why the string is getting altered after strcat()?

Question

this is the source code

int main()
{
    char str[]=\"dance\";
    char str1[]=\"hello\";
    char str2[]=\"abcd\";
    strcat(str1,str2);
    printf(\"%s\",s         


        

Answer 1

str1 has not enough space to concatenate the string str2. This invokes undefined behavior. You may get anything. Either expected or unexpected result.
Now try this:

#include 
#include  

int main(void) {
    char str[]="dance";
    char str1[10]="hello";
    char str2[]="abcd";
    strcat(str1,str2);
    printf("%s\n",str1);
    printf("%s\n",str);

    return 0;
}  

Output:

helloabcd
dance