Replace space with 20 in a string

/* C programme to replace a strings spaces with %20 */
#include <stdio.h>
int main(void)
{
    int i,j=0;
    char in[1000],out[1000];
    gets(in);
    for (i=0;in[i]!='\0';i++)
        {
        if (in[i] == ' ')
            {
            out[j++]='%';
            out[j++]='2';
            out[j++]='0';
        }
        /*
        WE CAN ALSO WRITE IT LIKE THAT
               if(in[i]==' ')
            {
                    out[j] = '%';
            out[j+1] = '2';
            out[j+2] = '0';
            j=j+3;
            }
        */
        else
        {
            out[j]=in[i]; //it is also correct out[j++]=in[i]; then we need not to write j++; later
            j++;
        }
    }
    out[j]='\0';
    puts(out);
    return 0;
}