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int main()
{
int a[3][4] = {
{0, 1, 2, 3} , // initializers for 0 row
{4, 5, 6, 7} , // initializers for 1 row
{8, 9, 10, 11} // initializers for 2 row
};
int b[3][4] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11};
printf("size of *a = %ld\n", sizeof(*a));
printf("size of *b = %ld\n", sizeof(*b));
//these prints out 16.
//why is this 16?
Hello, I am learning c in school and I have a question.
why is the size of a(and b) 16?? How does it work?
I am confused on how these C pointers work.
Here is what I am trying to do:
Create a struct that has a row, column, and grid (2d array of predefined size).
typedef struct
{
int row;
int column;
int (*matrix)[GRID_SIZE];
} parameters;
Now I create one
int grid[GRID_SIZE][GRID_SIZE] = {
{6, 5, 3, 1, 2, 8, 7, 9, 4},
{1, 7, 4, 3, 5, 9, 6, 8, 2},
{9, 2, 8, 4, 6, 7, 5, 3, 1},
{2, 8, 6, 5, 1, 4, 3, 7, 9},
{3, 9, 1, 7, 8, 2, 4, 5, 6},
{5, 4, 7, 6, 9, 3, 2, 1, 8},
{8, 6, 5, 2, 3, 1, 9, 4, 7},
{4, 1, 2, 9, 7, 5, 8, 6, 3},
{7, 3, 9, 8, 4, 6, 1, 2, 5}};
parameters *data = (parameters *) malloc(sizeof(parameters));
data->row = 1;
data->column = 2;
data->matrix = grid; // is this right?
int** test = data->matrix; //this is wrong.
My goal is to pass in that struct as data when creating a new thread. I am really clueless on how the pointer system works when creating the 2D array in the struct and then using it later. Any help is appreciated.
Edit... here is my new code.. looks like its working! :)
parameters *data = malloc(sizeof(parameters)); //make sure to free thread after
data->row = 1;
data->column = 2;
data->matrix = grid;
int (*test)[GRID_SIZE] = data->matrix;
printf("first value, %d\n", test[0][0]);
Something like this should work.
Define your struct like this:
typedef struct
{
int row;
int column;
int matrix[GRID_SIZE][GRID_SIZE];
} parameters;
Copy the matrix like this:
memcpy( data->matrix, grid, sizeof(grid) );
I'm supposed to make a method to test if two arrays have the same values in reverse order.
public static boolean areReversed (int [] t , int [] q)
{
boolean identical = false;
if(t.length != q.length){ identical = false;}
else{
for(int x = (t.length -1) , y = 0; -1 < x; x-- , y++)
{
if(t[y] == q[x])
{
identical = true;
}
}
}
return identical;
}
I test it by running it through these if statements with the following pre-determined arrays
int[] a = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
int[] b = {0, 9, 8, 7, 6, 5, 4, 3, 2, 1};
int[] c = {1, 2, 3, 4, 5, 6, 7, 8, 9};
int[] d = {9, 8, 7, 6, 5, 4, 3, 2, 1};
int[] e = {1, 2, 3, 4, 5, 4, 3, 2, 1};
int[] f = {1, 2, 3, 4, 4, 3, 2, 1};
int[] g = {1, 3, 5, 7, 9, 0, 2, 4, 6, 8};
int[] h = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
int[] i = {1, 1, 3, 3, 5, 5, 7, 7, 9, 9};
//test for areReversed, uncomment the next two lines to test your method
if(areReversed(a, b) && !areReversed(a, g) && !areReversed(a, c)) System.out.println("Basic areReversed method test PASSED");
else System.out.println("Basic areReversed method test FAILED");
My method doesn't pass the test, am I making an oversight somewhere in my code?
Yes, you are indexing to two arrays in the same order. You are essentially checking that they are equal, not that they are the reverse of each other.
Also, remember that a function can return at any point Think about this and think about how you could make your function more efficient in a case where the arrays look like this:
[1,2,3] and [4,2,4].
How many comparisons do you actually need to make before you can tell us the arrays are not the same.
This is what you should do:
public static boolean areReversed (int [] t , int [] q)
{
if(t.length != q.length) return false;
for(int x = (t.length -1) , y = 0; -1 < x; x-- , y++)
{
if(t[y] != q[x]) return false;
}
return true;
}
Note that I'm returning false from the method as and when it is known that the reversed arrays are unequal. Hence, at the end, if false isn't returned yet, the arrays have to be reversed equal, and thus, true is returned.
I made a question some hours ago but I get myself in a mess on what I had to do after finishing what I was asking on that question. All the solutions that the people gave me were ok, but useless for what I was really looking for as I didn't wrote the question as it has to be. I've to save an important position of a value, and it wasn't necessary to be saved on the other question to solve the problem. So here's the proper one.
(everything's is explained with an example above, understanding it is easy) I've a 8x8 matrix, and after choosing the row I desire, I want to get the three minimum elements of it, and choose one of this three randomly. Then, remove the row and column that contains this number. The thing is that I'dont know how to handle those three elements and remove the columns/rows. I just know how to get the minimum element, that is the following code.
int pieza[ROWS][COLS] = {
0, 2, 2, 5, 3, 2, 1, 1,
0, 4, 5, 2, 4, 3, 0, 0,
0, 4, 2, 2, 1, 2, 3, 2,
0, 3, 1, 5, 1, 2, 3, 4,
2, 5, 6, 5, 3, 1, 2, 7,
8, 2, 0, 0, 0, 2, 1, 1,
1, 2, 2, 1, 1, 6, 3, 4,
0, 1, 3, 2, 0, 0, 0, 0,
};
int myrow = 3; // the row I want to analyze
int index;
int min=0;
for (index=0;index<8;index++) {
printf("%d", piezas[myrow][index] );
if(piezas[myrow][index]<min)
min=piezas[myrow][index];
printf("\t\t");
}
printf("min: %d", min);
This is what I want to do. If the initial matrix is (which is always a nxn matrix):
{
0, 2, 2, 5, 3, 2, 1, 1,
0, 4, 5, 2, 4, 3, 0, 0,
0, 4, 2, 2, 1, 2, 3, 2,
0, 3, 1, 5, 1, 2, 3, 4,
2, 5, 6, 5, 3, 1, 2, 7,
8, 2, 0, 0, 0, 2, 1, 1,
1, 2, 2, 1, 1, 6, 3, 4,
0, 1, 3, 2, 0, 0, 0, 0,
};
And I choose row number 3:
0, 3, 1, 5, 1, 2, 3, 4,
The algorithm must choose the three minimum elements of that row.
0, 1, 1
And choose randomly one of these three. If, for example, it choose the first 'one'...
0, **1**, 1
... the algorithm must go to the 3th column of that line (becaue that was the position that was that '1') and remove the row and column, so the output matrix will be as follows, one dimension less than the original matrix (beucase you have removed a row and a column):
{
0, 2, 5, 3, 2, 1, 1,
0, 4, 2, 4, 3, 0, 0,
0, 4, 2, 1, 2, 3, 2,
2, 5, 5, 3, 1, 2, 7,
8, 2, 0, 0, 2, 1, 1,
1, 2, 1, 1, 6, 3, 4,
0, 1, 2, 0, 0, 0, 0,
};
I only know how to arrive to the line, but I'm having problems to handle three minimums because I'm having tons of problems pointers and I'm not a lot into C. Thanks in advance
Example to be sorted with the number of column.
#include <stdio.h>
#include <stdlib.h>
typedef struct pair {
int value, column;
} Pair;
int cmp(const void *a, const void *b){
Pair *pa = (Pair *)a;
Pair *pb = (Pair *)b;
return pa->value - pb->value;
}
int main(void){
int data[8] = {0, 3, 1, 5, 1, 2, 3, 4};
Pair data_pair[8];
int i;
for(i=0;i<8;++i){
data_pair[i].value = data[i];
data_pair[i].column = i;
}
qsort(data_pair, 8, sizeof(Pair), cmp);
for(i=0;i<3;++i)
printf("value = %d, column = %d\n", data_pair[i].value, data_pair[i].column);
return 0;
}
/* result
value = 0, column = 0
value = 1, column = 2
value = 1, column = 4
*/
#include <stdio.h>
#include <string.h>
#define SIZE 8
void delrow(int a[SIZE][SIZE], int row){
if(row < SIZE - 1)
memmove(&a[row], &a[row+1], (SIZE*SIZE - SIZE*(row+1))*sizeof(int));
};
void delcol(int a[SIZE][SIZE], int col){
int r;
if(col < SIZE - 1){
for(r=0;r<SIZE;++r){
memmove(&a[r][col], &a[r][col+1], (SIZE - (col+1))*sizeof(int));
}
}
}
int main(void){
int piezas[8][8] = {
0, 2, 2, 5, 3, 2, 1, 1,
0, 4, 5, 2, 4, 3, 0, 0,
0, 4, 2, 2, 1, 2, 3, 2,
0, 3, 1, 5, 1, 2, 3, 4,
2, 5, 6, 5, 3, 1, 2, 7,
8, 2, 0, 0, 0, 2, 1, 1,
1, 2, 2, 1, 1, 6, 3, 4,
0, 1, 3, 2, 0, 0, 0, 0,
};
//test
int row = 8, col = 8;
int r,c;
delrow(piezas, 3);
row -= 1;
for(r=0;r<row;++r){
for(c=0;c<col;++c)
printf("%2d", piezas[r][c]);
printf("\n");
}
printf("\n");
delcol(piezas, 1);
col -= 1;
for(r=0;r<row;++r){
for(c=0;c<col;++c)
printf("%2d", piezas[r][c]);
printf("\n");
}
return 0;
}
/* result
0 2 2 5 3 2 1 1
0 4 5 2 4 3 0 0
0 4 2 2 1 2 3 2
2 5 6 5 3 1 2 7
8 2 0 0 0 2 1 1
1 2 2 1 1 6 3 4
0 1 3 2 0 0 0 0
0 2 5 3 2 1 1
0 5 2 4 3 0 0
0 2 2 1 2 3 2
2 6 5 3 1 2 7
8 0 0 0 2 1 1
1 2 1 1 6 3 4
0 3 2 0 0 0 0
*/
Below is extract to get a nth smallest element from a selected row -Your first requirement before removing a row and column.
Copy the row
Sort the copied row (save index too while sorting)
Sorted index represents position of value in sorted order
pick 0 th or 1st or nth min indexing the sorted index.
------------ Very Draft code ---- try optimize -------
#include <stdio.h>
#include<memory.h>
void sortIndex(int *array, int *arrayIdx)
{
int i=0,j=0;
int temp=0;
int tempArr[4];
memcpy(tempArr, array, 4*sizeof(int));
for(i=0;i<4;i++)
{
printf("%d ",tempArr[i]);
}
printf("\n");
for(i=0;i<4;i++)
{
for(j=i+1;j<4;j++)
{
if(tempArr[i]>tempArr[j])
{
temp = arrayIdx[i];
arrayIdx[i]=arrayIdx[j];
arrayIdx[j]=temp;
temp = tempArr[i];
tempArr[i]=tempArr[j];
tempArr[j]=temp;
}
}
}
printf("Sorted array Index\n");
for(i=0;i<4;i++)
{
printf("%d ",arrayIdx[i]);
}
printf("\n");
printf("Sorted array Value\n");
for(i=0;i<4;i++)
{
printf("%d ",array[arrayIdx[i]]);
}
printf("\n");
}
int main ()
{
int array[4][4] = {{4,3,2,1},{7,5,4,3},{6,5,4,4},{5,5,2,1}};
int sortedIdx[4] = {0,1,2,3};
int i,ii;
for(i=0;i<4;i++)
{
for(ii=0;ii<4;ii++)
printf("%d ",array[i][ii]);
printf("\n");
}
printf("(Note:Count from 0). Which Row : ");
scanf("%d",&i);
sortIndex(array[i],sortedIdx);
printf("\n");
printf("(Nth smallest value)Give a N value (0 to 3): ");
scanf("%d",&ii);
printf(" (%d) smallest value in row (%d) is (%d)\n",ii,i,array[i][sortedIdx[ii]]);
printf("Now call function to remove Row (%d) and column (%d)\n",i,sortedIdx[ii]);
return 0;
}
I'm writing a variable list implementation. In my insertion step, I check if the array's size is maxed out, and if so I double the maximum capacity and call realloc() to allocate me some new memory. Going from size 2 to 4, 4 to 8, and 8 to 16 works fine, but going from size 16 to 32 gives me some random zeroes in my array. Can anyone tell me what's going on here? I know I could avoid using realloc() by mallocing some new space, using memcpy and then freeing the old pointer... and perhaps there's no performance hit from doing that. But my intuition tells me that there is, and in any case I thought that's what realloc was there for. Can anyone tell me what's going on? The key function in this code is the append function.
#include <stdio.h>
#include <stdlib.h>
#include "problem5.h"
#define FAILURE -1
#define SUCCESS 0
ArrayList ArrayList_Init(int n, int (*append) (ArrayList, int), void (*print) (ArrayList), void (*insert) (ArrayList, int, int), void (*destroy) (ArrayList), int (*valueOf) (ArrayList, int))
{
ArrayList newArrayList = (ArrayList) malloc(sizeof(ArrayList_));
newArrayList->max_size = n;
newArrayList->current_size = 0;
newArrayList->data = malloc(n*sizeof(int));
newArrayList->append = append;
newArrayList->destroy = destroy;
newArrayList->print = print;
newArrayList->insert = insert;
newArrayList->valueOf = valueOf;
return newArrayList;
}// init a new list with capacity n
int append_(ArrayList list, int val)
{
//if the array is at maximum capacity
//double the capacity
//update max_size
//insert the value in the first open spot in the array (aka index current_size)
//increment current_size
if (list->current_size == list->max_size) {
list->max_size *= 2;
if (( list->data = realloc(list->data, list->max_size) ) == NULL)
return FAILURE;
}
list->data[list->current_size] = val;
list->current_size++;
return SUCCESS;
}
void print_(ArrayList list)
{
int i;
printf("List of size %d, max size %d. Contents:\n", list->current_size, list->max_size);
for (i=0; i<list->current_size; i++)
printf("%d, ", list->data[i]);
printf("\n");
}
void insert_(ArrayList list, int val, int index) {
}// insert val into index
void destroy_(ArrayList list)
{
//free list memory
}
int valueOf_(ArrayList list, int index)
{
//return value of specified element
return 0;
}
int main()
{
ArrayList list;
int stat, count = 0;
list = ArrayList_Init(2, append_, print_, insert_, destroy_, valueOf_); // init a new list with capacity 8
do {
printf("Appending %d\n", count);
stat = list->append(list, count) ; // add val to end of the list
list->print(list);
} while (stat == SUCCESS && ++count < 20);
return 0;
}
And here's the output of this:
Appending 0
List of size 1, max size 2. Contents:
0,
Appending 1
List of size 2, max size 2. Contents:
0, 1,
Appending 2
List of size 3, max size 4. Contents:
0, 1, 2,
Appending 3
List of size 4, max size 4. Contents:
0, 1, 2, 3,
Appending 4
List of size 5, max size 8. Contents:
0, 1, 2, 3, 4,
Appending 5
List of size 6, max size 8. Contents:
0, 1, 2, 3, 4, 5,
Appending 6
List of size 7, max size 8. Contents:
0, 1, 2, 3, 4, 5, 6,
Appending 7
List of size 8, max size 8. Contents:
0, 1, 2, 3, 4, 5, 6, 7,
Appending 8
List of size 9, max size 16. Contents:
0, 1, 2, 3, 4, 5, 6, 7, 8,
Appending 9
List of size 10, max size 16. Contents:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
Appending 10
List of size 11, max size 16. Contents:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
Appending 11
List of size 12, max size 16. Contents:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
Appending 12
List of size 13, max size 16. Contents:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
Appending 13
List of size 14, max size 16. Contents:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
Appending 14
List of size 15, max size 16. Contents:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
Appending 15
List of size 16, max size 16. Contents:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
Appending 16
List of size 17, max size 32. Contents:
0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 16,
Appending 17
List of size 18, max size 32. Contents:
0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 16, 17,
Appending 18
List of size 19, max size 32. Contents:
0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 16, 17, 18,
Appending 19
List of size 20, max size 32. Contents:
0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 16, 17, 18, 19,
It is very bad to write so:
list->data = realloc(list->data, list->max_size)
You should use new variable and if memory was reallocated you write so:
List->data = temp;
It will protect you from memory leak.
And you forgot *sizeof(int)