[英]How to return a certain boolean value in a recursive function?
I want to make a recursive function that determines if a string's characters all consist of alphabets or not. 我想做一个递归函数,确定一个字符串的字符是否全部由字母组成。 I just can't figure it out.
我只是想不通。 Here's what I've done so far but it doesn't work properly.
这是我到目前为止所做的,但无法正常工作。
bool isAlphabetic(string s){
const char *c = s.c_str();
if ((!isalpha(c[0]))||(!isalpha(c[s.size()])))
{
return false;
}
else if (isalpha(c[0]))
{
isAlphabetic(c+1);
return true;
}
}
can anyone suggest a correct way? 谁能提出正确的方法?
Leaving aside the many partial strings you'll create (consider passing in just the string and a starting index instead), the isalpha(c[s.size()])
check will always fail, since that's the \\0
at the end of the string. 撇开您将要创建的许多部分字符串(考虑只传递字符串和起始索引),
isalpha(c[s.size()])
检查将始终失败,因为这是\\0
结尾字符串。 You're also ignoring the result of the recursive calls. 您还忽略了递归调用的结果。
bool isAlphabetic(string s){
if (s.size() < 1)
return true; // empty string contains no non-alphas
const char *c = s.c_str();
if (!isalpha(c[0]))
{
return false; // found a non-alpha, we're done.
}
else
{
return isAlphabetic(c+1); // good so far, try the rest of the string
}
}
Building on Paul's answer , here is a fixed implementation that won't copy any portion of the string. 以Paul的答案为基础 ,这是一个固定的实现,不会复制字符串的任何部分。 It accomplishes this by passing a reference to the
string
object and an index to the character to check; 它通过传递对
string
对象的引用和对要检查的字符的索引来实现此目的; recursion simply adds 1 to this index to check the next character, and so on until the end of the string is found. 递归只是向该索引加1以检查下一个字符,依此类推,直到找到字符串的结尾。
I have removed your call to c_str()
since it isn't needed. 我已经删除了您对
c_str()
调用,因为它不是必需的。 string
can be directly indexed. string
可以直接索引。
bool isAlphabetic(string const & s, int startIndex = 0) {
// Terminating case: End of string reached. This means success.
if (startIndex == s.size()) {
return true;
}
// Failure case: Found a non-alphabetic character.
if (!isalpha(s[startIndex])) {
return false;
}
// Recursive case: This character is alphabetic, so check the rest of the string.
return isAlphabetic(s, startIndex + 1);
}
Note that the empty string is considered alphabetic by this function. 请注意,此功能将空字符串视为字母。 You can change this by changing
return true
to return !s.empty()
. 您可以通过将
return true
更改为return !s.empty()
来进行更改。
Here a working example: 这是一个工作示例:
#include <iostream>
#include <string>
using namespace std;
bool isAlphabetic(string s)
{
if( s.empty() )
{
return false;
}
cout << "checking: " << s[0] << endl;
if( isalpha(s[0]) )
{
return true;
}
return isAlphabetic(&s[0]+1);
}
int main()
{
string word0 = "test";
if( isAlphabetic(word0) )
{
cout << word0 << " is alphabetic" << endl;
}
else
{
cout << word0 << " is NOT alphabetic" << endl;
}
string word1 = "1234";
if( isAlphabetic(word1) )
{
cout << word1 << " is alphabetic" << endl;
}
else
{
cout << word1 << " is NOT alphabetic" << endl;
}
string word2 = "1234w";
if( isAlphabetic(word2) )
{
cout << word2 << " is alphabetic" << endl;
}
else
{
cout << word2 << " is NOT alphabetic" << endl;
}
return 0;
}
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