Common array operations

Introduction

Arrays help us store many values in one place. Common operations let us add, find, or change these values easily.

You want to keep a list of student scores and find the highest score.

You need to add a new number to a list of temperatures.

You want to check if a certain number is in your list.

You want to change a value in your list after finding it.

You want to print all values stored in your list.

Syntax

C

/* Define an array and its size */
#define SIZE 100
int array[SIZE];
int size = 0; // current number of elements

/* Add element at the end */
if (size < SIZE) {
    array[size] = new_value;
    size++;
}

/* Find element by value */
int find_index = -1;
for (int i = 0; i < size; i++) {
    if (array[i] == value_to_find) {
        find_index = i;
        break;
    }
}

/* Update element at index */
if (index >= 0 && index < size) {
    array[index] = new_value;
}

/* Print all elements */
for (int i = 0; i < size; i++) {
    printf("%d ", array[i]);
}
printf("\n");

Arrays have fixed size in C, so you must check you don't add beyond capacity.

Indexing starts at 0, so first element is array[0].

Examples

This example shows adding two elements, finding one, updating it, and printing all.

C

int array[5];
int size = 0;

// Add elements
array[size++] = 10;
array[size++] = 20;

// Find 20
int find_index = -1;
for (int i = 0; i < size; i++) {
    if (array[i] == 20) {
        find_index = i;
        break;
    }
}

// Update element at found index
if (find_index != -1) {
    array[find_index] = 25;
}

// Print all
for (int i = 0; i < size; i++) {
    printf("%d ", array[i]);
}
printf("\n");

This example shows what happens if you try to add more elements than the array can hold.

C

int array[3];
int size = 0;

// Try to add 4 elements (one more than capacity)
for (int i = 1; i <= 4; i++) {
    if (size < 3) {
        array[size++] = i * 10;
    } else {
        printf("Cannot add %d, array full\n", i * 10);
    }
}

This example shows searching in an empty array returns -1 (not found).

C

int array[0];
int size = 0;

// Trying to find in empty array
int find_index = -1;
for (int i = 0; i < size; i++) {
    if (array[i] == 10) {
        find_index = i;
        break;
    }
}
printf("Find index: %d\n", find_index);

Sample Program

This program adds three numbers to an array, prints them, finds the number 10, updates it to 20, and prints the array again.

C

#include <stdio.h>

int main() {
    int array[5];
    int size = 0;

    // Add elements
    array[size++] = 5;
    array[size++] = 10;
    array[size++] = 15;

    printf("Array before update: ");
    for (int i = 0; i < size; i++) {
        printf("%d ", array[i]);
    }
    printf("\n");

    // Find element 10
    int find_index = -1;
    for (int i = 0; i < size; i++) {
        if (array[i] == 10) {
            find_index = i;
            break;
        }
    }

    // Update element if found
    if (find_index != -1) {
        array[find_index] = 20;
    }

    printf("Array after update: ");
    for (int i = 0; i < size; i++) {
        printf("%d ", array[i]);
    }
    printf("\n");

    return 0;
}

Important Notes

Adding elements is O(1) if you add at the end and have space.

Finding elements is O(n) because you may check each element.

Updating is O(1) once you know the index.

Common mistake: forgetting to check array size before adding causes bugs.

Use arrays when you know max size and want fast access by index.

Summary

Arrays store multiple values in order.

Common operations: add, find, update, print.

Always check array size to avoid errors.

Practice

1. What does the following C code do?
int arr[5] = {1, 2, 3, 4, 5};

easy

A. Creates an array of 5 integers with values 1 to 5

B. Creates a pointer to an integer

C. Declares a single integer variable

D. Creates a 2D array of integers

Solution

1. Step 1: Understand the declaration

   The code declares an integer array named arr with size 5.

2. Step 2: Check the initialization

   The array is initialized with values 1, 2, 3, 4, and 5 in order.

3. Final Answer:

   Creates an array of 5 integers with values 1 to 5 -> Option A

4. Quick Check:

   Array declaration with initialization = Creates an array of 5 integers with values 1 to 5 [OK]

Hint: Array size and values match initialization count [OK]

Common Mistakes:

• Confusing array with pointer declaration
• Thinking it's a single variable, not an array
• Assuming it's a 2D array

2. Which of the following is the correct syntax to access the third element of an array named arr?

easy

A. arr(3)

B. arr[2]

C. arr{3}

D. arr[3]

Solution

1. Step 1: Recall array indexing in C

   Arrays in C use zero-based indexing, so the first element is arr[0].

2. Step 2: Identify the third element index

   The third element is at index 2, so arr[2] accesses it.

3. Final Answer:

   arr[2] -> Option B

4. Quick Check:

   Third element index = 2 [OK]

Hint: Remember C arrays start at index 0 [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based indexing (arr[3])
• Using curly braces for access

3. What is the output of this C code?

int arr[4] = {10, 20, 30, 40};
int sum = 0;
for(int i = 0; i < 4; i++) {
    sum += arr[i];
}
printf("%d", sum);

medium

A. 100

B. 90

C. Syntax error

D. 40

Solution

1. Step 1: Sum all elements in the array

   The loop adds arr[0]=10, arr[1]=20, arr[2]=30, arr[3]=40 to sum.

2. Step 2: Calculate the total sum

   10 + 20 + 30 + 40 = 100.

3. Final Answer:

   100 -> Option A

4. Quick Check:

   Sum of array elements = 100 [OK]

Hint: Add all elements using loop index 0 to size-1 [OK]

Common Mistakes:

• Off-by-one error in loop causing wrong sum
• Confusing sum with last element value
• Syntax errors from missing semicolons

4. Identify the error in this code snippet:

int arr[3] = {1, 2, 3};
for(int i = 0; i <= 3; i++) {
    printf("%d ", arr[i]);
}

medium

A. Array size should be 4

B. printf format specifier is wrong

C. Loop condition should be i < 3, not i <= 3

D. Variable i should be declared outside the loop

Solution

1. Step 1: Analyze loop condition

   The loop runs from i=0 to i=3 inclusive, so it accesses arr[3].

2. Step 2: Check array bounds

   Array arr has size 3, valid indices are 0,1,2. Accessing arr[3] causes out-of-bounds error.

3. Final Answer:

   Loop condition should be i < 3, not i <= 3 -> Option C

4. Quick Check:

   Array index must be less than size [OK]

Hint: Loop index must be less than array size [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Assuming array size auto-expands
• Misunderstanding printf format specifier

5. You want to find the largest number in an integer array arr of size n. Which code snippet correctly does this?

hard

A. int max = arr[n]; for(int i = 0; i < n; i++) { if(arr[i] > max) max = arr[i]; }

B. int max = 0; for(int i = 0; i <= n; i++) { if(arr[i] > max) max = arr[i]; }

C. int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] < max) max = arr[i]; }

D. int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] > max) max = arr[i]; }

Solution

1. Step 1: Initialize max with first element

   Set max = arr[0] to start comparison from first element.

2. Step 2: Loop from second element to end

   Loop i from 1 to n-1, compare arr[i] with max, update max if arr[i] is larger.

3. Step 3: Check other options for errors

   int max = 0; for(int i = 0; i <= n; i++) { if(arr[i] > max) max = arr[i]; } uses max=0 which fails if all numbers are negative; loop condition i <= n causes out-of-bounds; int max = arr[n]; for(int i = 0; i < n; i++) { if(arr[i] > max) max = arr[i]; } initializes max with arr[n] which is invalid index; int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] < max) max = arr[i]; } finds minimum instead of maximum.

4. Final Answer:

   int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] > max) max = arr[i]; } -> Option D

5. Quick Check:

   Initialize max with first element and compare correctly [OK]

Hint: Start max with arr[0], loop i from 1 to n-1 [OK]

Common Mistakes:

• Using wrong loop bounds causing out-of-bounds access
• Initializing max with 0 ignoring negative values
• Confusing max with min logic