Using strcat()

Part of the Logic & Flow section of Coddy's C journey — lesson 17 of 63.

You've learned how to copy strings with strcpy(), but what if you want to join two strings together? This is called concatenation, and C provides the strcat() function from the <string.h> library for this purpose.

The strcat() function appends the contents of one string to the end of another string. It takes two arguments: the destination string (which will be modified) and the source string (which will be added to the end).

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

char greeting[20] = "Hello";
char name[] = " World";
strcat(greeting, name);  // greeting now contains "Hello World"

It's crucial that your destination string has enough space to hold both the original content and the new content being appended. The strcat() function finds the null terminator in the destination string, removes it, copies the source string to that location, and adds a new null terminator at the very end.

This function is essential for building longer strings from smaller pieces, such as creating full sentences from individual words or combining user input with predefined text.

Sometimes you need to convert a number into a string before you can concatenate it. For this, you can use sprintf() from <stdio.h> (which is already included in most C programs). sprintf() works just like printf(), but instead of printing to the screen it writes the formatted result into a char array.

For example, to convert an integer to a string:

int age = 25;
char ageStr[10];
sprintf(ageStr, "%d", age);  // ageStr now contains "25"

char result[30] = "Age: ";
strcat(result, ageStr);       // result now contains "Age: 25"

The first argument to sprintf() is the destination char array, the second is the format string (using the same format specifiers as printf()), and the remaining arguments are the values to insert.

Challenge

Easy

Write a C program that demonstrates string concatenation using the strcat() function to build personalized messages. Your program should:

Read an integer n representing the number of people to process
For each person:
- Read their first name from input
- Read their last name from input
- Create a character array named fullName with 100 elements
- Use strcpy() to copy the first name into fullName
- Use strcat() to append a single space " " to fullName
- Use strcat() to append the last name to fullName
- Create a character array named greeting with 150 elements
- Use strcpy() to copy "Hello, " into greeting
- Use strcat() to append the fullName to greeting
- Use strcat() to append "! Welcome to our program." to greeting
- Print the final greeting message
After processing all people, create a summary message:
- Create a character array named summary with 100 elements
- Use strcpy() to copy "Total people processed: " into summary
- Convert the number n to a string and use strcat() to append it to summary
- Print the summary message

Remember to include the <string.h> header to use the string functions.

Your output should display the results in the following format:

Hello, [FirstName LastName]! Welcome to our program.
Hello, [FirstName LastName]! Welcome to our program.
...
Total people processed: [n]

For example, if the input is:

2
John
Doe
Alice
Smith

Your output should be:

Hello, John Doe! Welcome to our program.
Hello, Alice Smith! Welcome to our program.
Total people processed: 2

This challenge tests your understanding of using strcat() to concatenate multiple strings together, combining it with strcpy() for initial string setup, and building complex strings from multiple components step by step.

Cheat sheet

The strcat() function from <string.h> concatenates (joins) two strings by appending the source string to the end of the destination string.

Syntax: strcat(destination, source)

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

char greeting[20] = "Hello";
char name[] = " World";
strcat(greeting, name);  // greeting now contains "Hello World"

Important: The destination string must have enough space to hold both the original content and the appended content. The function finds the null terminator in the destination, removes it, copies the source string to that location, and adds a new null terminator at the end.

The sprintf() function from <stdio.h> works like printf(), but writes the formatted result into a char array (string) instead of printing it. This makes it useful for converting integers (or other values) into strings.

Syntax: sprintf(destination, format, value)

#include <stdio.h>

int num = 42;
char numStr[10];
sprintf(numStr, "%d", num);  // numStr now contains "42"

Important: The destination array must be large enough to hold the resulting string, including the null terminator.

Try it yourself

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

int main() {
    // Read the number of people
    int n;
    scanf("%d", &n);
    
    // TODO: Write your code below
    // Process each person and create greeting messages
    // Then create and display the summary message
    
    return 0;
}