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C The processing of characters and strings in languages is frequent , but C The language itself has no string type , Strings are usually placed in constant strings , Or string array , String constants apply to string functions that do not modify them

String function with unlimited length

strlen function

Be careful ：size_t The meaning of shaping for unsigned

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include <string.h>
void main() {
	char c[] = "hello";// String is different from character array , There is..., by default '\0';
	char d[] = { 'h','e','l','l','o' };
	printf("%d\n", strlen(c));//5
	printf("%d\n", strlen(d));// Random value 
}

strcpy function

take source Copy string to destination On （destination If it is not an empty string, it will be source String override ）, The return value is destination The first address
The source string must be in '\0' end
The of the source string will be '\0' Copy to target string
The target string space must be large enough , To ensure that the source string can be stored
The target space has to be variable
Learn to simulate

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include<string.h>
void main() {
	char arr1[20] = { 0 };
	char arr2[] = "hello world";
	strcpy(arr1, arr2);// The return value is arr1 The first address 
	printf("%s", arr1);
}

strcat function

Append the source string data to the target string , The return value is destination The first address
The source string must be in '\0' end
The target space must be large enough , Can accommodate the contents of the source string
The target space must be modifiable
You can't add to yourself （ reason ：'\0' Being replaced cannot pause the loop ）

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include<string.h>
void main() {
	char arr1[20]="hello ";
	strcat(arr1, "world");// Append string , The return value is the first address of the target string 
	printf("%s\n", arr1);//hello world
}

strcmp function

Comparison of two strings

== Compare , The comparison is 2 Address value of a string
strcmp（str1,str2） Function to compare whether the contents of two strings are equal , If the contents of two strings are equal , Then this function will return 0

The return value of this function is as follows ：

If the return value is less than 0, said str1 Less than str2.
If the return value is greater than 0, said str1 Greater than str2.
If the return value is equal to 0, said str1 be equal to str2.

Comparison method ： Compare ASCII Code values are compared one by one from beginning to end when they appear for the first time ASCII Different code values , Then this character ASCII The string where the code big character is located is larger than another string .

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include<string.h>
void main() {
	char arr1[] = "abc";
	char arr2[] = "abc";
	if (0==strcmp(arr1,arr2))
	{
		printf(" The contents of two strings are equal ");
	}
	else if(0<strcmp(arr1,arr2))
	{
		printf("arr1 Greater than arr2");
	}
	else
	{
		printf("arr1 Less than arr2");
	}
}

Length restricted string function

strncpy function

Before the source string num Characters are copied into the target string , The return value is the first address of the target string
If the number of copies of the original array exceeds the length of the source string , Then the subsequent copy uses '\0' Add
The target space must be large enough , Can accommodate the contents of the source string
The target space must be modifiable

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include<string.h>
void main() {
	char arr1[20] = "hello";
	char arr2[] = "hi world";
	printf("%s\n", strncpy(arr1, arr2,2));//hillo
}

strncat function

Before the source string num Characters are appended to the target string , The return value is the first address of the target string
If the number of copies of the original array exceeds the length of the source string , Only append to the source string length
You can add to yourself
The target space must be large enough , Can accommodate the contents of the source string
The target space must be modifiable

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include<string.h>
void main() {
	char arr1[20] = "hello ";
	char arr2[] = "world hahaha";
	printf("%s\n", strncat(arr1, arr2,5));//hello world
}

strncmp function

take str1 String and str2 String comparison num Characters , The return value type is the same as strcmp

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include<string.h>
void main() {
	char* p = "abcd";
	char* q = "abed";
	int ret = strncmp(p, q, 2);// Compare 2 Characters 
	int res = strncmp(p, q, 3);// Compare 3 Characters 
	printf("%d\n", ret);//0
	printf("%d\n", res);//-1
}

Other string functions

strstr function

stay str1 Find whether the string contains str2 character string , If it exists, the string at the first occurrence position is returned , If it doesn't exist , Is returned null, If str2 If an empty string is passed, it returns str1

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include<string.h>
void main() {
	char arr1[] = "abcdefabcdef";
	char arr2[] = "bcd";
	// stay arr1 Find out whether it contains arr2
	printf("%s\n", strstr(arr1, arr2));//bcdefabcdef
}

strtok function

seq It's a string , Defines the set of characters used as separators
The first parameter specifies a string , It contains 0 One or more by seq A mark separated by one or more separators in a string .
strtok Function found str The next mark in , And use it ’\0' Replace end , Returns a pointer to the tag .（ Be careful ：strtok Function changes the string being manipulated , So it's using strtok The string segmented by the function is generally a temporary copy of the content , And modifiable ）.
strtok The first argument of the function is not null, Function will find str The first mark in ,strtok The function will save its position in the string
strtok The first argument to the function is null, The function will start at the same position in the string that is saved , Find next tag
If there are no more tags in the string , Then return to null The pointer

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include<string.h>
void main() {
	char arr1[] = "abcdefabcdef";
	char tmp[30] = { 0 };
	strcpy(tmp, arr1);// Temporary copy 
	char arr2[] = "cf";// Set of delimiters 
	char* ret = NULL;
	for (ret = strtok(tmp, arr2); ret != NULL; ret = strtok(NULL, arr2)) {
		printf("%s\n", ret);
	}
}

strerror function

The error code will be set when calling the library function fails ,errnum Indicates the error code , Pass the error code to strerror Function , This function will return the first address of the error message string corresponding to the error code .（ Translate the error code into error message ）

Use ：

Import header file ：include <string.h>、include <errno.h>

#include <stdio.h>
#include<string.h>
void main() {
	printf("%s\n", strerror(0));//No error
	printf("%s\n", strerror(1));//Operation not permitted
	printf("%s\n", strerror(2));//No such file or directory
	printf("%s\n", strerror(3));//No such process
	printf("%s\n", strerror(4));//Interrupted function call
}

Be careful ：errno For in C Global error code in language , To use, you must import header files include <errno.h>

#include <stdio.h>
#include<string.h>
#include <errno.h>
int main()
{
    FILE* pFile;
    pFile = fopen("test.txt", "r");
    if (pFile == NULL)
        printf("Error opening file test.txt: %s\n", strerror(errno));
    // When fopen There will be an error code when the library function call fails , Then put the error code in the back errno In this variable , then strerror The function will be based on the error code , Print out the corresponding error message 
    return 0;
}

Character classification function

isdigit function

Judge whether the function is a character function

The parameters inside pass characters ASCII Code value , If the character is a numeric character , Then return a non-zero value , If the character is not a numeric character , Then return to 0;

Use ：

Import header file ：include <ctype.h>

#include <stdio.h>
#include <ctype.h>
void main() {
	char c = '6';// Numeric character 
	char d = '@';// Nonnumeric character 
	int ret = isdigit(c);
	int res = isdigit(d);
	printf("c=%d,d=%d", ret,res);//c=4,d=0
}

islower function

Judge whether a character is lowercase

The parameters inside pass characters ASCII Code value , If the character is lowercase , Then return a non-zero value , If the character is not lowercase , Then return to 0;

Use ：

Import header file ：include <ctype.h>

#include <stdio.h>
#include <ctype.h>
void main() {
	char c = 'A';// Uppercase characters 
	char d = 'a';// Lowercase characters 
	int ret = islower(c);
	int res = islower(d);
	printf("c=%d,d=%d", ret,res);//c=0,d=2
}

Other character classification functions （ similar ）

Character conversion function

tolower function

Character to lowercase

The parameters in it are in uppercase characters ASCII Code value , This function will then convert the upper case character to the lower case character ASCII The code value returns .

Use ：

Import header file ：include <ctype.h>

#include <stdio.h>
#include <ctype.h>
void main() {
	char arr[20] = "HELLOWORLD";
	int i = 0;
	while (arr[i]!='\0')
	{
		if (isupper(arr[i])) {
			arr[i] = tolower(arr[i]);
			printf("%c", arr[i]);
		}
		i++;
	}
}
//helloworld

Be careful ：toupper Functions are similar to this （ Capitalize characters ）

Memory function

memcpy function

Memory copy function

hold source The data pointed to by space is copied to destination Go inside the space , Inside num The parameter indicates how much space to copy （ In bytes ）, The return value is destination The first address
memcpy Functions can only copy non overlapping memory

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include <string.h>
void main() {
	int arr1[10] = { 1,2,3,4,5,6,7,8,9,10 };
	int arr2[10] = { 0 };
	memcpy(arr2, arr1, 20);// take arr1 Inside 20 byte （5 Elements ） copy to arr2 in 
	int i = 0;
	for (i = 0; i < 10; i++) {
		printf("%d ", arr2[i]);
	}
	//1 2 3 4 5 0 0 0 0 0
}

#include <stdio.h>
#include <string.h>
void main() {
	int arr1[10] = { 1,2,3,4,5,6,7,8,9,10 };
	int arr2[10] = { 0 };
	int* p=(memcpy(arr2, arr1, 20));// take arr1 Inside 20 byte （5 Elements ） copy to arr2 in 
	printf("%d ", *p);//1
}

memmove function

Memory move function

hold source The spatially oriented data is moved to destination Go inside the space , Inside num The parameter indicates how much space to move （ In bytes ）, The return value is destination The first address
memmove Function can handle overlapping memory

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include <string.h>
void main() {
	int arr1[10] = { 1,2,3,4,5,6,7,8,9,10 };
	memmove(arr1+2, arr1, 20);// take arr1 Inside 20 byte （5 Elements ） Move to arr1+2 in 
	int i = 0;
	for (i = 0; i < 10; i++) {
		printf("%d ", arr1[i]);
	}
	//1 2 1 2 3 4 5 8 9 10, use memcpy Can't achieve 
}

memcmp function

Memory compare function

ptr1 And in memory ptr2 Front in memory num If the bytes are the same, then it returns 0, If ptr1 Data in memory and ptr2 The data in memory is different （ front num Within bytes ） In different data , if ptr1>ptr2 Return greater than 0 Number of numbers , if ptr1<ptr2 Then return less than 0 Number of numbers .

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include <string.h>
void main() {
	float arr1[] = { 1.0,2.0,3.0,4.0 };
	float arr2[] = { 1.0,3.0};
	int ret = memcmp(arr1, arr2, 4);// Compare the former 4 Bytes 
	int res = memcmp(arr1, arr2, 8);// Compare the former 8 Bytes 
	printf("ret=%d,res=%d\n", ret, res);//ret=0,res=-1
}

memset function

Memory setting function （ Set memory in bytes ）

hold ptr The front of the memory pointed to num” byte “ All the contents of are set to what we want value value , return ptr The first address of the memory pointed to
Integer array settings cannot be used memset

Use ：

Import header file ：include <string.h>

#include <stdio.h>
#include <string.h>
void main() {
	char arr[] = "hello world";
	char* c=memset(arr, 'x', 5);// take arr An array of former 5 Bytes changed to x
	printf("%s\n", c);//xxxxx world
}

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