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Pointer to array

Learn to access array elements with pointer arithmetic.

Pointer arithmetic with arrays

Adding an integer to a pointer jumps to the next element. p + 1 moves forward by the size of the pointer's type; 4 bytes for int.

int arr[3] = {10, 20, 30};
int *p = arr;
printf("%d", *(p + 2));  /* 30 */
printf("%d", p[2]);       /* 30: same thing */

p[i] and *(p + i) mean exactly the same thing. Index notation is more readable, pointer notation more flexible.

p[i] and *(p+i) are the same

In C the p[i] notation is actually shorthand for *(p + i). They are fully equivalent; the compiler produces the same machine code.

int arr[4] = {5, 10, 15, 20};
int *p = arr;
printf("%d", p[0]);      /* 5 */
printf("%d", *(p + 1));   /* 10 */
printf("%d", p[2]);      /* 15 */

Which form you choose is a matter of style. Index notation is easier for beginners.

Practice Mix both notations

Define int arr[4] = {5, 10, 15, 20};. Start with int *p = arr;. With four printf calls print p[0], *(p + 1), p[2] and *(p + 3) on separate lines. Output should be 5, 10, 15, 20.

Starter code

main.c
#include <stdio.h>

int main(void) {
    int arr[4] = {5, 10, 15, 20};
    /* bind p to arr; print p[0], *(p+1), p[2], *(p+3) */
    return 0;
}

Sample solution

main.c
#include <stdio.h>

int main(void) {
    int arr[4] = {5, 10, 15, 20};
    int *p = arr;
    printf("%d\n", p[0]);
    printf("%d\n", *(p + 1));
    printf("%d\n", p[2]);
    printf("%d\n", *(p + 3));
    return 0;
}

Expected output

5
10
15
20

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Access the third element

Question 
int arr[5] = {1,2,3,4,5};
int *p = arr;
printf("%d", *(p + ____));  /* prints 3 */
Answer 
int arr[5] = {1,2,3,4,5};
int *p = arr;
printf("%d", *(p + 2));  /* prints 3 */

Explanation: *(p + 2) gives the third element (3).

2 Match access forms

Term Meaning
p[0] first element
p[1] second element
*(p+2) third element
p+3 fourth address

3 p = arr; p[2] is equal to which?

  • *(p + 1)
  • *(p + 3)
  • *(p + 2) Correct answer
  • *p

Explanation: p[2] and *(p + 2) are the same.

All lessons in Beginner

1 C basics

  1. What is C?
  2. Your first program
  3. Compile and run
  4. Comments
  5. printf and escapes
  6. Summary - C basics

2 Variables

  1. Declaring variables
  2. Variable assignment
  3. const
  4. Variable scope
  5. typedef
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. char
  4. sizeof
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Menu loop
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 I/O

  1. printf formatting
  2. scanf
  3. Multiple inputs
  4. getchar and putchar
  5. puts and fputs
  6. Summary - I/O

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. break and continue
  5. Nested loops
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Array size
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 Strings

  1. String basics
  2. String literals
  3. strlen
  4. strcpy and strcat
  5. strcmp
  6. Summary - Strings

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function prototypes
  5. Recursion
  6. Summary - Functions

13 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. NULL
  5. Pointer arithmetic
  6. Summary - Pointer basics

14 Arrays and pointers

  1. Array name as pointer
  2. Pointer to array
  3. Passing arrays to functions
  4. Pointer loop
  5. Multidim array pointer
  6. Summary - Arrays and pointers

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String functions

  1. strchr
  2. strstr
  3. strtok
  4. sprintf
  5. sscanf
  6. Summary - String functions

17 Structs

  1. Defining structs
  2. Using structs
  3. typedef struct
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Memory and stack

  1. Stack and heap
  2. Automatic storage
  3. malloc
  4. free
  5. calloc and realloc
  6. Summary - Memory and stack

19 Preprocessor

  1. #define
  2. #include
  3. Conditional compilation
  4. Macro functions
  5. Header files
  6. Summary - Preprocessor

20 Beginner final

  1. Project design
  2. Data model
  3. Functions
  4. Memory management
  5. Putting it together