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“Essential Knowledge Points for C Language Beginners: 100 Articles Series“

The following notes finally enter the practical series, which is also the most important and difficult part of C language. Keep it up, young learners!

34. Summary of Character Arrays and String Usage Techniques in C Language: Thoroughly Understand String Processing!

1. Character Arrays

A character array is the basic data structure for storing characters in C language and can be used to store strings. However, note that the last character must be ‘\0’; otherwise, it is a character array, not a string.

char str[10];  // Can store up to 9 characters + 1 '\0'

String Characteristics

• Ends with a null character<span>'\0'</span> (ASCII 0)
• Actual length ≤ array length – 1
• Stored in contiguous memory

2. String Initialization Methods

1. Character-by-character Initialization

char str1[5] = {'H', 'e', 'l', 'l', 'o'};  // Not a string, just a character array (no '\0')
char str2[6] = {'H', 'e', 'l', 'l', 'o', '\0'};  // Valid string

2. String Literal Initialization

char str3[6] = "Hello";  // Automatically adds '\0'
char str4[] = "World";   // Automatically calculates length (6)

3. Pointer Initialization

char *str5 = "Hello";  // Stored in read-only data segment

3. String Input and Output

1. Standard Input and Output

char name[20];
printf("Enter name: ");
scanf("%19s", name);  // Limit length to prevent overflow
printf("Hello, %s\n", name);

2. Safe Input Functions

fgets(name, sizeof(name), stdin);  // Includes newline character

3. Character-by-character Processing

int i = 0;
while (str[i] != '\0') {
    putchar(str[i++]);
}

4. String Processing Functions

1. Common Functions in <string.h>

Function	Description	Example
strlen	Calculate string length	strlen(“Hello”) → 5
strcpy	String copy	strcpy(dest, src)
strncpy	Safe string copy	strncpy(dest, src, n)
strcat	String concatenation	strcat(s1, s2)
strncat	Safe string concatenation	strncat(s1, s2, n)
strcmp	String comparison	strcmp(s1, s2)
strchr	Find first occurrence of a character	strchr(str, ‘l’)
strstr	Find first occurrence of a substring	strstr(str, “ll”)

2. Usage Example

char s1[20] = "Hello";
char s2[] = " World";
strcat(s1, s2);  // s1 becomes "Hello World"

5. Strings and Pointers

1. Pointer Traversal of Strings

char *p = "Hello";
while (*p) {
    // Postfix ++ has higher precedence than dereference *, postfix returns original value first, then increments
    putchar(*p++);
}

2. Pointer Array Storing Strings

char *colors[] = {"Red", "Green", "Blue"};
printf("%s", colors[1]);  // Outputs "Green"

6. Common Errors

1. Buffer Overflow

char buf[10];
scanf("%s", buf);  // Dangerous! Input too long will overflow

Limit length:

scanf("%9s", buf);  // Limit length

2. String Not Terminated, Insufficient Space Lacking ‘\0’

char str[5] = "Hello";  // No space for '\0'

Correction:

char str[6] = "Hello";  // Correct

3. Modifying Read-Only Strings

// Strings initialized in pointer form are read-only
char *p = "constant";
*p = 'C';  // Runtime error!

Correction:

char arr[] = "modifiable";
arr[0] = 'M';  // Correct

7. Common Program Examples for Strings

1. String Length Function

size_t my_strlen(const char *s) {
    size_t n = 0;
    while (*s++) n++;
    return n;
}

2. String Reversal Implementation

void reverse(char *str) {
    int i = 0, j = strlen(str)-1;
    while (i < j) {
        char temp = str[i];
        str[i++] = str[j];
        str[j--] = temp;
    }
}

3. Splitting Strings

char str[] = "apple,orange,banana";
char *token = strtok(str, ",");
while (token) {
    printf("%s\n", token);
    // Pass NULL to continue splitting the same string from the last end position
    token = strtok(NULL, ",");
}

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Essential Knowledge Points for C Language Beginners: Summary of Character Arrays and String Usage Techniques

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