Week 11: Strings and String Functions

Overview

Welcome to Week 11 of C programming! This week, we will explore how to declare and manipulate strings in C. By the end of this tutorial, you will:

• Understand how strings are stored in memory (char[] and char *)
• Use input/output functions like fgets, puts, and scanf
• Work with common string functions such as strlen, strcat, strcmp, strncpy
• Convert strings to numbers using atoi, atof, etc.

Time Breakdown

• String declaration and usage
• Pointer vs Array in strings
• Built-in string functions
• String-to-number conversion
• Exercises & Q/A

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Examples

Example 1: Basic String Input and Output

#include <stdio.h>

int main(void) {
    char str[100];
    printf("Enter a sentence: ");
    fgets(str, sizeof(str), stdin);
    printf("You typed: ");
    puts(str);
    return 0;
}

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Example 2: Pointer vs Array

#include <stdio.h>

int main(void) {
    char arr[] = "Hello";
    char *ptr = "World";
    puts(arr);
    puts(ptr);
    return 0;
}

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Example 3: String Functions

#include <stdio.h>
#include <string.h>

int main(void) {
    char a[20] = "Hi";
    char b[] = " there";
    strcat(a, b);
    printf("Concatenated: %s\n", a);
    printf("Length: %lu\n", strlen(a));
    return 0;
}

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Example 4: String to Number

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    char str[] = "123";
    int num = atoi(str);
    printf("Converted: %d\n", num);
    return 0;
}

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Exercises

Exercise 1: Read and Count Characters

1. Ask the user to enter a sentence using fgets().
2. Print the sentence and also display how many characters it has using strlen().

Example Output:

Enter a sentence: I like pizza.
You typed: I like pizza.
Length: 14

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Exercise 2: Modifying char[] vs char* Strings

1. Declare a string using char arr[] and another using char *ptr.
2. Try modifying the first character of both. Print the results and observe which one works.

Example Output:

Modified arr: Xello
Modifying ptr caused a crash or is not allowed.

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Exercise 3: Use String Functions Together

1. Ask the user to enter two words.
2. Compare them using strcmp() and print the result.
3. Concatenate them using strcat().

Example Output:

Enter first word: Hello
Enter second word: World
Comparison result: not equal
Joined: HelloWorld

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Exercise 4: Convert and Sum Two Numbers

1. Ask the user to enter two numbers as strings.
2. Convert them to integers using atoi().
3. Print their sum.

Example Output:

Enter two numbers: 12 34
Sum: 46

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Exercise 5: Count Words

1.Count how many words are in a sentence. Words are separated by spaces.

Example Output:

Enter a sentence: I like C programming a lot
Words counted: 5