One-dimensional arrays in C

Introduction

One-dimensional arrays help you store many values of the same type in a single list. This makes it easy to organize and use data like numbers or letters together.

When you want to keep a list of student scores in a test.

When you need to store daily temperatures for a week.

When you want to hold a sequence of characters as a word or sentence.

When you want to process a list of items one by one.

When you want to perform calculations on a fixed number of values.

Syntax

C

/* Define an array of integers with 5 elements */
int numbers[5];

/* Define and initialize an array of characters */
char letters[4] = {'a', 'b', 'c', 'd'};

The number inside the square brackets [] tells how many items the array can hold.

Array indexes start at 0, so the first element is numbers[0].

Examples

This creates an array of 3 integers but does not set their values yet.

C

int scores[3];

// Empty array of 3 integers, values are garbage until assigned

Here, the array is created and filled with 5 prime numbers right away.

C

int primes[5] = {2, 3, 5, 7, 11};

// Array with 5 prime numbers initialized

This array holds the word "Hello" as characters, ending with '\0' to mark the end.

C

char word[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

// Array holding a string with null terminator

Some compilers allow zero-length arrays as an extension, but they cannot hold any elements.

C

int emptyArray[0];

// Edge case: array with zero length (not standard in C, but allowed as a compiler extension in some compilers)

Sample Program

This program creates an array to hold temperatures for 5 days. It prints all the temperatures, changes the temperature for day 3, and prints the updated list.

C

#include <stdio.h>

int main() {
    // Create an array of 5 integers
    int temperatures[5] = {23, 25, 22, 20, 24};

    // Print all temperatures
    printf("Temperatures for the week:\n");
    for (int index = 0; index < 5; index++) {
        printf("Day %d: %d degrees\n", index + 1, temperatures[index]);
    }

    // Change the temperature of day 3
    temperatures[2] = 26;

    // Print updated temperatures
    printf("\nUpdated temperatures:\n");
    for (int index = 0; index < 5; index++) {
        printf("Day %d: %d degrees\n", index + 1, temperatures[index]);
    }

    return 0;
}

Important Notes

Accessing an array index outside its range causes undefined behavior and can crash your program.

Arrays have fixed size; you cannot change their length after creation.

Time complexity to access any element by index is O(1), which means very fast.

Summary

One-dimensional arrays store multiple values of the same type in order.

Use arrays when you need to keep and work with lists of data.

Remember array indexes start at 0 and size is fixed.

Practice

1. What is the starting index of a one-dimensional array in C?

easy

A. 0

B. 1

C. -1

D. Depends on the array size

Solution

1. Step 1: Understand array indexing in C

   In C, arrays always start indexing from 0, meaning the first element is at index 0.

2. Step 2: Confirm the starting index

   This is a fixed rule in C and does not depend on array size or other factors.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Array indexing starts at 0 [OK]

Hint: Remember: C arrays always start at zero index [OK]

Common Mistakes:

• Thinking arrays start at 1 like in some other languages
• Assuming negative indexes are valid
• Believing index depends on array size

2. Which of the following is the correct way to declare an integer array of size 5 in C?

easy

A. int arr = 5;

B. int arr();

C. array int arr[5];

D. int arr[5];

Solution

1. Step 1: Recall C array declaration syntax

   In C, arrays are declared by specifying the type, name, and size in square brackets, like int arr[5];

2. Step 2: Check each option

   int arr[5]; matches the correct syntax. int arr(); uses parentheses which is for functions. int arr = 5; assigns a single int, not an array. array int arr[5]; uses invalid keyword 'array'.

3. Final Answer:

   int arr[5]; -> Option D

4. Quick Check:

   Array declaration uses square brackets [OK]

Hint: Use square brackets for array size in declaration [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Assigning a single value instead of declaring array
• Using invalid keywords like 'array'

3. What will be the output of the following code?

int arr[3] = {10, 20, 30};
printf("%d", arr[1]);

medium

A. 10

B. 30

C. 20

D. Compilation error

Solution

1. Step 1: Identify the array elements and indexes

   The array arr has elements: arr[0]=10, arr[1]=20, arr[2]=30.

2. Step 2: Understand the printf statement

   printf prints the value at arr[1], which is 20.

3. Final Answer:

   20 -> Option C

4. Quick Check:

   arr[1] = 20 [OK]

Hint: Remember array index 1 is second element [OK]

Common Mistakes:

• Confusing index 1 with index 0
• Expecting output 10 or 30 incorrectly
• Thinking code causes compilation error

4. Find the error in this code snippet:

int arr[4];
arr[4] = 10;

medium

A. Array index out of bounds

B. Missing semicolon

C. Wrong data type

D. No error

Solution

1. Step 1: Check array size and indexing

   The array arr has size 4, so valid indexes are 0,1,2,3 only.

2. Step 2: Identify the invalid index usage

   arr[4] accesses the fifth element, which is outside the array bounds causing undefined behavior.

3. Final Answer:

   Array index out of bounds -> Option A

4. Quick Check:

   Index must be less than array size [OK]

Hint: Index must be less than array size [OK]

Common Mistakes:

• Using index equal to array size
• Ignoring zero-based indexing
• Assuming no error for out-of-bounds access

5. Given the array declaration int nums[5] = {1, 2, 3, 4, 5};, which code snippet correctly calculates the sum of all elements?

hard

A. int sum = 0;
for(int i = 0; i <= 5; i++) sum += nums[i];

B. int sum = 0;
for(int i = 0; i < 5; i++) sum += nums[i];

C. int sum = 0;
for(int i = 1; i < 5; i++) sum += nums[i];

D. int sum = 0;
for(int i = 0; i < 4; i++) sum += nums[i];

Solution

1. Step 1: Understand array size and valid indexes

   The array nums has 5 elements with indexes 0 to 4.

2. Step 2: Analyze each loop condition

   int sum = 0; for(int i = 0; i <= 5; i++) sum += nums[i]; uses i <= 5 which accesses nums[5] (out of bounds). int sum = 0; for(int i = 0; i < 5; i++) sum += nums[i]; uses i < 5 which correctly iterates from 0 to 4. int sum = 0; for(int i = 1; i < 5; i++) sum += nums[i]; starts at 1, missing nums[0]. int sum = 0; for(int i = 0; i < 4; i++) sum += nums[i]; stops at 4, missing nums[4].

3. Final Answer:

   int sum = 0; for(int i = 0; i < 5; i++) sum += nums[i]; -> Option B

4. Quick Check:

   Loop from 0 to less than size sums all elements [OK]

Hint: Loop from 0 to less than array size for all elements [OK]

Common Mistakes:

• Using <= size causing out-of-bounds access
• Starting loop at 1 missing first element
• Stopping loop early missing last element