In C++, strncmp() compares two C-strings the same way as strcmp(), but it stops after count characters. It also stops early if it reaches a '\0'.
All three arguments are required. A result of 0 means the compared part matches. The rest of each string can still be different. Include <cstring>.
Use the third argument when you only want to compare the start of the strings.
strncmp("Hello!", "Hello?", 5);| Call | What it does |
|---|---|
strncmp("Hello!", "Hello?", 5) | Compares only the first 5 characters. Those match, so the result is 0. |
int strncmp(const char* lhs, const char* rhs, size_t count);| Parameter | Description |
|---|---|
| first | Required. The first string. |
| second | Required. The second string. |
| count | Required. The most characters to compare. |
| Return Value | Description |
|---|---|
| 0 | The compared characters are the same. |
| Less than 0 | The first string comes before the second. |
| Greater than 0 | The first string comes after the second. |
See if a string starts with "Hello".
#include <iostream>
#include <cstring>
int main() {
const char* text = "Hello World!";
if (std::strncmp(text, "Hello", 5) == 0) {
std::cout << "Starts with Hello\n";
}
return 0;
}"Hello" matches the start of text.0 means equal for that prefix.With a small count, longer differences are ignored.
#include <iostream>
#include <cstring>
int main() {
const char* a = "Hello World!";
const char* b = "Hello!";
int r5 = std::strncmp(a, b, 5);
int r10 = std::strncmp(a, b, 10);
std::cout << "First 5: " << (r5 == 0 ? "equal" : "different") << '\n';
std::cout << "First 10: " << (r10 == 0 ? "equal" : "different") << '\n';
return 0;
}Hello, so count 5 matches.count controls how far the compare goes.std::stringClearer modern alternative for the same idea.
#include <iostream>
#include <string>
int main() {
std::string text = "Hello World!";
if (text.compare(0, 5, "Hello") == 0) {
std::cout << "Starts with Hello\n";
}
return 0;
}compare(0, 5, "Hello") checks positions 0–4.0 means that slice matches.strncmp for C APIs; prefer std::string in new C++.Copy at most n characters into a destination buffer.
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