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2020-04-23 12:18:18
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例如，A=[1,2,3,4]、B="1256U"。
若要取A的前两位，可以A=A[0:2]，要取A的后三位，可以A=A[1:4]。
切片操作可以理解为截取[起始位置:结束位置(本身不包含，因此实际上会-1)]的部分。
再进一步，不一定两个位置都要写，有些情况下可以省略，例如A=A[0:2]可以写为A=A[:2]，:的左部分不写的话，默认是0；A=A[1:4]可以写为A=A[1:]，:的右部分不写的话，默认是列表的长度。
如果对列表（数组）下标从0开始的概念非常清晰，并且记住:的右半部分会-1，列表切片操作就非常简单。
同理，字符串截取也可以这样，例如B[:2]就等于"12"，B[2:]就等于"56U"。
补充一点，A[-2:]代表截取A的最后两位，A[-2]代表截取A的倒数第二位。也就是说负号的操作和平时的一样，只不过顺序是从尾往头，而平时的顺序是从头往尾。
PAT乙级:1023 组个最小数中就用到列表切片的操作。
1.index
index函数可以查找一个值在列表中的索引。例如A=[2,5,6,9,8,3,2]，则A.index(2)=0。但是列表A中实际上有两个2，index也提供查找的范围，假如我们要找到第二个2的索引，可以A.index(2,1(起始位置),len(A)(结束位置))，这样index函数就等同于在A[1:]中查找2的索引。
但其实有更好的方法，如果我们要查找到第二个2的索引，可以使用rfind函数。rfind函数是从列表的右边往左边查找，而index函数、find函数都是从列表的左边往右边查找。使用rfind函数查找2就可以写为A.rfind(2)。
PAT乙级:1047 编程团体赛中就同时用到了index函数和find函数。
2.count
count函数可以统计一个值在列表中出现的次数，例如A=[2,3,5,6,9,2,2]，则A.count(2)=3。
PAT乙级:1021 个位数统计中就用到了count函数。
3.find
find函数的功能与index函数一致，但是在找不到的情况下，find函数会返回-1，而index函数会直接报错，所以在某些情况下find函数更适合。
CCF-CSP_20150903:模板生成系统中就用到了find函数。
4.replace
replace可以把字符串中的某种值替换为别的值然后返回新字符串，例如A="AAACCCWW68"，则A.replace('A','D')的结果等于"DDDCCCWW68"，但这样并没有改变A的值，需要同时进行赋值操作，A=A.replace('A','D')。
PAT乙级:1039 到底买不买中就用到了replace函数。
之后会在刷题的过程中继续更新Python解题的一些小技巧和常用的API函数。
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