String Manipulation and User Input in C Programming

Introduction

In C programming, the string.h library provides several functions for manipulating strings. This blog post will explore the usage of string manipulation functions in C, along with a detailed explanation of user input using scanf and fgets. We will analyze a code snippet that showcases these concepts and discuss each aspect of the code step by step.

Code Snippet

Including string.h

The first line of the code snippet #include <string.h> is a preprocessor directive that includes the string.h header file. This file contains function declarations for string manipulation, such as strlen, strcpy, strcat, and strcmp. By including this header file, we gain access to these functions, which we will utilize in the code snippet.

Creating a Directory

To begin, let's create a file named intermediate.c using the command line. Follow the instructions below based on your operating system:

• Open the command prompt or PowerShell.
• Run the following commands:

cd basic-programming/c-programming

• Open the file in your favorite text editor.

code intermediate.c

or

vim intermediate.c # installed by default on macOS

or

emacs -Q -nw intermediate.c

or

nano intermediate.c # installed by default on Linux

Note:
Make sure you save changes in the file.

String Manipulation using string.h Functions

In this section, we'll analyze the code related to string manipulation functions provided by the string.h library.

// String Manipulation
strcat(firstText, secondText);
strcpy(secondStr, thirdStr);
const int str13Cmp = strcmp(firstStr, thirdStr);
const int str3greetingCmp = strcmp(thirdStr, greeting);

In the code above, we perform various string manipulation operations:

• strcat(firstText, secondText);: This line uses thestrcatfunction to concatenate thesecondTextstring at the end of thefirstTextstring. The result is that thefirstText string becomes "Hello World!".

• strcpy(secondStr, thirdStr);: Here, we use the strcpy function to copy the contents of the thirdStr string into the secondStr array. This operation creates an independent copy of the string.

• const int str13Cmp = strcmp(firstStr, thirdStr);: The strcmp function compares the contents of the firstStr and thirdStr strings. It returns an integer value indicating the result of the comparison. If the two strings are equal, it returns 0. If firstStr is lexicographically less than thirdStr, it returns a negative value. If firstStr is lexicographically greater than thirdStr, it returns a positive value. The result is stored in the str13Cmp variable.

• const int str3greetingCmp = strcmp(thirdStr, greeting);: Similarly, this line compares the thirdStr and greeting strings using strcmp and stores the result in the str3greetingCmp variable.

User Input using scanf

Now, let's look at the code related to user input using the scanf function.

// User Input using scanf
printf("Please type your age and grade: ");
scanf("%d %c", &myAge, &myChar);
printf("Please enter your first name: ");
scanf("%19s", myFirstName);

In the code above, we use scanf to retrieve user input:

• scanf("%d %c", &myAge, &myChar);: This line prompts the user to enter their age and grade. The format string "%d %c" specifies that we expect an integer followed by a character as input. The & operator is used to pass the memory addresses of myAge and myChar variables to store the values entered by the user.

• scanf("%19s", myFirstName);: This line prompts the user to enter their first name. The format string "%19s" specifies that we expect a string input with a maximum length of 19 characters (to avoid buffer overflow). The entered string is stored in the myFirstName character array.

User Input using fgets

Here's the code that demonstrates user input using the fgets function.

// User Input using fgets
printf("Please enter your full name: ");
getchar();  // Clear the newline character from the previous input
fgets(myFullName, sizeof(myFullName), stdin);
myFullName[strcspn(myFullName, "\n")] = '\0';

In the code above, we utilize fgets to obtain user input:

• fgets(myFullName, sizeof(myFullName), stdin);: This line reads a line of input from the user, storing it in the myFullName character array. The sizeof(myFullName) argument specifies the maximum number of characters to read, preventing buffer overflow. The stdin stream indicates that the input is read from the standard input (keyboard).

• myFullName[strcspn(myFullName, "\n")] = '\0';: Since fgets captures the newline character (\n) when the user presses Enter, this line removes the newline character from the myFullName string by replacing it with a null character (\0). This step ensures that the string ends at the intended input and doesn't include the newline character.

Memory Address and Pointers

This section focuses on memory addresses and pointers used in the code.

// Memory Address and Pointers
printf("My age is: %d\n", myAge);
printf("My character is: %c\n", myChar);
printf("My first name is: %s\n", myFirstName);
printf("My full name is: %s\n", myFullName);
printf("%p\n", (void*)&myAge);
printf("%p\n", (void*)ptr);
printf("%d\n", *ptr);

In the code above, we work with memory addresses and pointers:

• printf("My age is: %d\n", myAge);: This line displays the value of the myAge variable, representing the age entered by the user.

• printf("My character is: %c\n", myChar);: Here, we print the value of the myChar variable, which corresponds to the grade entered by the user.

• printf("My first name is: %s\n", myFirstName);: This line outputs the contents of the myFirstName character array, which contains the user's first name.

• printf("My full name is: %s\n", myFullName);: Similarly, we display the content of the myFullName character array, which holds the user's full name.

• printf("%p\n", (void*)&myAge);: This line prints the memory address of the myAge variable using the %p format specifier. The & operator retrieves the address of the variable.

• printf("%p\n", (void*)ptr);: Here, we print the memory address stored in the ptr pointer variable. The (void*) cast is used to ensure proper formatting of the memory address.

• printf("%d\n", *ptr);: This line dereferences the ptr pointer and prints the value stored at the memory address it points to. Since ptr points to myAge, it effectively displays the value of myAge.

Output
Finally, the code snippet includes several printf statements to display the output.

// Output
printf("%d\n", alphabetLength);
printf("%d\n", alphabetByte);
printf("%s\n", firstText);
printf("%s\n", secondStr);
printf("%d\n", str13Cmp);
printf("%d\n", str3greetingCmp);

The code above produces the following output:

• alphabetLength: This line prints the length of the alphabet string, which is determined using the strlen function.

• alphabetByte: Similarly, this line displays the size (in bytes) of the alphabet array, obtained using the sizeof operator.

• firstText: This line outputs the contents of the firstText character array, which now contains the concatenated string "Hello World!".

• secondStr: Here, we print the contents of the secondStr character array, which was assigned the value of the thirdStr string using strcpy.

• str13Cmp: This line displays the result of comparing the firstStr and thirdStr strings using strcmp. It indicates whether the two strings are equal or different.

• str3greetingCmp: Similarly, this line shows the result of comparing the thirdStr and greeting strings using strcmp.

Practive

Assuming you have an account or are already logged in on Replit.

• Try the Replit Editor online.
• Click the Fork button.
• Edit the current code.

Conclusion

In this blog post, we explored the usage of string manipulation functions provided by the string.h library in C programming. We discussed functions such as strlen, strcpy, strcat, and strcmp, along with their purpose and how they are utilized in the code snippet. Additionally, we examined user input using scanf and fgets, which allow the program to receive input from the user. Finally, we covered memory addresses and pointers, highlighting their significance and demonstrating their usage in the code snippet.

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References

String and Character Array
Introduction to C Pointers
Using Pointers in C
What is return type of getchar(), fgetc() and getc()?