Copying first half of dynamic array into second half failed in C?-CodePudding

I wanted to copy the first half of my dynamic array into the second half of the same array, but the output here is from ptr[100] to ptr[200] 9.90. I think I coded everything fine but I dont know where the problme is. Can you help me?


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

/* run this program using the console pauser or add your own getch, system("pause") or input loop */

int main(int argc, char *argv[]) {
    int i;

    int j;
    
    double * ptr = calloc(100, sizeof(double));
    
    if (ptr == NULL)
        return 0;

        for(i = 0; i < 100; i  ) {
            ptr [i] = i / 10.0;
        }
    
    
    for(i = 0; i < 100; i  ) {
        printf("ptr[%d] = %.2f\n", i, ptr[i]);
        
        }
        
        if (realloc(ptr, 200) == NULL)
            return 0;


            for (i = 0; i < 100; i  ) 
                for (j = 100; j < 201; j  ) 
                    ptr [j] = ptr[i];
                
        
            
            
            for (j = 100; j < 201; j  ) 
                    printf("ptr[%d] = %.2f\n", j, ptr[j]);
        
        
    return 0;
}

CodePudding user response：

You are underallocating on the realloc. You didn't make enough space for 200 doubles, so attempting to access ptr[x] is undefined behavior for values of x > 99. Also, realloc may move the memory, so you need to assign the new value back to ptr. For example:

ptr = realloc(ptr, 200 * sizeof *ptr);
if( ptr == NULL ){
    perror("realloc");
    exit(1);
}

And after this, accessing ptr[200] is undefined behavior. You need to reduce the scope of the loop index to for( j = 100; j < 200; j )

CodePudding user response：

You are overwriting for every copy

for (i = 0; i < 100; i  ) // for every item in the first half
    for (j = 100; j < 201; j  )  // into every cell
        ptr [j] = ptr[i];

shoud be

for (i = 0; i < 100; i  ) 
        ptr [i 100] = ptr[i];

and issue 2:

you didn't raallocate enough memory : change realloc(ptr, 200) to realloc(ptr, 200 * sizeof(double))