void load(int *n, int *x, int **arr)
{
arr = (int**)malloc(sizeof(int*)*(*n));
for(int i = *n; i >= 0; i--)
{
scanf("%d", &arr[i]);
}
}
int main()
{
int n = 0, x = 0;
int *arr;
load(&n, &x, &arr);
printf("%d", arr[1]);
return EXIT_SUCCESS;
}
The program compiles properly, but it throws windows error during the printf() in main function. Displaying just "arr" gives random big numbers. What is wrong here?
arr = (int**)malloc(sizeof(int*)*(*n));
doesn't change anything in main, it only overwrites the copy of the pointer (address of arr in main) that load receives.
What the function should do is change arr in main, for that, you have to dereference the argument,
*arr = (int*)malloc(sizeof(int)*(*n)); // cast for C++ compiler left in
to change the value of arr in main. (The object that the argument arr of load points to, that is arr in main, needs to be changed, hence you need to modify *arr in load.)
The scans should then be
scanf("%d", &(*arr)[i]);
or (equivalent)
scanf("%d", *arr + i);
#include <stdio.h>
#include <stdlib.h>
void load(int *n, int *x, int **arr)
{
int i = 0;
*arr = (int*) malloc(*n * sizeof(int));
if(!*arr) {
perror("Can not allocate memory!");
return;
}
for(i = *n; i >= 0; i--)
{
scanf("%d", *arr + i);
}
return;
}
int main()
{
int n = 0, x = 0;
int *arr;
int i;
/* You probably need to initialize n */
n = 5;
load(&n, &x, &arr);
for(i = n; i >= 0; i--)
{
printf("%d - %d\n", i, arr[i]);
}
return EXIT_SUCCESS;
}
Related
double fun(int size) {
double *arr = (double*)calloc(size, sizeof(double));
assert(arr);
for (int i = 0; i < size; i++) {
scanf_s("%lf", &arr[i]);
}
return *arr;
}
void main() {
int size = 5;
printf("%lf", fun(size));
}
trying to print my array using the main function, it prints only the 1st element from the array..
someone have an idea how to fix it?
double fun(int size)
Consider what the function is returning. A single double. The line *arr will get the value of the first element in the arr pointer and return it. What I think you mean to do is iterate over each element in a pointer to a double returned by the function.
double *fun(int size) {
double *arr = (double*)calloc(size, sizeof(double));
assert(arr);
for (int i = 0; i < size; i++) {
scanf_s("%lf", &arr[i]);
}
return arr;
}
void main() {
int size = 5;
double *arr = fun(size);
for (int i = 0; i < size; i++) {
printf("%lf", arr[i]);
}
}
For n=3 and a={1,2,3},b={4,5,6} its supposed to calculate 1*4+2*5+3*6.
I don't understand why does it work because p is a pointer and p=produs(a,b,n) means that the address of p becomes the value returned by produs.
#include <stdio.h>
#include <conio.h>
void citire(int *x,int *n)
{
for(int i=1; i<=*n; i++)
scanf("%d",&x[i]);
}
int produs(int *a,int*b,int n)
{
int produs=0;
for(int i=1;i<=n;i++)
produs=a[i]*b[i]+produs;
return produs;
}
int main()
{
int n;
int*p;
scanf("%d",&n);
int *a=(int*)malloc(n*sizeof(int));
int *b=(int*)malloc(n*sizeof(int));
citire(a,&n);
citire(b,&n);
p=produs(a,b,n);
printf("%d",p);
return 0;
}
When you do:
size_t size = 10;
int* x = calloc(size, sizeof(int));
You get an array x with 10 items in it, indexed 0..9, not 1..10. Here calloc is used to make it abundantly clear what's being requested instead of doing multiplication that can be mysterious or obtuse.
As such, to iterate:
for (int i = 0; i < size; ++i) {
x[i] ...
}
You have a number of off-by-one errors in your code due to assuming arrays are 1..N and not 0..(N-1).
Putting it all together and cleaning up your code yields:
#include <stdio.h>
#include <stdlib.h>
void citire(int *x, size_t s)
{
for(int i=0; i < s; i++)
scanf("%d", &x[i]);
}
int produs(int *a, int* b, size_t s)
{
int produs = 0;
for(int i = 0; i < s; i++)
produs = a[i] * b[i] + produs;
return produs;
}
int main()
{
int n;
scanf("%d",&n);
int* a = calloc(n, sizeof(int));
int* b = calloc(n, sizeof(int));
citire(a, n);
citire(b, n);
// produs() returns int, not int*
int p = produs(a,b,n);
printf("%d", p);
return 0;
}
You're using pointers in places where pointers don't belong. In C passing a pointer to a single value means "this is mutable", but you don't change those values, so no pointer is necessary nor advised.
Try and use size_t as the "size of thing" type. That's what's used throughout C and it's an unsigned value as negative indexes or array lengths don't make any sense.
This program is supposed to take an array, and sort it from lowest to highest value. My program won't sort any values though. I believe the error is in the selectionSort. The values i and j are present in the function, I printed them out inside the function but they are not passed into the swap function. I tried making i and j pointers but it didn't work. I just have no clue on what to do next. Any help would be appreciated.
#include <stdio.h>
#define N 5
void selectionSort(int *a, int n);
int *findLargest(int *a, int n);
void swap(int *p, int *q);
int main(void)
{
int i;
int a[N];
printf("Enter %d numbers: ", N);
for (i = 0; i < N; i++) {
scanf("%d", &a[i]);
}
selectionSort(a, N);
printf("In sorted order:");
for (i = 0; i < N; i++) {
printf(" %d", a[i]);
}
printf("\n");
return 0;
}
void selectionSort(int *a, int n)
{
int *p = a;
int i;
int j;
if (n == 1) {
return;
}
i = *(p+n-1);
j = *findLargest(a, n);
swap(&i, &j);
selectionSort(a, n - 1);
}
int *findLargest(int *a, int n)
{
int *p;
int *p_max = a;
for(p = a + 1; p < a + n - 1; p++) {
if ( *p > *p_max)
p_max = p;
}
return p_max;
}
void swap(int *p, int *q)
{
int temp = *(p-1);
*(p-1) = *q;
*q = temp;
}
The problem is in your call of swap: you swap the content of two local variables
int i;
int j;
... // Some other code, then
swap(&i, &j);
This has no effect on the original array. You should be passing p+n-1 and findLargest(a, n) directly, or store their results in pointers, not in ints:
swap(p+n-1, findLargest(a, n));
In addition, your swap is broken: rather than swapping the content of two pointers, it assumes that p points one element past the target location. This is a bad assumption to make in a general-purpose function, such as swap, and it also leads to undefined behavior in your program.
void swap(int *p, int *q) {
int temp = *p;
*p = *q;
*q = temp;
}
I have a pointer to a pointer ("paths") and I want to reallocate each pointer (each "path"). But I get a crash. Generally I am trying to find all possible powers of a number, which one can compute for some amount of operations (e.g for two operations we can get power of three and four (one operation for square of a number, then another one either for power of three or four)). I figured out how to do it on paper, now I am trying to implement it in code. Here is my try:
#include <stdio.h>
#include <stdlib.h>
void print_path(const int *path, int path_length);
int main(void)
{
fputs("Enter number of operations? ", stdout);
int operations;
scanf("%i", &operations);
int **paths, *path, npaths, npath;
npaths = npath = 2;
path = (int*)malloc(npath * sizeof(int));
paths = (int**)malloc(npaths * sizeof(path));
int i;
for (i = 0; i < npaths; ++i) // paths initialization
{
int j;
for (j = 0; j < npath; ++j)
paths[i][j] = j+1;
}
for (i = 0; i < npaths; ++i) // prints the paths, all of them are displayed correctly
print_path(paths[i], npath);
for (i = 1; i < operations; ++i)
{
int j;
for (j = 0; j < npaths; ++j) // here I am trying to do it
{
puts("trying to reallocate");
int *ptemp = (int*)realloc(paths[j], (npath + 1) * sizeof(int));
puts("reallocated"); // tried to write paths[j] = (int*)realloc...
paths[j] = ptemp; // then tried to make it with temp pointer
}
puts("memory reallocated");
++npath;
npaths *= npath; // not sure about the end of the loop
paths = (int**)realloc(paths, npaths * sizeof(path));
for (j = 0; j < npaths; ++j)
paths[j][npath-1] = paths[j][npath-2] + paths[j][j];
for (j = 0; j < npaths; ++j)
print_path(paths[j], npath);
puts("\n");
}
int c;
puts("Enter e to continue");
while ((c = getchar()) != 'e');
return 0;
}
void print_path(const int *p, int pl)
{
int i;
for (i = 0; i < pl; ++i)
printf(" A^%i -> ", p[i]);
puts(" over");
}
I am not sure the problem resides with the call to realloc(), rather you are attempting to write to locations for which you have not created space...
Although you create memory for the pointers, no space is created (allocate memory) for the actual storage locations.
Here is an example of a function to allocate memory for a 2D array of int:
int ** Create2D(int **arr, int cols, int rows)
{
int space = cols*rows;
int y;
arr = calloc(space, sizeof(int));
for(y=0;y<cols;y++)
{
arr[y] = calloc(rows, sizeof(int));
}
return arr;
}
void free2DInt(int **arr, int cols)
{
int i;
for(i=0;i<cols; i++)
if(arr[i]) free(arr[i]);
free(arr);
}
Use example:
#include <ansi_c.h>
int main(void)
{
int **array=0, i, j;
array = Create2D(array, 5, 4);
for(i=0;i<5;i++)
for(j=0;j<4;j++)
array[i][j]=i*j; //example values for illustration
free2DInt(array, 5);
return 0;
}
Another point here is that it is rarely a good idea to cast the return of [m][c][re]alloc() functions
EDIT
This illustration shows my run of your code, just as you have presented it:
At the time of error, i==0 & j==0. The pointer at location paths[0][0] is uninitialized.
EDIT 2
To reallocate a 2 dimension array of int, you could use something like:
int ** Realloc2D(int **arr, int cols, int rows)
{
int space = cols*rows;
int y;
arr = realloc(arr, space*sizeof(int));
for(y=0;y<cols;y++)
{
arr[y] = calloc(rows, sizeof(int));
}
return arr;
}
And here is a test function demonstrating how it works:
#include <stdio.h>
#include <stdlib.h>
int ** Create2D(int **arr, int cols, int rows);
void free2DInt(int **arr, int cols);
int ** Realloc2D(int **arr, int cols, int rows);
int main(void)
{
int **paths = {0};
int i, j;
int col = 5;
int row = 8;
paths = Create2D(paths, col, row);
for(i=0;i<5;i++)
{
for(j=0;j<8;j++)
{
paths[i][j]=i*j;
}
}
j=0;
for(i=0;i<5;i++)
{
for(j=0;j<8;j++)
{
printf("%d ", paths[i][j]);
}
printf("\n");
}
//reallocation:
col = 20;
row = 25;
paths = Realloc2D(paths, col, row);
for(i=0;i<20;i++)
{
for(j=0;j<25;j++)
{
paths[i][j]=i*j;
}
}
j=0;
for(i=0;i<20;i++)
{
for(j=0;j<25;j++)
{
printf("%d ", paths[i][j]);
}
printf("\n");
}
free2DInt(paths, col);
getchar();
return 0;
}
The realloc() does not fail. What fails is that you haven't allocated memory for the new pointers between paths[previous_npaths] and paths[new_npaths-1], before writing to these arrays in the loop for (j = 0; j < npaths; ++j).
Here's my main:
int main()
{
int i;
int *a = readData(N);
int *f = frequency(a, N, MAX);
int c = 0;
printf("%9s %9s\n", "Number", "Frequency");
for( i = 0; i <= MAX; i ++)
{
c += f[i];
printf("%9d %9d\n", i, f[i]);
}
printf("total frequency for all words: %d\n", c);
free(f);
free(a);
printf("All memory freed!\n");
return(0);
}
and here is the first supporting function
int * readData(int size)
{
int i, *array = malloc(N*sizeof(int));
for(i =0; i<N;i++)
scanf("%d", &array[i]);
return array;
}
and the second supporting function
int * frequency(int *input, int size, int max)
{
int i, x;
int *farray = malloc(max+1*sizeof(int));
for(i = 0; i<=max;i++)
farray[i]=0;
for(i = 0; i<1099; i++)
{
x = input[i];
farray[x]++;
}
return farray;
}
Everything works great! When I run it through valgrind, it tells me that all my memory was freed. But, when I run the program normally, it crashes at the end with a core dump when I free the allocated memory. Why? Thanks in advance! I'm new at C.
It's true that all the memory is freed, but the size you allocated is not correct.
int *farray = malloc(max+1*sizeof(int));
Here, the size is max plus one sizeof(int)), it should be:
int *farray = malloc((max+1)*sizeof(int));