Inserting a string into another string no library functions - c

Question

I am attempting to complete a homework assignment, part of which is to design a function that inserts a string into another string. All functions, with the exception of strlen(), are off limits to me. Specifically the problem is this: I am having trouble implementing the part of the function that "makes room" in the target string for the inserted string. Inserting the string always results in a trampling of array elements. Can anyone point out what I am doing wrong here? Thanks for helping me learn!
Edit: Integer n is the location in the string that I am supposed to insert the other string.
void insertstring(char *str, char *ins, int n)
{
int i;
int scoot=strlen(ins);
char *p=str+n;
for (i=strlen(str); i > n; --i) { //This is where I'm doing it wrong I think
str[i+scoot]=str[i]; //
}
do {
*p = *ins;
++p;
++ins;
}
while (*ins != '\0');
}

Answer 1

An elegant solution uses reversing to achieve the desired result. Assume your target string, str, is composed of two blocks, AB, where A is the block before the insertion point, and B is the block after insertion point. Furthermore, assume that the string to insert, ins, is denoted by a third block C.
It can be seen that if you concatenate C to AB and then reverse B, reverse C, and reverse both B and C, you get ACB, which is the desired result. More explicitly, this is what you have after appending C to AB:
ABC
And this is what you want:
ACB
ACB can be obtained by:
Reverse B, to get B_r (B reversed);
Reverse C, to get C_r - at this point we have AB_rC_r;
Reverse both B and C, that is, compute A(B_rC_r)_r, which yields ACB.
Here's the code that implements this algorithm:
void reverse(char *, char *, char *);
/* Insert string C between blocks AB in str */
void insertstring(char *str, char *ins, int n) {
/* 1 - Append C to the end of str */
int i = strlen(str);
char *p = str+i, *q = ins;
while ((*p++ = *q++));
p--;
/* 2 - Reverse C and B */
reverse(str, str+i, p-1); /* C */
reverse(str, str+n, str+i-1); /* B */
/* 3 - Reverse B_rC_r */
reverse(str, str+n, p-1);
}
void reverse(char *str, char *begin, char *end) {
char tmp;
while (begin < end) {
tmp = *begin;
*begin = *end;
*end = tmp;
begin++;
end--;
}
}
And some sample code to test it:
#include <stdio.h>
#include <string.h>
int main() {
void insertstring(char *, char *, int);
char test[128] = "A string to test.";
printf("Before: %s\n", test);
insertstring(test, "hello!", 4);
printf("After: %s\n", test);
return 0;
}
This will insert "hello!" beginning in test[4]. Here's the output:
$ ./test
Before: A string to test.
After: A sthello!ring to test.
Consider taking this approach: the code is short and elegant. This technique is described in Programming Pearls, 2nd edition, as a good way to perform vector rotation. According to the author, Brian Kernighan and P.J. Plauger used precisely this method in their Software Tools in Pascal to move lines within a text editor.

Answer 2

First you have change
for (i=strlen(str); i > n; --i)
to
for (i=strlen(str); i >= n; --i)
because you have to move str[n] too.
The other problem is when you insert ins:
do {
*p = *ins;
++p;
++ins;
}
while (*ins != '\0');
Here you copy the terminating '\0' from ins to str so the rest is lost. Change it to
while (*ins != '\0') {
*p = *ins;
++p;
++ins;
}

Answer 3

After mentioning about n, update for loop as
for (i=n+scoot; i >= n; i--)
You want to move strlen(ins) number of character ahead from location n.

Answer 4

Tried it with single loop,
go to index , copy src content into temp_array and simultaneously copying medi into src
once medi is copied completely , start putting the content of temp_array till it become empty
void insert_in_middle (char *src, char *medi, int index)
{
int i=0, j = index, k=0;
char temp_array[50];
while(src[j] != '\0' || temp_array[k] != '\0')
{
temp_array[i] = src[j];
if(medi[i] != '\0')
{
src[j] = medi[i];
}
else if(temp_array[k] != '\0')
{
src[j] = temp_array[k];
k++;
}
i++; j++;
}
printf("src[%s]\n", src);
printf("medi[%s]\n",medi);
printf("temp_array[%s]\n",temp_array);
}

Answer 5

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *str_ins();
int main()
{
char *test = "Hello, world!";
printf("%s\n",test);
str_ins(&test,strstr(test,"world"),"my ");
printf("%s\n",test);
}
char *str_ins(a, b, c) // insert string c into a at point b
char **a, *b, *c;
{
char *stemp;
int pos = strlen(*a) - strlen(b);
stemp = (char *) malloc(strlen(*a)+strlen(c)+1);
strncpy(stemp,*a,pos);
strcpy(stemp+pos,c);
strcat(stemp,b);
free(a);
*a = stemp;
return *a;
}