Module 6: Arrays and Strings in C

Learning Objectives

By the end of this module, you will be able to:

• Understand what arrays are and why they are used.
• Work with one-dimensional, two-dimensional, and multidimensional arrays.
• Traverse arrays using loops.
• Understand strings and character arrays.
• Learn the difference between gets() and fgets().
• Use common string functions such as strlen(), strcpy(), strcat(), and strcmp().
• Build practical programs using arrays and strings.
• Develop matrix and string manipulation applications.

Introduction

Imagine storing the marks of 100 students.

Without arrays, you would need to create 100 separate variables:

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1int mark1;
2int mark2;
3int mark3;
4...
5int mark100;

Managing this many variables becomes difficult.

Arrays solve this problem by storing multiple values of the same data type under a single name.

Similarly, strings are simply arrays of characters that allow us to store and manipulate text.

Arrays and strings are among the most frequently used concepts in C programming and form the foundation for many advanced data structures.

Arrays

An array is a collection of elements of the same data type stored in contiguous memory locations.

Example:

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1Marks = [85, 90, 78, 92, 88]

Instead of using five separate variables, we use one array.

Advantages of Arrays

• Store multiple values using one variable.
• Easy access using indexes.
• Efficient memory usage.
• Simplifies loops.
• Foundation for matrices, stacks, queues, and many data structures.

Array Indexing

Array indexing starts from 0.

Example

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1Index : 0   1   2   3   4
2Value : 10 20 30 40 50

Declaring an Array

Syntax

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1data_type array_name[size];

Example

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1int numbers[5];

Initializing an Array

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1int numbers[5] = {10,20,30,40,50};

Or

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1int numbers[] = {10,20,30,40,50};

The compiler automatically determines the array size.

Accessing Array Elements

Example

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1#include <stdio.h>
2
3int main()
4{
5    int marks[] = {80,85,90};
6
7    printf("%d", marks[1]);
8
9    return 0;
10}

Output

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185

One-Dimensional (1D) Array

A one-dimensional array stores values in a single row.

Example

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1Roll Numbers
2
310
420
530
640
750

Program

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1#include <stdio.h>
2
3int main()
4{
5    int numbers[5] = {10,20,30,40,50};
6
7    for(int i=0;i<5;i++)
8    {
9        printf("%d ", numbers[i]);
10    }
11
12    return 0;
13}

Output

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110 20 30 40 50

Input in Array

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1#include <stdio.h>
2
3int main()
4{
5    int marks[5];
6
7    printf("Enter 5 marks:\n");
8
9    for(int i=0;i<5;i++)
10    {
11        scanf("%d",&marks[i]);
12    }
13
14    printf("\nMarks:\n");
15
16    for(int i=0;i<5;i++)
17    {
18        printf("%d ",marks[i]);
19    }
20
21    return 0;
22}

Two-Dimensional (2D) Array

A two-dimensional array stores data in rows and columns.

It is commonly used to represent matrices.

Example

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11 2 3
24 5 6
37 8 9

Declaration

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1int matrix[3][3];

Initialization

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1int matrix[2][3] =
2{
3    {1,2,3},
4    {4,5,6}
5};

Accessing Elements

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1printf("%d", matrix[1][2]);

Output

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16

Input and Output of 2D Array

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1#include <stdio.h>
2
3int main()
4{
5    int matrix[2][2];
6
7    for(int i=0;i<2;i++)
8    {
9        for(int j=0;j<2;j++)
10        {
11            scanf("%d",&matrix[i][j]);
12        }
13    }
14
15    printf("\nMatrix:\n");
16
17    for(int i=0;i<2;i++)
18    {
19        for(int j=0;j<2;j++)
20        {
21            printf("%d ",matrix[i][j]);
22        }
23
24        printf("\n");
25    }
26
27    return 0;
28}

Multidimensional Arrays

Arrays can have more than two dimensions.

Example

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1int data[2][3][4];

Applications:

• Image Processing
• 3D Graphics
• Scientific Computing
• Machine Learning

Array Traversal

Traversal means visiting every element of an array.

Example

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1#include <stdio.h>
2
3int main()
4{
5    int numbers[]={5,10,15,20};
6
7    for(int i=0;i<4;i++)
8    {
9        printf("%d\n",numbers[i]);
10    }
11
12    return 0;
13}

Strings

A string is a sequence of characters terminated by the null character ('\0').

Example

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1Hello

Memory representation

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1H e l l o \0

Character Arrays

Strings are stored using character arrays.

Example

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1char name[20];

Initialization

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1char name[]="Ankit";

Equivalent to

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1A n k i t \0

Reading Strings

Using scanf()

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1char name[20];
2
3scanf("%19s",name);

This reads input only until the first whitespace.

gets() vs fgets()

gets()

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1gets(name);

Problems:

• Unsafe
• No bounds checking
• Removed from the C standard
• Can cause buffer overflow

fgets()

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1fgets(name,sizeof(name),stdin);

Advantages

• Safe
• Limits input length
• Prevents buffer overflow

Recommendation

Always prefer fgets() over gets() in modern C programs.

puts()

Displays a string followed by a newline.

Example

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1#include <stdio.h>
2
3int main()
4{
5    char name[]="Tech3Space";
6
7    puts(name);
8
9    return 0;
10}

Output

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1Tech3Space

Common String Functions

To use string functions include:

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1#include <string.h>

strlen()

Returns the length of a string (excluding the null character).

Example

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1#include <stdio.h>
2#include <string.h>
3
4int main()
5{
6    char name[]="Programming";
7
8    printf("%zu",strlen(name));
9
10    return 0;
11}

Output

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111

strcpy()

Copies one string into another.

Example

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1char source[]="Hello";
2char destination[20];
3
4strcpy(destination,source);
5
6printf("%s",destination);

Output

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1Hello

strcat()

Concatenates (joins) two strings.

Example

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1char first[30]="Hello ";
2char second[]="World";
3
4strcat(first,second);
5
6printf("%s",first);

Output

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1Hello World

strcmp()

Compares two strings.

Example

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1char first[]="Apple";
2char second[]="Apple";
3
4printf("%d",strcmp(first,second));

Output

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10

Return values:

Practice Project 1: Matrix Addition

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1#include <stdio.h>
2
3int main()
4{
5    int a[2][2],b[2][2],sum[2][2];
6
7    printf("Enter first matrix:\n");
8
9    for(int i=0;i<2;i++)
10        for(int j=0;j<2;j++)
11            scanf("%d",&a[i][j]);
12
13    printf("Enter second matrix:\n");
14
15    for(int i=0;i<2;i++)
16        for(int j=0;j<2;j++)
17            scanf("%d",&b[i][j]);
18
19    printf("\nSum Matrix:\n");
20
21    for(int i=0;i<2;i++)
22    {
23        for(int j=0;j<2;j++)
24        {
25            sum[i][j]=a[i][j]+b[i][j];
26            printf("%d ",sum[i][j]);
27        }
28
29        printf("\n");
30    }
31
32    return 0;
33}

Practice Project 2: Matrix Multiplication

Matrix multiplication is performed by multiplying rows of the first matrix with columns of the second matrix.

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1Result[i][j] = Σ(A[i][k] × B[k][j])

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1#include <stdio.h>
2
3int main()
4{
5    int a[2][2], b[2][2], result[2][2] = {0};
6
7    printf("Enter first 2x2 matrix:\n");
8    for(int i=0;i<2;i++)
9        for(int j=0;j<2;j++)
10            scanf("%d",&a[i][j]);
11
12    printf("Enter second 2x2 matrix:\n");
13    for(int i=0;i<2;i++)
14        for(int j=0;j<2;j++)
15            scanf("%d",&b[i][j]);
16
17    for(int i=0;i<2;i++)
18    {
19        for(int j=0;j<2;j++)
20        {
21            for(int k=0;k<2;k++)
22            {
23                result[i][j] += a[i][k] * b[k][j];
24            }
25        }
26    }
27
28    printf("\nResult Matrix:\n");
29
30    for(int i=0;i<2;i++)
31    {
32        for(int j=0;j<2;j++)
33        {
34            printf("%d ", result[i][j]);
35        }
36        printf("\n");
37    }
38
39    return 0;
40}

Practice Project 3: String Reverse

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1#include <stdio.h>
2#include <string.h>
3
4int main()
5{
6    char str[100];
7
8    printf("Enter a string: ");
9    fgets(str,sizeof(str),stdin);
10
11    int len=strlen(str);
12
13    if(str[len-1]=='\n')
14    {
15        str[--len]='\0';
16    }
17
18    printf("Reversed String: ");
19
20    for(int i=len-1;i>=0;i--)
21    {
22        printf("%c",str[i]);
23    }
24
25    return 0;
26}

Practice Project 4: Palindrome Checker

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1#include <stdio.h>
2#include <string.h>
3
4int main()
5{
6    char str[100];
7    int flag=1;
8
9    printf("Enter a string: ");
10    fgets(str,sizeof(str),stdin);
11
12    int len=strlen(str);
13
14    if(str[len-1]=='\n')
15    {
16        str[--len]='\0';
17    }
18
19    for(int i=0;i<len/2;i++)
20    {
21        if(str[i]!=str[len-i-1])
22        {
23            flag=0;
24            break;
25        }
26    }
27
28    if(flag)
29        printf("Palindrome");
30    else
31        printf("Not Palindrome");
32
33    return 0;
34}

Common Mistakes to Avoid

• Accessing array indexes outside their valid range.
• Forgetting that array indexing starts from 0.
• Using gets(), which is unsafe and removed from modern C standards.
• Forgetting to include <string.h> when using string functions.
• Assuming scanf("%s") can read strings containing spaces.
• Forgetting that strings end with the null character ('\0').
• Copying strings with = instead of using strcpy().

Best Practices

• Use meaningful array names such as studentMarks or salesData.
• Validate array indexes before accessing elements.
• Prefer fgets() over gets() for reading strings safely.
• Always allocate enough memory for the null terminator in strings.
• Use library functions like strlen(), strcpy(), and strcmp() instead of writing custom implementations unless for learning purposes.
• Keep matrix dimensions consistent when performing operations such as addition and multiplication.

Module Summary

In this module, you learned:

• What arrays are and how to declare, initialize, and access them.
• How to work with one-dimensional, two-dimensional, and multidimensional arrays.
• How to traverse arrays using loops.
• How strings are represented as character arrays in C.
• The difference between gets() and the safer fgets().
• How to display strings using puts().
• How to use common string library functions: strlen(), strcpy(), strcat(), and strcmp().
• How to build practical applications such as matrix addition, matrix multiplication, string reversal, and palindrome checking.

After completing this module, you'll be ready to learn Pointers in C, including memory addresses, pointer arithmetic, dynamic memory allocation, and the relationship between pointers, arrays, and functions.