ## 将判断外提 bad ``` for (i=0;i<cout;i++){ if(sumType == SUMTYPE_NET){ netSum = netSum+amount[i]; }else{ grossSUm = grossSum = amount[i]; } } ``` good ``` if(sumType == SUMTYPE_NET){ for (i=0;i<cout;i++){ netSum = netSum+amount[i]; } }else{ for (i=0;i<cout;i++){ grossSUm = grossSum = amount[i]; } } ``` > 此代码虽然提升了性能,但是违背了可读性,所以视情况而定 ## 合并循环 bad ``` VB 示例 For i=0 to employCount -1 employeeName(i)="" Next For i=0 to employCount -1 employeeEarnings(i)=0 Next ``` good ``` For i=0 to employCount -1 employeeName(i)="" employeeEarnings(i)=0 Next ``` ## 尽可能减少在循环内部做的工作 bad ``` // C++ 示例 for ( i=0;i<rateCount;i++){ netRate[i] = baseRate[i] * rates->discounts->factors->net } ``` good ``` // C++ 示例 quantityDiscount = rates->discounts->factors->net; for ( i=0;i<rateCount;i++){ netRate[i] = baseRate[i] * quantityDiscount; } ``` ## 哨兵值 所谓“哨兵”就是用一个特殊的值来作为数组的边界key，可以少用一条判断语句 哨兵法可以用于链表到数组 ### 示例1:数组 我们可以将数组的第一个值作为”哨兵“，数据存储下标index从1开始，则list的0号位表示暂无元素，位”哨兵“Key ``` // item 为需要查找的值 def searchItemFromArray(array, item): if len(array) == 0: return -1 firstItem = array[0] # 如果第一個元素等於查找值，直接返回 # 如果不是,则把第一个元素当做哨兵 if firstItem == item: return 0 # 赋值第一个元素为哨兵值 index = len(array) - 1 array[0] = item while array[index] != item: index -= 1 # 查找結束，把數組的首位元素改回來 array[0] = firstItem return index if(index > 0) else -1 ``` ## 示例2 bad 每次循环都要进行好几次判断 ``` found = FALSE; i = 0; while( (!found) && (i<count) ){ if(item[i] == testValue){ found = TRUE: }else{ i++; } } ``` good 把需要查找的值赋给超出范围之外的元素, ``` // 设置捎兵值, 在数组末尾捎兵值预留空间 initaValue = item[count]; item[count] = testValue; i = 0; while(item[i] != testValue){ i++; } if(i<count){ ... } ``` ## 把最忙的循环放在最内层 bad ``` //java 示例: for( column = 0; column<100;column++){ for( row = 0;row <5;row++){ sum+sum+table[row][colnum] } } ``` good ``` // java 示例: for( row = 0;row <5;row++){ for( column = 0; column<100;column++){ sum+sum+table[row][colnum] } } ```