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 <iostream>
#include <string>
#include <cstring>
using std::cout; using std::cin;
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 <iostream>
#include <string>
#include <cstring>
using std::cout; using std::cin;
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 <iostream>
#include <string>
#include <cstring>
using std::cout; using std::cin;
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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