How can I concatenate characters to a string in C?

Question

It's an exercise where I have to build a function which returns a string called "secret identity" composed with your birth date, your name and your mother's name (for example, if "02/12/2007", "LUCY TOLKIEN" and "JENNIFER" it returns "20070212LT*J") but I'm struggling to concatenate the characters (like "L" and "T" of "LUCY TOLKIEN") to the string called "secret identity". I hope I could explain it well. There's what I did by far:

int length(char * s) {
    int i, n = 0;
    for (i = 0; *(s + i) != '\0'; i++) {
        n++;
    }

    return n;
}

void concatenate(char * s, char * t) {
    int i = 0;
    int j;

    while (*(s+i) != '\0') {
        i++;
    }

    for (j = 0; *(t+i) != '\0'; j++) {
        *(s + i) = *(t + j);
        i++;
    }
    *(s + i + 1) = '\0';
}

void copy(char * dest, char * orig) {
    int i;
    for (i = 0; *(orig + i) != '\0'; i++) {
        *(dest + i) = *(orig + i);
    }

    *(dest + i) = '\0';
}

void geraIdentidade(void) {
    char * ident;
    int lname, ldate, lmom;

    char name[80];
    printf("Name: ");
    scanf(" %[^\n]s", name);

    lname = length(name);

    char date[11];
    printf("Date: ");
    scanf(" %[^\n]s", date);

    ldate = length(date);

    char mom[20];
    printf("Name (mom): ");
    scanf(" %[^\n]s", mom);

    lmom = length(mom);

    char day[3], month[3], year[5];
    int i, j, k; 

    for (i = 0; date[i] != '/'; i++) {
        day[i] = date[i];
    day[i + 1] = '\0';
    }

    for (j = 3, i = 0; date[j] != '/'; j++, i++) {
        month[i] = date[j];
    month[i + 1] = '\0';
    }

    for (k = 6, i = 0; k <= 9; k++, i++) {
        year[i] = date[k];
    year[i + 1] = '\0';
    }

    ident = (char*)malloc((lmom + ldate + lname) * sizeof(char)); //change lenght

    if (ident != NULL) {
        copy(ident, year);
        concatenate(ident, month);
        concatenate(ident, day);
    }
    else {
        return NULL;
    }

    printf("%s\n", ident);


}

int main(void) {

    geraIdentidade();

    return 0;
}

Answer 1

For my opinion, 3 functions in your code:

int length(char * s)
void concatenate(char * s, char * t)
void copy(char * dest, char * orig)

You can make the code easier to do when you use some C standard functions in <string.h>:

size_t strlen(const char *s); // for length
char *strcpy(char *dest, const char *src); // for copy 
char *strcat(char *dest, const char *src); // for concatenation

When you want to concatenate string and character, you just need to convert character to string by adding the \0 character to the character you want to concatenate. For example, if you want to concatenate T to string 20070212L:

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

int main()
{
    char str[11] = "20070212L";
    char ch[2] = "\0";
    ch[0] = 'T';
    strcat(str, ch);
    printf("str = %s", str);
    return 0;
}

The output:

str = 20070212LT