String Basics and Memory Representation in DSA C

Mental Model

A string is a sequence of characters stored one after another in memory, ending with a special marker to show where it stops.

Analogy: Think of a string like a train of connected boxes, each holding one letter, with a special box at the end that says 'this is the last one'.

S -> [H] -> [E] -> [L] -> [L] -> [O] -> [\0]
↑start pointer

Dry Run Walkthrough

Input: string: "HELLO" stored in memory

Goal: Understand how each character is stored in order and how the string ends

Step 1: Store 'H' at first memory location

S -> [H] -> [] -> [] -> [] -> [] -> []
↑start pointer

Why: First character must be stored at the start

Step 2: Store 'E' at next memory location

S -> [H] -> [E] -> [] -> [] -> [] -> []
↑start pointer

Why: Characters are stored one after another

Step 3: Store 'L' at next memory location

S -> [H] -> [E] -> [L] -> [] -> [] -> []
↑start pointer

Why: Continue storing characters in order

Step 4: Store second 'L' at next memory location

S -> [H] -> [E] -> [L] -> [L] -> [] -> []
↑start pointer

Why: Store repeated characters as normal

Step 5: Store 'O' at next memory location

S -> [H] -> [E] -> [L] -> [L] -> [O] -> []
↑start pointer

Why: Store last character before end marker

Step 6: Store '\0' (null character) to mark end

S -> [H] -> [E] -> [L] -> [L] -> [O] -> [\0]
↑start pointer

Why: Null character shows where string ends in memory

Result:

S -> [H] -> [E] -> [L] -> [L] -> [O] -> [\0]
String ends with null character to mark termination

Annotated Code

DSA C

#include <stdio.h>

int main() {
    char str[] = "HELLO"; // string stored with null terminator

    // Print each character and its memory index
    for (int i = 0; str[i] != '\0'; i++) {
        printf("Index %d: %c\n", i, str[i]);
    }

    // Print the null terminator explicitly
    printf("Index %d: %d (null terminator)\n", 5, str[5]);

    return 0;
}

for (int i = 0; str[i] != '\0'; i++) {

Traverse characters until null terminator to print each character

printf("Index %d: %c\n", i, str[i]);

Print character at current index

printf("Index %d: %d (null terminator)\n", 5, str[5]);

Print the null terminator value explicitly to show string end

Output

Index 0: H

Index 1: E

Index 2: L

Index 3: L

Index 4: O

Index 5: 0 (null terminator)

Complexity Analysis

Time: O(n) because we visit each character once to print

Space: O(n) because the string uses memory proportional to its length plus one for the null terminator

vs Alternative: Compared to storing characters without a null terminator, this method allows easy detection of string end but requires extra space for the terminator

Edge Cases

Empty string ""

Only the null terminator is stored at the first position, indicating no characters

DSA C

for (int i = 0; str[i] != '\0'; i++) {

String with one character "A"

Character 'A' stored at index 0, null terminator at index 1

DSA C

for (int i = 0; str[i] != '\0'; i++) {

When to Use This Pattern

When you see problems about storing or manipulating text in low-level languages, think about strings as arrays of characters ending with a null marker to know where they stop.

Common Mistakes

Mistake: Forgetting the null terminator at the end of the string
Fix: Always ensure strings end with '\0' so functions know where the string ends

Summary

A string is a sequence of characters stored in memory ending with a null character to mark its end.

Use this when you need to store and process text in languages like C that do not store string length separately.

The key insight is the null terminator '\0' which tells the program where the string stops.