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I have written code which allows me to modify the elements of a 1D array within my function by passing the element of the array:
I print the original array
I pass each element of the array to the function
Within the function I add the value 50 to each element of the array
I then call the function, and print out to screen the modified element value (i.e the value of each element +50)
I have been able to do this for a 1D array, with example values in the array being (10,20,30) and the valued printed after modification being (60,70,80).
What I am hoping to do is adapt that code to work for 2D arrays, you will see my attempt at doing this below. This code focuses on the use of int, but once I understand how to achieve this I am hoping to adapt for the use of a 2D string as well.
With the code below:
My objective is
Print to screen the original 2D array
Pass each element of the 2D array to the function
Within the function add the value 50 to each element of the array
Then call the function, and print out the modified element values to the screen(expected result displayed on screen 60,61,etc,.)
So far I have been able to print the original 2D array to the screen. It is the function I think I am messing up and would appreciate any advice. Thank you.
#include <stdio.h>
#include <string.h>
#define M 4
#define N 2
int function(int **arr);
int main() {
int i, a;
int arr[N][M] = {10, 11, 12, 13, 14, 15, 16, 17};
// the int array first
for(i = 0; i < N; i++) {
for(size_t j = 0; j < M; j++) {
// Accessing each variable
printf("value of arr[%d] is %d\n", i, arr[i][j]);
}
}
printf("\n ***values after modification***\n");
a = function(&arr[i][0]);
// int array print results
for(int i = 0; i < N; i++) {
for(size_t j = 0; j < M; j++) {
printf("value of arr %d\n", arr[i][j]);
}
}
return 0;
}
int function(int **arr) {
int i;
int j;
for(int i = 0; i < 3; i++) {
for(size_t j = 0; j < 5; j++) {
arr[i][j] = arr[i][j] + 50;
}
}
}
My apologies in advance for silly mistakes I am very new to C.
Thank you in advance.
The function int function(int **arr) does not return an int so make it void.
When you call it, a = function(&arr[i][0]);, you do not use a after the assignment. I suggest that you remove a from the program completely since it's not used anywhere.
The call to the function, function(&arr[i][0]);, should simply be function(arr);
The function signature needs to include the extent of all but the outermost dimension:
void function(int arr[][M])
Inside the function, you use 3 and 5 instead of N and M. That accesses the array out of bounds.
In function, the i and j you declare at the start of the function are unused. Remove them.
arr[i][j] = arr[i][j] + 50; is better written as arr[i][j] += 50;
When initializing a multidimensional array, use braces to make it simpler to read the code:
int arr[N][M] = {{10, 11, 12, 13}, {14, 15, 16, 17}};
In main you mix int and size_t for the indexing variables. I suggest you settle for one type.
Remove unused header files (string.h)
Example:
#include <stdio.h>
#define N 2
#define M 4
void function(int arr[][M]) {
for(int i = 0; i < N; i++) {
for(size_t j = 0; j < M; j++) {
arr[i][j] += 50;
}
}
}
int main() {
int arr[N][M] = {{10, 11, 12, 13}, {14, 15, 16, 17}};
for(size_t i = 0; i < N; i++) {
for(size_t j = 0; j < M; j++) {
printf("value of arr[%zu][%zu] is %d\n", i, j, arr[i][j]);
}
}
printf("\n ***values after modification***\n");
function(arr);
// int array print results
for(size_t i = 0; i < N; i++) {
for(size_t j = 0; j < M; j++) {
printf("value of arr[%zu][%zu] is %d\n", i, j, arr[i][j]);
}
}
}
Since you print the array more than once, you could also add a function to do so to not have to repeat that code in main:
void print(int arr[][M]) {
for(size_t i = 0; i < N; i++) {
for(size_t j = 0; j < M; j++) {
printf("value of arr[%zu][%zu] is %d\n", i, j, arr[i][j]);
}
}
}
Two-Dimensional arrays in C (and C++) are actually one-dimensional arrays whose elements are one-dimensional arrays. The indexing operator [] has left-to-right semantics, so for a type arr[N][M] the first index (with N elements) is evaluated first. The resulting expression, e.g. arr[0], the first element in arr, is a one-dimensional array with M elements. Of course that array can be indexed again , e.g. arr[0][1], resulting in the second int in the first sub-array.
One of the quirks in the C language is that if you use an array as a function argument, what the function sees is a pointer to the first element. An array used as an argument "decays" or, as the standard says, is "adjusted" that way. This is no different for two-dimensional arrays, except that the elements of a two-dimensional array are themselves arrays. Therefore, what the receiving function gets is a pointer to int arr[M].
Consider: If you want to pass a simple integer array, say intArr[3], to a function, what the function sees is a pointer to the first element. Such a function declaration might look like void f(int *intPtr) and for this example is simply called with f(intArr). An alternative way to write this is void f(int intPtr[]). It means exactly the same: The parameter is a pointer to an int, not an array. It is pointing to the first — maybe even only — element in a succession of ints.
The logic with 2-dimensional arrays is exactly the same — except that the elements, as discussed, have the type "array of M ints", e.g. int subArr[M]. A pointer argument to such a type can be written in two ways, like with the simple int array: As a pointer like void f(int (*subArrPtr)[M]) or in array notation with the number of top-level elements unknown, like void f(int arr[][M]). Like with the simple int array the two parameter notations are entirely equivalent and interchangeable. Both actually declare a pointer, so (*subArrPtr)[M] is, so to speak, more to the point(er) but perhaps more obscure.
The reason for the funny parentheses in (*subArrPtr)is that we must dereference the pointer first in order to obtain the actual array, and only then index that. Without the parentheses the indexing operator [] would have precedence. You can look up precedences in this table. [] is in group 1 with the highest priority while the dereferencing operator * (not the multiplication!) is in group 2. Without the parentheses we would index first and only then dereference the array element (which must therefore be a pointer), that is, we would declare an array of pointers instead of a pointer to an array.
The two possible, interchangeable signatures for your function therefore are
void function( int (*arrArg)[M] ); // pointer notation
void function( int arrArg[][M] ); // "array" notation (but actually a pointer)
The entire program, also correcting the problems Ted mentioned, and without printing the original values (we know them, after all), is below. I have also adapted the initialization of the two-dimensional array so that the sub-arrays become visible. C is very lenient with initializing structures and arrays; it simply lets you write consecutive values and fills the elements of nested subobjects as the come. But I think showing the structure helps understanding the code and also reveals mistakes, like having the wrong number of elements in the subarrays. I have declared the function one way and defined it the other way to show that the function signatures are equivalent. I also changed the names of the defines and of the function to give them more meaning.
#include<stdio.h>
#define NUM_ELEMS_SUBARRAY 4
#define NUM_ELEMS_ARRAY 2
/// #arrArg Is a pointer to the first in a row of one-dimensional
/// arrays with NUM_ELEMS_SUBARRAY ints each.
void add50ToElems(int arrArg[][NUM_ELEMS_SUBARRAY]);
int main()
{
// Show the nested structure of the 2-dimensional array.
int arr[NUM_ELEMS_ARRAY][NUM_ELEMS_SUBARRAY] =
{
{10, 11, 12, 13},
{14, 15, 16, 17}
};
// Modify the array
add50ToElems(arr);
// print results
for (int i = 0; i < NUM_ELEMS_ARRAY; i++) {
for (int j = 0; j < NUM_ELEMS_SUBARRAY; j++)
{
printf("value of arr[%d][%d]: %d\n", i, j, arr[i][j]);
}
}
return 0;
}
// Equivalent to declaration above
void add50ToElems(int (*arrArg)[NUM_ELEMS_SUBARRAY])
{
for (int i = 0; i < NUM_ELEMS_ARRAY; i++)
{
for (size_t j = 0; j < NUM_ELEMS_SUBARRAY; j++)
{
//arrArg[i][j] = arrArg[i][j] + 50;
arrArg[i][j] += 50; // more idiomatic
}
}
}
Why is it wrong to pass a two-dimensional array to a function expecting a pointer-to-pointer? Let's consider what void f(int *p) means. It receives a pointer to an int which often is the beginning of an array, that is, of a succession of ints lying one after the other in memory. For example
void f(int *p) { for(int i=0; i<3; ++i) { printf("%d ", p[i]); }
may be called with a pointer to the first element of an array:
static int arr[3];
void g() { f(arr); }
Of course this minimal example is unsafe (how does f know there are three ints?) but it serves the purpose.
So what would void f(int **p); mean? Analogously it is a pointer, pointing to the first in a succession of pointers which are lying one after the other in memory. We see already why this will spell disaster if we pass the address of a 2-dimensional array: The objects there are not pointers, but all ints! Consider:
int arr1[2] = { 1,2 };
int arr2[2] = { 2,3 };
int arr3[2] = { 3,4 };
// This array contains addresses which point
// to the first element in each of the above arrays.
int *arrOfPtrToStartOfArrays[3] // The array of pointers
= { arr1, arr2, arr3 }; // arrays decay to pointers
int **ptrToArrOfPtrs = arrOfPtrToStartOfArrays;
void f(int **pp)
{
for(int pi=0; pi<3; pi++) // iterate the pointers in the array
{
int *p = pp[pi]; // pp element is a pointer
// iterate through the ints starting at each address
// pointed to by pp[pi]
for(int i=0; i<2; i++) // two ints in each arr
{
printf("%d ", pp[pi][i]); // show double indexing of array of pointers
// Since pp[pi] is now p, we can also say:
printf("%d\n", p[i]); // index int pointer
}
}
}
int main()
{
f(ptrToArrOfPtrs);
}
f iterates through an array of pointers. It thinks that the value at that address, and at the subsequent addresses, are pointers! That is what the declaration int **pp means.
Now if we pass the address of an array full of ints instead, f will still think that the memory there is full of pointers. An expression like int *p = pp[i]; above will read an integer number (e.g., 1) and think it is an address. p[i] in the printf call will then attempt to access the memory at address 1.
Let's end with a discussion of why the idea that one should pass a 2-dimensional array as a pointer to a pointer is so common. One reason is that while declaring a 2-dimensional array argument as void f(int **arr); is dead wrong, you can access the first (but only the first) element of it with e.g. int i = **arr. The reason this works is that the first dereferencing gives you the first sub-array, to which you can in turn apply the dereferencing operator, yielding its first element. But if you pass the array as an argument to a function it does not decay to a pointer to a pointer, but instead, as discussed, to a pointer to its first element.
The second source of confusion is that accessing elements the array-of-pointers uses the same double-indexing as accessing elements in a true two-dimensional array: pp[pi][i] vs. arr[i][j]. But the code produced by these expressions is entirely different and spells disaster if the wrong type is passed. Your compiler warns about that, by the way.
I have a function in which I make a 3D array and fill in all the values. I also have to pass a pointer to the function which will assign the memory location of the 3D array to that function so that it can be used outside of that function. Currently, I am doing something which does not seem to work, can someone guide me to the best possible resolution?
int (*arr)[4];
void assign_3D(int (*arr)[4])
{
int local[2][3][4]; //probably we should pass *local?
memset(local, 0, sizeof(int)*2*3*4); // fill the local array with numbers
arr = local;
}
printf("%d\n", arr[1][2][3]);
I know I have written horrible code above. But I am learning :).
It is not possible to assign arrays. You are also using the wrong type for the argument (int (*)[5] is not what a int [2][3][4] decays into, use int (*)[3][4] as the argument type). Once you have the correct type, you can use memcpy() to do the assignment:
#include <string.h>
#include <stdio.h>
int arr[2][3][4];
void assign_3D(int (*arr)[3][4]) {
int local[2][3][4];
memset(local, 0, sizeof(local)); //pass local here, because it is not a pointer but an array. Passing *local would only initialize the first element of the array, i. e. the first 2D slice of it.
// fill the local array with numbers
memcpy(arr, local, sizeof(local));
}
int main() {
assign_3D(arr);
printf("%d\n", arr[1][2][3]);
}
But you can also return a newly allocated array from your function:
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
typedef int arrayType[2][3][4];
arrayType* create_3D() {
arrayType* result = malloc(sizeof(*result)); //here we need to dereference because result is a pointer and we want memory for the array, not the pointer.
memset(result, 0, sizeof(*result));
(*result)[1][2][3] = 7; // fill the local array with numbers
return result; //that's easy now, isn't it?
}
int main() {
arrayType* array = create_3D();
printf("%d\n", (*array)[1][2][3]);
free(array); //cleanup
}
Edit:
You mention that the size of the first dimension is not know before the function is run. In that case, you have to use the malloc() approach, but a bit differently:
#include <stddef.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
typedef int sliceType[3][4];
sliceType* create_3D(size_t* firstDimSize) {
*firstDimSize = 2;
size_t arraySize = *firstDimSize*sizeof(sliceType);
sliceType* result = malloc(arraySize);
memset(result, 0, arraySize);
result[1][2][3] = 7; // fill the local array with numbers
return result;
}
int main() {
size_t firstDim;
sliceType* array = create_3D(&firstDim);
printf("%d\n", array[1][2][3]);
free(array); //cleanup
}
There are two different ways to allocate a 3D array. You can allocate it either as a 1D array of pointers to a (1D array of pointers to a 1D array). This can be done as follows:
int dim1, dim2, dim3;
int i,j,k;
int *** array = (int ***)malloc(dim1*sizeof(int**));
for (i = 0; i< dim1; i++) {
array[i] = (int **) malloc(dim2*sizeof(int *));
for (j = 0; j < dim2; j++) {
array[i][j] = (int *)malloc(dim3*sizeof(int));
}
}
Sometimes it is more appropriate to allocate the array as a contiguous chunk. You'll find that many existing libraries might require the array to exist in allocated memory. The disadvantage of this is that if your array is very very big you might not have such a large contiguous chunk available in memory.
const int dim1, dim2, dim3; /* Global variables, dimension*/
#define ARR(i,j,k) (array[dim2*dim3*i + dim3*j + k])
int * array = (int *)malloc(dim1*dim2*dim3*sizeof(int));
To access your array you just use the macro:
ARR(1,0,3) = 4;
i have declared a pointer to a group of 3-d array which I have shared below.I have a problem in accessing elements of the 3-d array using pointers to the 3-d array.
#include <stdio.h>
void main()
{
int m,row,col;
int *ptr,*j;
int array[2][5][2]={10,20,30,40,50,60,70,80,90,100,18,21,3,4,5,6,7,81,9,11};
int (*p)[5][2]; // pointer to an group of 3-d array
p=array;
for(m=0;m<2;m++)
{
ptr=p+m;
for(row=0;row<5;row++)
{
ptr=ptr+row;
for(col=0;col<2;col++)
{
printf("\n the vale is %d",*(ptr+col));
}
}
}
}
output:
the value is 10
the value is 20
the value is 20
the value is 30
the value is 40
the value is 50
the value is 70
the value is 80
the value is 18
the value is 21
the value is 18
the value is 21
the value is 21
the value is 3
the value is 4
the value is 5
the value is 7
the value is 81
the value is -1074542408
the value is 134513849
my question is how to access the elements of 3-d array using pointer to an array and in my case the output shows my code not accessing the elements 90,100,9,11 and how do i can access this in the above code.Thanks in advance.
Although flattening the arrays and accessing them as 1-d arrays is possible, since your original question was to do so with pointers to the inner dimensions, here's an answer which gives you pointers at every level, using the array decay behaviour.
#include <stdio.h>
/* 1 */
#define TABLES 2
#define ROWS 5
#define COLS 2
/* 2 */
int main()
{
/* 3 */
int array[TABLES][ROWS][COLS] = {
{ {10, 20}, {30, 40}, {50, 60}, {70, 80}, {90, 100} },
{ {18, 21}, {3, 4}, {5, 6}, {7, 81}, {9, 11} }
};
/* pointer to the first "table" level - array is 3-d but decays into 2-d giving out int (*)[5][2] */
/* name your variables meaningully */
int (*table_ptr)[ROWS][COLS] = array; /* try to club up declaration with initialization when you can */
/* 4 */
size_t i = 0, j = 0, k = 0;
for (i = 0; i < TABLES; ++i)
{
/* pointer to the second row level - *table_ptr is a 2-d array which decays into a 1-d array */
int (*row_ptr)[COLS] = *table_ptr++;
for (j = 0; j < ROWS; ++j)
{
/* pointer to the third col level - *row_ptr is a 1-d array which decays into a simple pointer */
int *col_ptr = *row_ptr++;
for (k = 0; k < COLS; ++k)
{
printf("(%lu, %lu, %lu): %u\n", (unsigned long) i, (unsigned long) j, (unsigned long) k, *col_ptr++); /* dereference, get the value and move the pointer by one unit (int) */
}
}
}
return 0; /* report successful exit status to the platform */
}
Inline code comments elaborated with reference
It's good practise to have the dimensions defined commonly somewhere and use it elsewhere; changing at one place changes it at all places and avoids nasty bugs
main's retrun type is int and not void
It's recommended not to avoid the inner braces
Use size_t to hold size types
Problems in your code
For the line ptr=p+m;, GCC throws assignment from incompatible pointer type; reason is p is of type int (*)[5][2] i.e. pointer to an array (size 5) of array (size 2) of integers, which is assigned to ptr which is just an integer pointer. Intead if you change it to int (*ptr) [5]; and then do ptr = *(p + m);. This is what my code does (I've named p as table_ptr), only that it doesn't use m but it increments p directly.
After this at the third level (inner most loop), you need a integer pointer say int *x (in my code this is col_ptr) which you'd do int *x = *(ptr + m1). Bascially you need to have three different pointers, each for one level: int (*) [5][2], int (*) [2] and int *. I've named them table_ptr, row_ptr and col_ptr.
Rewritten your code below and just used the pointer p to print everything.
#include <stdio.h>
void main()
{
int m,row,col;
int array[2][5][2]={10,20,30,40,50,60,70,80,90,100,18,21,3,4,5,6,7,81,9,11};
int (*p)[5][2]; // pointer to an group of 3-d array
p=array;
for(m=0;m<2;m++)
{
for(row=0;row<5;row++)
{
for(col=0;col<2;col++)
{
printf("\n the vale is %d", *((int*)(p+m) + (row*2) + col));
}
}
}
}
You can easily access all the elements simply by a looping through 2*5*2 = 20 and using a pointer to the first element of array, i.e, array[0][0][0] assuming 3D array as 1D array of arrays of arrays of int's.
#include <stdio.h>
void main()
{
int m; //row,col;
int *ptr; //,*j;
int array[2][5][2]={10,20,30,40,50,60,70,80,90,100,18,21,3,4,5,6,7,81,9,11};
//int (*p)[5][2]; // pointer to an group of 3-d array
//p=array;
ptr = &array[0][0][0];
for(m=0;m <2;m++)
{
for (m = 0; m < 20; m++)
/* ptr=ptr+m;
for(row = 0;row < 5;row ++)
{
ptr=ptr+row;
for(col=0;col<2;col++)
{
printf("\n the vale is %d",*(ptr+col));
}
}*/
printf("\n the vale is %d", *(ptr++));
}
}
I commented some parts of your code and left it in the modified code to let you clear what I have done.
#include<stdio.h>
int main()
{
int array[2][2][2]={1,2,3,4,5,6,7,8};
int *p;
p=&array[0][0][0];
int i=0,j,k;
/*Accessing data using pointers*/
for(i=0;i<2;i++)
{
for(j=0;j<2;j++)
{
for(k=0;k<2;k++)
{
printf("%d\n",*p);
p++;
}
}
}
return 0;
}
This is a sample code where elements in a 2X2X2 array is been accessed using pointers.
Hope it helps !!!!
Consider if I am having an input, where number of arrays to have as input is not fixed and the number of elements in each array is also not fixed. So each time my input varies,
Example 1: 1st input
1st array= [2,3,4]
2nd array =[6,7,8,9]
3rd array=[5,3,12]
Example 2: 2nd input
1st array= [6,3,4,8]
2nd array =[6,7,4,9]
3rd array=[1,2,12]
4th array= [20,21,22,23,25]
The solution required is, 1st array is considered as a reference array, the next set of arrays are to be checked with respect to the first array(reference), the requirement is second array should not have a common element with respect to first array and it goes on for next check third array should not have a common element with first array.
from 1st Example
1st array= [2,3,4] -- reference array
1st array= [2,3,4] is compared to 2nd array =[6,7,8,9]
1st array= [2,3,4] is compared to 3rd array=[5,3,12]
solution needed:
+ print 1st array( reference array)
+ if no common elements found between 1st array and 2nd array, from ex. no common elements found, so print out the 2nd array.
+ same for 3rd array, from ex. there is common element(3 is present in both first and third array), so dont print.
I ave tried this by storing the input in 2-dimensional array, but i messed up .
Please guide me for computing this with your algorithm/code.
Notice how i had to create an array to contain the length of the various arrays, and a parameter to tell how long that array is.
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
void print_array(int *a, int size) {
int i;
for (i = 0; i<size; i++) {
printf("%d ", a[i]);
}
printf("\n");
}
bool is_in(int *a, int size, int elem) {
int i;
for (i=0; i<size; i++) {
if (a[i] == elem) {
return true;
}
}
return false;
}
void f(int **a, int a_siz, int* sizes) {
print_array(a[0],sizes[0]);
for (int i=1; i< a_siz; i++) {
bool common_element = false;
for (int k=0; k< sizes[i]; k++) {
if (is_in(a[0],sizes[0],a[i][k])) {
common_element = true;
break;
}
}
if (! common_element) {
print_array(a[i],sizes[i]);
}
}
}
int main() {
int s = 3;
int sizes[] = {3,4,3};
int **a = malloc(sizeof(int*)*3);
for (int i=0; i<s; i++) {
a[i] = malloc(sizeof(int*)*sizes[i]);
}
a[0][0] = 2;
a[0][1] = 3;
a[0][2] = 4;
a[1][0] = 4;
a[1][1] = 7;
a[1][2] = 8;
a[1][3] = 9;
a[2][0] = 5;
a[2][1] = 44;
a[2][2] = 12;
f(a,s,&sizes);
}
Since the question states that the length of the array is not a constant, the idea of static 2-dimensional array might not work. You can go with creating a 2 dimensional array using DMA (malloc and calloc functions)
You can prefer a straight forward algorithm which compares each element in reference array with other arrays.
Let us say I have the following method prototype:
void mix_audio(int *vocal_data_array, int *instrumental_data_array, int *mixed_audio_array, FOURTH ARGUMENT)
{
}
How would I:
Initialize an array_of_arrays before the above argument so as to pass it as the fourth argument?
In the method, make it so that the first value of my array_of_arrays is the array called vocal_data, that the second value of my array is instrumental_data_array and the third value is mixed_audio_array.
How would I later then loop through all the values of the first array within the array_of_arrays.
I hope I'm not asking too much here. I just thought it would be simple syntax that someone could spit out pretty quickly :)
Thanks!
EDIT 1
Please note that although I've showed by my example an array_of_arrays of length 3 I'm actually looking to create something that could contain a variable length of arrays.
Simple array of arrays and a function showing how to pass it. I just added fake values to the arrays to show that something was passed to the function and that I could print it back out. The size of the array, 3, is just arbitrary and can be changed to whatever sizing you want. Each array can be of a different size (known as a jagged array). It shows your three criteria:
Initialization, Assigning values to each index of arrayOfArrays, The function demonstrates how to extract the data from the array of arrays
#include <stdio.h>
void mix_audio(int *arr[3]);
int main() {
int *arrayOfArrays[3];
int vocal[3] = {1,2,3};
int instrumental[3] = {4,5,6};
int mixed_audio[3] = {7,8,9};
arrayOfArrays[0] = vocal;
arrayOfArrays[1] = instrumental;
arrayOfArrays[2] = mixed_audio;
mix_audio(arrayOfArrays);
return(0);
}
void mix_audio(int *arr[3]) {
int i;
int *vocal = arr[0];
int *instrumental = arr[1];
int *mixed_audio = arr[2];
for (i=0; i<3; i++) {
printf("vocal = %d\n", vocal[i]);
}
for (i=0; i<3; i++) {
printf("instrumental = %d\n", instrumental[i]);
}
for (i=0; i<3; i++) {
printf("mixed_audio = %d\n", mixed_audio[i]);
}
}
From your question it sounds like you actually want a struct containing your arrays, something like:
struct AudioData {
int* vocal_data_array;
unsigned int vocal_data_length;
int* instrumental_data_array;
unsigned int instrumental_data_length;
int* mixed_audio_array;
unsigned int mixed_audio_length;
};
For the array allocation using the example of an array of integers:
int** x = malloc (sizeof (int*) * rows);
if (! x) {
// Error
}
for (int i = 0; i < rows; ++i) {
x[i] = malloc (sizeof (int) * columns);
if (! x[i]) {
// Error
}
}