C program copy string without using strcpy() with enough memory - c

Question

I am trying to have this output:
Comparing results of concat and strcat ...
strcmp("Plain old stringTroy", "Plain old stringTroy") says: 0
The strcmp returns 0 if the two string arguments are identical. If the result is 0, then the concat behaves exactly like the library function strcat.
this is what I have for concat method.
#define MAXSIZE 32
void concat(char dest[], char src[])
{
int i=length(src);
int j=0;
for(j; j<src[j] !='\0'; j++) {
dest[i+j] = src[j];
}
dest[i+j] = '\0';
}
length method is:
int length(char str[])
{
// Add code here to return the length of the
// string str without using the strlen function
// Do not count the null character '\0'
// in computing the length of the string
int len=0;
int i;
for(i=0;i<str[i];i++) {
len++;
}
return len;
}
This is my main
int main()
{
// Variable declarations for all parts
char str2[] = "Troy";
char str4[] = "Plain old string";
char str6[MAXSIZE];
// Part 6
printf("\n----- Part 6 -----\n");
// Make a copy of the destination string first, to be reused later
strcpy(str6, str4);
concat(str4, str2);
strcat(str6, str2);
printf("Comparing results of concat and strcat ...\n");
printf("strcmp(\"%s\", \"%s\") says: %d\n",
str4, str6, strcmp(str4, str6)
);
return 0;
}
This is my output when I run it:
----- Part 6 -----
Comparing results of concat and strcat ...
strcmp("PlaiTroy", "Plain old stringTroy") says: -1
The first string is not the same as the second string which is why I am getting a -1. My problem is in my concat method but I can't seem to understand why it won't execute well. Is it because of the spaces? Is 0 and '\0' not executing well?

Answer 1

There are multiple problems in your code:
The loop test in the length function is incorrect: instead of i < str[i], it should be:
for (i = 0; str[i] != '\0'; i++)
the same problem in the concat function. Change the loop to:
for (j = 0; src[j] != '\0'; j++) {
also in the concat function, i should be the length of dst, not that of src. You might use len instead of i for this variable.
The array str4 in function main does not have any space available at the end for concat to append anything. Define it with a larger size this way:
char str4[MAXSIZE] = "Plain old string";
Here is the corrected version:
#include <stdio.h>
#include <string.h>
#define MAXSIZE 32
void concat(char dest[], char src[]) {
int len = length(dest);
int j;
for (j = 0; src[j] != '\0'; j++) {
dest[len + j] = src[j];
}
dest[len + j] = '\0';
}
int length(char str[]) {
int len = 0;
int i;
for (i = 0; i < str[i]; i++) {
len++;
}
return len;
}
int main(void) {
// Variable declarations for all parts
char str2[MAXSIZE] = "Troy";
char str4[MAXSIZE] = "Plain old string";
char str6[MAXSIZE];
// Part 6
printf("\n----- Part 6 -----\n");
// Make a copy of the destination string first, to be reused later
strcpy(str6, str4);
concat(str4, str2);
strcat(str6, str2);
printf("Comparing results of concat and strcat ...\n");
printf("strcmp(\"%s\", \"%s\") says: %d\n",
str4, str6, strcmp(str4, str6));
return 0;
}

Answer 2

You have several problems in both functions:
concat
for(j; j<src[j] !='\0'; j++) {
What is the for exit condition here?, src[j] != '\0' is enough.
dest[i+j] = src[j];
Here you add data with an offset of i, but I is the length of src, not dst.
So the corrected function could be:
void concat(char dest[], char src[])
{
/* descriptive variable name */
int len_dst = length(dst);
int j=0;
/* clear exit condition */
for(; src[j] != '\0'; j++) {
dest[len_dst+j] = src[j];
}
dest[len_dst+j] = '\0';
}
length
for(i=0;i<str[i];i++) {
Same remark, what is this exit condition? src[i] != '\0' is enough
So the corrected function could be:
int length(char str[])
{
int len=0;
int i;
/* clear exit condition */
for ( i=0; str[i] != '\0'; i++) {
len++;
}
return len;
}
main
And warning in main function:
char str4[] = "Plain old string";
concat(str4, str2); /* <- erases what is after str4 */
You do not have enough space to store result. Write something like:
char str2[] = "Troy";
char str4[MAXSIZE] = "Plain old string"; /* <-- reserve spaces after str4*/
/* ... */
concat(str4, str2);