How to compare strings in c language

Comparison method: 1. bcmp(), compares the first n bytes of a string to see if they are equal; 2. strcmp(), compares strings case-sensitively; 3. strcmp(), not size-sensitive Written comparison string; 4. strncmp() or strnicmp(), case-sensitive comparison of the first n characters of the string.

The operating environment of this tutorial: windows7 system, c99 version, Dell G3 computer.

Use bcmp()--Compare the first n bytes of memory (string) to see if they are equal

The prototype is:

int bcmp(const void *s1, const void * s2, int n);

[Parameters] s1, s2 are the two pieces of memory (or two strings) that need to be compared, and n is the length to be compared.

[Return value] If the first n bytes of s1 and s2 are equal or n is equal to 0, then return 0, otherwise return a non-0 value.

bcmp() function does not check for NULL.

In fact, bcmp() and memcmp() have the same function, used to compare whether the first n bytes of the memory block are equal, but the two parameters s1 and s2 are pointers, and strangely located in string. h file, it can also be used to compare strings.

Example:

#include <stdio.h>
#include <string.h>
int main ()
{
    char *s1 = "Golden Global View";
    char *s2 = "Golden Global View";
    if( !bcmp(s1, s2, 7) )
        printf("s1 equal to s2 in first 7 bytes");
    else
        printf("s1 not equal to s2 in first 7 bytes");
    return 0;
}

Use strcmp()--Compare strings (case sensitive)

The prototype is:

int strcmp(const char *s1, const char *s2);

[Parameters] s1, s2 are the two strings that need to be compared.

The comparison of string sizes is determined by the order in the ASCII code table, and this order is also the value of the characters. strcmp() first subtracts the first character value of s1 from the first character value of s2. If the difference is 0, it will continue to compare the next character. If the difference is not 0, the difference will be returned. For example, comparing the strings "Ac" and "ba" will return the difference (-33) between the characters "A" (65) and 'b' (98).

[Return value] If the strings of parameters s1 and s2 are the same, 0 will be returned. If s1 is greater than s2, a value greater than 0 is returned. If s1 is less than s2, a value less than 0 is returned.

Note: strcmp() performs comparisons in binary and does not consider multi-byte or wide-byte characters.

Example:

#include <string.h>
main(){
    char *a = "aBcDeF";
    char *b = "AbCdEf";
    char *c = "aacdef";
    char *d = "aBcDeF";
    printf("strcmp(a, b) : %d\n", strcmp(a, b));
    printf("strcmp(a, c) : %d\n", strcmp(a, c));
    printf("strcmp(a, d) : %d\n", strcmp(a, d));
}

Output:

strcmp(a, b) : 32
strcmp(a, c) :-31
strcmp(a, d) : 0

Use strictmp()--Compare strings (case-insensitive)

The prototype is:

int strcmp(char *s1, char *s2);

[Parameter description] s1, s2 are the two strings that need to be compared.

The comparison of string sizes is determined by the order in the ASCII code table, and this order is also the value of the characters. strictmp() first subtracts the first character value of s1 from the first character value of s2. If the difference is 0, it will continue to compare the next character. If the difference is not 0, the difference will be returned. For example, comparing the strings "Ac" and "ba" will return the difference (-33) between the characters "A" (65) and 'b' (98).

[Return value] If the strings of parameters s1 and s2 are the same, 0 will be returned. If s1 is greater than s2, a value greater than 0 is returned. If s1 is less than s2, a value less than 0 is returned.

Note: strictmp() is unique to Windows. The function with the same function under Linux is strcasecmp(), which is included in the header file strings.h.

Another function that compares strings without case sensitivity is strcmpi(). strcmpi() is the macro definition of strictmp(), but this function is not actually provided.

Example: Compare the size of two strings.

#include<stdio.h>
#include<string.h>
int main(void)
{
    /*定义两个字符串*/
    char* s1 = "http://see.xidian.edu.cn/cpp/u/biaozhunku/";
    char* s2 = "http://see.xidian.edu.cn/cpp/shujujiegou/";
    /*比较字符串（不分大小写）*/
    int result = stricmp(s1,s2);  // 也可以改成strcmpi()
    if(result>0)
    {
        printf("Result: s1 > s2");
    }
    else if(result<0)
    {
        printf("Result: s1 < s2");
    }
    else if(result == 0)
    {
        printf("Result: s1 = s2");
    }   
    getchar();/*等待用户输入个字符，然后退出*/
    return 0;
}

Output:

Result: s1 > s2

Use strncmp()--Compare the first n characters of the string (case sensitive)

Its prototype is:

int strncmp ( const char * str1, const char * str2, size_t n );

[Parameters] str1, str2 are the two strings that need to be compared, and n is the number of characters to be compared.

The comparison of string sizes is determined by the order in the ASCII code table, and this order is also the value of the characters. strncmp() first subtracts the first character value of s2 from the first character value of s1. If the difference is 0, then it will continue to compare the next character until the character end mark '\0'. If the difference is not 0, then Return the difference. For example, comparing the strings "Ac" and "ba" will return the difference (-33) between the characters "A" (65) and 'b' (98).

Note: The characters to be compared include the string end mark '\0', and once it encounters '\0', the comparison will end, no matter what n is, the following characters will not continue to be compared.

[Return value] If the first n characters of str1 and str2 are the same, return 0; if s1 is greater than s2, return a value greater than 0; if s1 is less than s2, return a value less than 0.

Note: If the two characters are different, GCC returns the difference between the ASCII codes corresponding to the characters, and VC returns -1 or 1. But what is embarrassing is that strnicmp(), strcmp(), and strictmp() all return -1 or 1 under GCC and VC, not the ASCII difference.

Example: Compare two sets of strings.

#include<stdio.h>
#include<string.h>
int main(void){
    char* s1 = "http://see.xidian.edu.cn/cpp/u/xitong/";
    char* s2 = "HTTP://see.xidian.edu.cn/cpp/u/xitong/";
    char* s3 = "abc";
    char* s4 = "abcxyz";
    char* s5 = "123456";
    char* s6 = "123";
    printf("s1-s2=%d\n", strncmp(s1, s2, 20));  // 是否区分大小写
    printf("s3-s4=%d\n", strncmp(s3, s4, 100));  // s3的长度小于s4
    printf("s5-s6=%d\n", strncmp(s5, s6, 100));  // s5的长度大于s6
    return 0;
}

Running results under VC6.0:

s1-s2=1
s3-s4=-1
s5-s6=1

Running results under GCC:

s1-s2=32
s3-s4=-120
s5-s6=52

Use strnicmp()--Compare the first n characters of the string (Case-sensitive)

The prototype is:

int strnicmp ( const char * str1, const char * str2, size_t n );

[Parameters] str1, str2 are the two strings that need to be compared, and n is the number of characters to be compared.

[Return value] If the first n characters of str1 and str2 are the same, return 0; if s1 is greater than s2, return a value greater than 0; if s1 is less than s2, return a value less than 0.

The difference from strncmp() is that if the two characters are different, both GCC and VC return -1 or 1 instead of returning the corresponding ASCII difference.

For other instructions about strnicmp(), please see strncmp(). This is only an example and will not be repeated.

另外一个不区分大小写比较字符串前n个字符的函数是strncmpi()，strncmpi()是strnicmp()的宏定义，实际未提供此函数。

示例：对6组字符串进行比较。

#include<stdio.h>
#include<string.h>
int main(void){
    char* s1 = "http://see.xidian.edu.cn/cpp/u/xitong/";
    char* s2 = "HTTP://see.xidian.edu.cn/cpp/u/xitong/";
    char* s3 = "abc";
    char* s4 = "abcxyz";
    char* s5 = "123456";
    char* s6 = "123";
    printf("s1-s2=%d\n", strnicmp(s1, s2, 20));  // 是否区分大小写
    printf("s3-s4=%d\n", strnicmp(s3, s4, 100));  // s3的长度小于s4
    printf("s5-s6=%d\n", strnicmp(s5, s6, 100));  // s5的长度大于s6
    return 0;
}

VC6.0下运行结果：

s1-s2=0
s3-s4=-1
s5-s6=1

GCC下运行结果：

s1-s2=0
s3-s4=-1
s5-s6=1

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