CodeBlanks Logo Icon CodeBlanks
Login Register

Array access

Learn to access and modify array elements by index.

Array access

You access array elements by index. Index starts at 0; for an array of length n the last index is n - 1.

int scores[5] = {85, 92, 78, 90, 88};
printf("%d\n", scores[0]);  // 85
printf("%d\n", scores[4]);  // 88
scores[2] = 80;  // change 78 to 80

If the index is out of bounds the behavior is undefined; that's why loops use i < n, not i <= n. You can both read and write an element.

Modifying elements

You can change an array element with =. The old value is lost, the new value is written to that index.

int nums[3] = {10, 20, 30};
nums[1] = 99;
printf("%d %d %d\n", nums[0], nums[1], nums[2]);
// Output: 10 99 30

Here nums[1] was 20; we set it to 99. The other elements stay unchanged. This is useful for updating elements based on conditions inside loops.

Practice Modify an element and print

Write a C program. Declare int nums[3] = {10, 20, 30};. Change the middle element with nums[1] = 99;. Print the three values with printf("%d %d %d\n", nums[0], nums[1], nums[2]);. Put #include <stdio.h> at the top.

Starter code

main.c
#include <stdio.h>

int main(void) {
    // Declare nums, change the middle element to 99 and print
    return 0;
}

Sample solution

main.c
#include <stdio.h>

int main(void) {
    int nums[3] = {10, 20, 30};
    nums[1] = 99;
    printf("%d %d %d\n", nums[0], nums[1], nums[2]);
    return 0;
}

Expected output

10 99 30

Questions and answers

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

1 Access the first element

Question 
printf("%d
", scores[____]);
Answer 
printf("%d
", scores[0]);

Explanation: scores[0] is the first element.

2 What's the last index of scores[5]?

main.c
int scores[5] = {1,2,3,4,5};
  • 0
  • 5
  • 4 Correct answer
  • 6

Explanation: A 5-element array's last index is 4.

3 Match the index concepts

Term Meaning
scores[0] First element
scores[4] Last element (size 5)
nums[1] = 99 Modification
scores[5] Out of bounds

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