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strcpy and strcat

Learn to copy with strcpy and concatenate with strcat.

Copying strings with strcpy

In C, you cannot copy strings later with =; you must use the strcpy function from <string.h>.

char src[] = "Hello";
char dst[20];
strcpy(dst, src);
/* dst: "Hello" */

The destination array must be at least one element larger than the source string (for the null). Otherwise, overflow occurs and behavior becomes undefined.

Joining strings with strcat

strcat appends the source to the end of the destination. The destination array must be large enough to hold the result.

char greeting[20] = "Hello, ";
strcat(greeting, "Anna");
/* greeting: "Hello, Anna" */

strcat finds the null in the destination, overwrites it, and places a new null at the end. If the destination is too small, overflow occurs; sizing matters.

Practice Join first and last name

Define char full[40] and copy "Anna" into it with strcpy. Then append " Smith" with strcat. Print with printf %s; output should be Anna Smith.

Starter code

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

int main(void) {
    char full[40];
    /* copy "Anna" with strcpy */
    /* append " Smith" with strcat */
    /* print with printf %s */
    return 0;
}

Sample solution

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

int main(void) {
    char full[40];
    strcpy(full, "Anna");
    strcat(full, " Smith");
    printf("%s\n", full);
    return 0;
}

Expected output

Anna Smith

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which of these assignments causes a compile error?

  • char a[] = "Hello";
  • char b[20]; strcpy(b, a);
  • char c[20] = a; Correct answer
  • char d[] = {'A','l','i','\0'};

Explanation: You cannot copy between arrays with =; strcpy is needed.

2 strcpy call

Question 
strcpy(dst, ____);  /* copy from src to dst */
Answer 
strcpy(dst, src);  /* copy from src to dst */

Explanation: The second argument is the source, the first is the destination.

3 Match strcpy and strcat

Term Meaning
strcpy copy
strcat append
dst destination array
src source array

All lessons in Beginner

1 C basics

  1. What is C?
  2. Your first program
  3. Compile and run
  4. Comments
  5. printf and escapes
  6. Summary - C basics

2 Variables

  1. Declaring variables
  2. Variable assignment
  3. const
  4. Variable scope
  5. typedef
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. char
  4. sizeof
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Menu loop
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 I/O

  1. printf formatting
  2. scanf
  3. Multiple inputs
  4. getchar and putchar
  5. puts and fputs
  6. Summary - I/O

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. break and continue
  5. Nested loops
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Array size
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 Strings

  1. String basics
  2. String literals
  3. strlen
  4. strcpy and strcat
  5. strcmp
  6. Summary - Strings

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function prototypes
  5. Recursion
  6. Summary - Functions

13 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. NULL
  5. Pointer arithmetic
  6. Summary - Pointer basics

14 Arrays and pointers

  1. Array name as pointer
  2. Pointer to array
  3. Passing arrays to functions
  4. Pointer loop
  5. Multidim array pointer
  6. Summary - Arrays and pointers

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String functions

  1. strchr
  2. strstr
  3. strtok
  4. sprintf
  5. sscanf
  6. Summary - String functions

17 Structs

  1. Defining structs
  2. Using structs
  3. typedef struct
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Memory and stack

  1. Stack and heap
  2. Automatic storage
  3. malloc
  4. free
  5. calloc and realloc
  6. Summary - Memory and stack

19 Preprocessor

  1. #define
  2. #include
  3. Conditional compilation
  4. Macro functions
  5. Header files
  6. Summary - Preprocessor

20 Beginner final

  1. Project design
  2. Data model
  3. Functions
  4. Memory management
  5. Putting it together