String concatenation

Learn to concatenate strings with strcat and sprintf.

Concatenation with strcat

strcat(dest, src) appends the src string to the end of dest. dest must be large enough; otherwise a buffer overflow occurs.

Example: char dest[20] = "Merhaba"; strcat(dest, " dünya");. Here dest is a 20-byte array; strcat appends " dünya". Result: "Merhaba dünya". dest cannot be a string literal; strcat modifies it. So use a writable array like char dest[20].

Formatted concatenation with sprintf

sprintf works like printf but writes the output into a string instead of the screen. It's ideal for joining several pieces at once.

Example: char buf[30]; sprintf(buf, "%s %s", "Merhaba", "dünya");. This joins two strings with a space between. sprintf is powerful but dangerous: it doesn't check the output size. That's why modern C prefers snprintf; you'll see it in the safety lesson. For now, learning sprintf for formatted joins is enough.

Exercise Concatenate two strings

Declare a writable array char dest[20] = "Merhaba";. Append the second string with strcat(dest, " dünya");. Print with printf("%s\n", dest);. Include <string.h>.

Show example solution

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

int main(void) {
    char dest[20] = "Merhaba";
    strcat(dest, " dünya");
    printf("%s\n", dest);
    return 0;
}

Test yourself

Match the concatenation tools

strcat(dest, src)

Append
sprintf(dest, fmt, ...)

Formatted write
strcpy(dest, src)

Copy
strlen(s)

Measure length
Which string does strcat modify?
- Only src
- Both
- Only dest
- Neither
Show answer

Only dest — strcat appends src to dest; only dest changes.

Complete the strcat call

char dest[20] = "Merhaba";
____(dest, " dünya");

Show answer

char dest[20] = "Merhaba";
strcat(dest, " dünya");

Next lesson: String search

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