如何在 C++ 中忽略大小寫的比較兩個字串
本文將演示如何在 C++ 中比較兩個字串而忽略字母大小寫的多種方法。
使用 strcasecmp 函式比較兩個忽略大小寫的字串
strcasecmp 是 C 標準庫函式，可以使用 <cstring> 標頭檔案包含在 C++ 原始檔中。該函式本身以逐個位元組為單位進行操作，並在對應的字串評估時，返回一個小於或等於或大於 0 的整數。
即，如果忽略大小寫情況下兩個字串相等，返回值為 0。在其他情況下，當找到第一個不同的字元時，將其與值比較到其字母位置，並返回相應的結果。
#include <cstring>
#include <iostream>
#include <string>
using std::cin;
using std::cout;
using std::endl;
using std::string;
int main() {
string text1 = "Hey! Mr. Tambourine man, play a song for me";
string text2 = "hey! Mr. Tambourine man, PLAY a song for me";
string text3 = "Hey! Mrs. Tambourine man, play a song for me";
if (strcasecmp(text1.c_str(), text2.c_str()) == 0) {
cout << "strings: text1 and text2 match." << endl;
} else {
cout << "strings: text1 and text2 don't match!" << endl;
}
if (strcasecmp(text1.c_str(), text3.c_str()) == 0) {
cout << "strings: text1 and text3 match." << endl;
} else {
cout << "strings: text1 and text3 don't match!" << endl;
}
return EXIT_SUCCESS;
}
輸出：
strings: text1 and text2 match.
strings: text1 and text3 don't match!
使用 strncasecmp 函式比較兩個忽略大小寫的字串
strncasecmp 是一個上述函式的另一個變體，可用於從兩個字串中比較給定數量的字元，並且忽略大小寫。該函式採用整數值表示需要從第一個字元開始，取需要比較的最大字元數。下面的例子演示瞭如何比較兩個字串的前 5 個字元，忽略大小寫。
#include <cstring>
#include <iostream>
#include <string>
using std::cin;
using std::cout;
using std::endl;
using std::string;
constexpr int BYTES_TO_COMPARE = 5;
int main() {
string text1 = "Hey! Mr. Tambourine man, play a song for me";
string text3 = "hey! Mrs. Tambourine man, PLAY a song for me";
if (strncasecmp(text1.c_str(), text3.c_str(), BYTES_TO_COMPARE) == 0) {
printf("The first %d characters of strings: text1 and text3 match.\n",
BYTES_TO_COMPARE);
}
return EXIT_SUCCESS;
}
輸出：
The first 5 characters of strings: text1 and text3 match.
使用自定義的 toLower 函式和 == 操作符來比較兩個字串，忽略大小寫
另外，我們也可以將字串轉換為相同的情況，然後使用簡單的 == 運算子進行比較。作為一種任意的選擇，這個例子提供了用使用者定義的函式來降低兩個字串，該函式返回 string 物件。最後，if 語句可以包含比較表示式。
#include <cstring>
#include <iostream>
#include <string>
using std::cin;
using std::cout;
using std::endl;
using std::string;
std::string toLower(std::string s) {
std::transform(s.begin(), s.end(), s.begin(),
[](unsigned char c) { return std::tolower(c); });
return s;
}
int main() {
string text1 = "Hey! Mr. Tambourine man, play a song for me";
string text2 = "Hey! Mr. Tambourine man, play a song for me";
string text3 = "Hey! Mrs. Tambourine man, play a song for me";
if (toLower(text1) == toLower(text2)) {
cout << "strings: text1 and text2 match." << endl;
} else {
cout << "strings: text1 and text2 don't match!" << endl;
}
if (toLower(text1) == toLower(text3)) {
cout << "strings: text1 and text3 match." << endl;
} else {
cout << "strings: text1 and text3 don't match!" << endl;
}
return EXIT_SUCCESS;
}
輸出：
strings: text1 and text2 match.
strings: text1 and text3 don't match!
Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.
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