在 Dynamic 中使用指针将一个字符串复制到另一个字符串中

Question

Trying to copy a string to another string. As a basic learner I have tried maximum from my side to get the output but in the end of the program(point1) my logic is not working proper. Kindly refer my Input and output given below to get clear idea.

This Program copy a string.

#include<stdio.h>
#include<stdlib.h>

int main()
{
    int n1,n2,loc;
    char *p1, *p2;

    printf("Enter size of p1\n");
    scanf("%d", &n1);

    p1 = (char*) malloc (n1 * sizeof(char));

    printf("Enter the P1 String\n");
    fflush(stdin);
    gets(p1);

    printf("\nEnter the size of p2\n");
    scanf("%d", &n2);

    p2 = (char*) malloc (n2 * sizeof(char));

    printf("Enter the P2 String\n");
    fflush(stdin);
    gets(p2);

    printf("\nEnter the Location to copy\n");
    scanf("%d", &loc);

    for(int i=loc-1;i<=n1;i++) //point 1
    {
       *(p1+i) = *(p1+i)+n2;
    }

    for(int i=0;i<=n2;i++)
    {
       *(p2+i) = *(p1+i)+loc;
    }

    printf("\n Final copy is\n");
        printf("%d",p1);

    free(p1);
    free(p2);

    return 0;
}

Expected:

Input:
google
microsoft

output:
Goomicrosoftgle

Actual:

Input:
google
microsoft

output:
[Some garbage values including given string]

Answer 1

• First of all, don't use gets() as it is unsafe to use and also deprecated
• p1 and p2 are dynamically allocated in the memory and they have their own size of n1 and n2 respectively. So, when you add more characters from p2 into p1 , the size of output needs to be increased, otherwise they wouldn't fit in that memory space
• For string input using scanf the allocated memory should have to be one more than the actual length as one null terminating character is inserted at the end
• In your final print statement, you are using %d which is the format specifier for integer type, so you should use %s as you are printing out whole string

So, this should work:

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int n1, n2, loc;
    char *p1, *p2, *output;

    printf("Enter size of p1: ");
    scanf("%d", &n1);

    p1 = malloc((n1 + 1) * sizeof(char));

    printf("Enter the P1 String: ");
    scanf("%s", p1);

    printf("\nEnter the size of p2: ");
    scanf("%d", &n2);

    p2 = malloc((n2 + 1) * sizeof(char));

    printf("Enter the P2 String: ");
    scanf("%s", p2);

    printf("\nEnter the Location to copy: ");
    scanf("%d", &loc);

    output = malloc((n1 + n2 + 1) * sizeof(char));

    for (int i = 0; i < loc; i++)
        *(output + i) = *(p1 + i);

    for (int i = loc - 1; i <= n1; i++)
        *(output + i + n2) = *(p1 + i);

    for (int i = 0; i < n2; i++)
        *(output + i + loc) = *(p2 + i);

    printf("\nFinal copy is: ");
    printf("%s\n", output);

    free(p1);
    free(p2);
    free(output);

    return 0;
}

Here's the output:

Enter size of p1: 6
Enter the P1 String: google

Enter the size of p2: 9
Enter the P2 String: microsoft

Enter the Location to copy: 3

Final copy is: goomicrosoftgle

Answer 2

Here is the correct logic of copy a string using dynamic method.

output = malloc((n1 + n2 + 1) * sizeof(char)); //After allocating a memory to both sizes of string along with adding memory for NULL.