Strings

What is a string?

A C string is not a separate data type. It is a char array whose last meaningful byte is '\0'. Everything after the visible text is either unused capacity or must still leave room for that terminator.

char name[6] = "Ravi";
/* Memory idea: 'R' 'a' 'v' 'i' '\0'  (and maybe unused slots) */

• Character — one letter/digit/symbol in the array
• Null terminator '\0' — marks the end of the string
• Length — number of characters before '\0' (strlen)
• Capacity — size of the array (must be ≥ length + 1)

Why strings?

• Store text — names, messages, file paths, commands
• One name for many characters — easier than separate char variables
• Library support — strlen, strcpy, strcmp, …
• I/O with %s — read/write whole words or lines as text
• Foundation for larger programs — menus, parsers, and string-of-strings tables

Applications of strings

• User input — login names, search queries, addresses
• Messages & UI text — titles, errors, prompts
• File / path names — open files by string path
• Token / word processing — count words, reverse, search
• Tables of text — list of student names (string of strings)

Initialization of strings

With a string literal (preferred for fixed text):

char s1[]  = "Hello";           /* size becomes 6 (incl. '\0') */
char s2[20] = "Hello";        /* length 5; rest of buffer unused/zeroed with this form */
char s3[6]  = {'H', 'i', '\0'};  /* explicit chars */

String built-in functions (string.h)

Include #include <string.h>. Common starter functions:

#include <string.h>

char a[40] = "Learn";
char b[40];

strcpy(b, a);           /* b becomes "Learn" */
strcat(b, "Code");     /* b becomes "LearnCode" */
/* strlen(b) == 9 ; strcmp(a, b) != 0 */

Reading strings

Word with scanf (stops at space):

char word[20];
scanf("%19s", word);   /* width limits overflow; no & on array name */

Line with fgets (keeps spaces; may keep newline):

char line[100];
fgets(line, sizeof line, stdin);

Writing strings

Print whole string:

printf("%s\n", word);
puts(word);   /* prints string + newline */

Print character by character:

int i;
for (i = 0; word[i] != '\0'; i++)
    putchar(word[i]);
putchar('\n');

String of strings

A string of strings is a collection of strings — often a 2D char array where each row is one string (null-terminated within that row).

char names[3][20] = {
    "Asha",
    "Ravi",
    "Neha"
};

/* names[0] → "Asha" , names[1] → "Ravi" , names[2] → "Neha" */
printf("%s\n", names[1]);  /* prints Ravi */

• names[i] — the i-th string
• names[i][j] — character j inside string i
• Each row needs space for text + '\0' (here up to 19 chars + null)

What is a string? — terminator & memory

Library functions stop at the first '\0'. If you overwrite the terminator or never write one, %s / strlen walk into neighboring memory (undefined behavior).

char s[5];
s[0] = 'O'; s[1] = 'K'; s[2] = '\0';  /* valid short string */

Why strings? — vs char-by-char & vs other languages

• C gives you raw control and speed, but you manage buffers yourself
• No automatic resizing — overflowing a buffer is your bug to prevent
• Same model underpins argv, file APIs, and network text protocols

Applications — parsing & tables

• Split / scan words inside a line (manual loops or library helpers)
• Menus: array of option strings; print and match user choice with strcmp
• CSV-like rows: each line is a string; columns may be further split

Initialization — literals, empty, and partial

char empty[] = "";              /* only '\0' */
char buf[8] = {0};           /* all zeros → empty string */
char msg[8] = {'H', 'i'};  /* rest zeroed → "Hi" */

Built-in functions — deeper rules

if (strcmp(a, b) == 0)
    puts("equal");
else if (strcmp(a, b) < 0)
    puts("a before b");
else
    puts("a after b");

Reading strings — scanf vs fgets

• scanf("%s") — one word; skips leading whitespace; use field width
• fgets — up to size−1 chars or until newline; stores '\0'; may keep '\n'
• Mixing scanf and fgets can leave a leftover newline — clear or redesign input flow

/* Strip trailing newline after fgets */
size_t n = strlen(line);
if (n > 0 && line[n - 1] == '\n')
    line[n - 1] = '\0';

Writing strings — format & manual loops

• printf("%s", s) requires a valid terminator
• To write into a string: assign chars, or strcpy/sprintf (watch buffer size)
• Building text char-by-char: write chars, then set s[i] = '\0' when done

String of strings — 2D array vs array of pointers

Fixed-width rows (2D char array):

char table[3][16];
scanf("%15s", table[0]);  /* read into first row */

Array of pointers (often to literals):

char *days[] = {
    "Mon", "Tue", "Wed"
};
puts(days[1]);  /* Tue — do not modify the literal text */

Common mistakes