I am currently working on a simple code to store and display top-right triangular matrix. Well, everything was fine till I tried to input 4x4 matrix structure and gave the input. The first array of structure's (called a) last value changed although I did not put any code to change ANY of the values in a. It happens in the mReorder() function. Then I tried some try-and-errors find out the problem in the 3rd row of mReorder() function. I wonder why and how to solve it.
Here is my complete code:
#include<stdio.h>
//CMO fashion
typedef struct
{
int row;
int col;
int val;
}term;
#define MAX_TERMS 10
term a[MAX_TERMS], b[MAX_TERMS];
void mReorder(void);
int main()
{
int n, i, j;
printf("Enter the number of rows: ");
scanf("%d", &n);
if (n<1 || n>MAX_TERMS)
{
printf("\nInvalid number of rows!!");
exit(0);
}
i=nCount(n);
mRead(n,i);
for (j=0; j<i+1; j++) printf("\n%d\t%d\t%d", a[j].col, a[j].row, a[j].val);
mReorder();
for (j=0; j<i+1; j++) printf("\n%d\t%d\t%d", a[j].col, a[j].row, a[j].val);
printf("\n");
for (j=0; j<i+1; j++) printf("\n%d\t%d\t%d", b[j].col, b[j].row, b[j].val);
mDisplay();
return 0;
}
void mReorder(void)
{
int i, j, k, m=1;
b[0].col=a[0].col;
b[0].row=a[0].row;
b[0].val=a[0].val;
for(i=0; i<a[0].col; i++)
for (j=1; j<=a[0].val; j++)
if (a[j].row==i)
{
b[m].col=a[j].col;
b[m].row=a[j].row;
b[m].val=a[j].val;
m++;
}
}
void mDisplay(void)
{
int i, j, k, m=1;
printf("\nThe resulting matrix is:\n");
for (i=0; i<b[0].col; i++)
{
//printf("\na");
for (k=0; k<i; k++) printf("%5c", '-');
for (j=i; j<b[0].col; j++)
{
printf("%5d", b[m].val);
m++;
}
printf("\n");
}
}
void mRead(int n, int x)
{
int i, j, m=1, val;
printf("\nEnter %d elements of the matrix: \n", x);
a[0].row=a[0].col=n;
a[0].val=x;
for (i=0; i<n; i++)
{
for (j=0; j<=i; j++)
{
scanf("%d", &val);
a[m].row=j;
a[m].col=i;
a[m].val=val;
m++;
}
}
}
int nCount(int n)
{
if (n==1)
return 1;
return (n+nCount(n-1));
}
Can you explain what's going on here?
You allocate enough space for 10 term items, but nCount(4) returns 10, and nCount(5) returns 15, etc. If you specify a value bigger than 4, you overflow your array boundaries, leading to undefined behaviour — which is something to be avoided at all costs. In practice, one of your two arrays tramples over the other, but what happens when you access the other array out of bounds is entirely up to the compiler. It may appear to work; it may crash horribly; it may corrupt other data structures.
Nominally, since you allocate 10 elements in the arrays a and b, you should be OK with the 4x4 data, but in mRead(), you set m = 1 to start with, so you end up writing to a[10] in the last iteration of the loop, which is outside the bounds of the array. Remember, C arrays are indexed from 0, so an array defined as SomeType array[N]; has elements from array[0] to array[N-1].
Note that you can rewrite nCount() as a simple (non-recursive) function:
static inline int nCount(int n) { return (n + 1) * n / 2; }
(which would need to appear before it is called, of course). Or, if you're stuck using an archaic compiler that doesn't support C99 or C11, drop the inline keyword.
Related
void interclas(int *ptr,int *vec, int *c, int n) {
int i,j,tmp;
tmp=0;
for (i=0;i++;i<n)
for (j=0;i++;j<n)
{
if (vec[j]<=ptr[i])
c[tmp]=vec[j];
else
c[tmp]=ptr[i];
tmp++;
}
}
int main() {
int i,n;
int *ptr,*vec,*c;
printf("Nr. of elements of initial arrays : 5 \n");
n=5;
vec=(int*)malloc( n * sizeof(int));
ptr=(int*)malloc( n * sizeof(int));
c=(int*)malloc( 2 * n * sizeof(int));
for (i=0;i<n;i++) {
scanf("%d",&ptr[i]);
}
for (i=0;i<n;i++) {
scanf("%d",&vec[i]);
}
printf("\n");
printf("Initial arrays are : ");
for (i=0;i<n;i++) {
printf("%d ",ptr[i]);
}
printf("\n");
for (i=0;i<n;i++) {
printf("%d ",vec[i]);
}
interclas(ptr,vec,&c,n);
printf("Merged array is : ");
for (i=0;i<10;i++) {
printf("%d ",c[i]);
}
return 0;
}
So I'm trying to merge two sorted arrays into one new one using pointers with the function 'interclas'. I tried using the same method to sort an array with a pointer in a function and it worked just fine. Now as you can see, it stores the adress of the variable rather than the variable itself.
If I run this, it stores the adresses of the arrays. How can I fix this? (I'm still new to pointers)
In your method's body, change:
for (i=0;i++;i<n)
for (j=0;i++;j<n)
to this:
for (i=0; i<n; i++)
for (j=0; j<n; j++)
and then change the call to your method, from this:
interclas(ptr, vec, &c, n);
to this:
interclas(ptr, vec, c, n);
since the prototype expects a pointer to an int, for the third parameter.
The logic of your method is also flawed, try to put some printfs (e.g. printf("here i = %d, j = %d, ptr[i] = %d, vec[j] = %d, tmp = %d\n", i, j, ptr[i], vec[j], tmp);) to see what values your variables have at its iteration - you only get the first two elements of the first array to be merged!
If you think about it, what you'd like to do is to go through the first element of array ptr and vec, and store the minimum of this two. If now that min was of array ptr, you'd like the next element of ptr to be taken into account, otherwise the next element of vec.
Take a pencil and a paper and sketch that algorithm - you'll see that it goes out nicely, but some leftover elements might be left behind, and not get inserted in the output array.
Driven from that observation, after traversing both the arrays and comparing elements, we will loop over the first array, if needed, to collect elements that were not visited. Similarly for the second array.
Coding that thought gives something like this:
void interclas(int *ptr,int *vec, int *c, int n) {
int i = 0, j = 0, tmp = 0;
// Traverse both arrays simultaneously,
// and choose the min of the two current elements.
// Increase the counter of the array who had
// the min current element.
// Increase the counter for the output array in
// any case.
while(i < n && j < n)
{
if(ptr[i] < vec[j])
{
c[tmp++] = ptr[i++];
}
else
{
c[tmp++] = vec[j++];
}
}
// Store remaining elements of first array
while (i < n)
c[tmp++] = ptr[i++];
// Store remaining elements of second array
while (j < n)
c[tmp++] = vec[j++];
}
Not the source of your problem, but Do I cast the result of malloc? No.
I have written code that allows you to enter one dimension of a NxN double array. It will then print random numbers in a 2D array and it finds the maximum and minimum number of each row. It then prints them and their coordinates (row and column).
ATTENTION!!!!
I have altered my code in such a way that it finds the minimum number of the maximum. I now don't know how to find it's coordinates
My code is as follows:
int N, i, j, min=1000, max, m , o;
time_t t;
int masyvas[100][100], minmax[100];
printf("Enter one dimension of a NxN array\n");
scanf("%d", &N);
srand((unsigned) time(&t));
for (i=0; i<N; i++)
{
for (j=0; j<N; j++)
{
masyvas[i][j] = rand() % 10;
printf("%4d", masyvas[i][j]);
}
printf("\n");
}
int k, l, idkeymax, idkeymin;
for(k=0; k<N; k++)
{
max=-1000;
for(l=0; l<N; l++)
{
if(max<masyvas[k][l])
{
max=masyvas[k][l];
}
}
minmax[k]=max;
}
for(m=0; m<N; m++)
{if(minmax[m]<min)
min=minmax[m];
}
printf("maziausias skaicius tarp didziausiu yra %d eiluteje %d stulpelyje %d\n",min);
Here's the pseudo code of what you need to do.
for row in grid {
row_max = max_in_row(row)
grid_min = min(grid_min, row_max)
}
Step one is to write a routine that finds the max and location in a list. You could do this as one big function, but it's much easier to understand and debug in pieces.
You also need the index where it was found. Since C can't return multiple values, we'll need a struct to store the number/index pair. Any time you make a struct, make routines to create and destroy it. It might seem like overkill for something as trivial as this, but it will make your code much easier to understand and debug.
typedef struct {
int num;
size_t idx;
} Int_Location_t;
static Int_Location_t* Int_Location_new() {
return calloc(1, sizeof(Int_Location_t));
}
static void Int_Location_destroy( Int_Location_t* loc ) {
free(loc);
}
Now we can make a little function that finds the max number and position in a row.
static Int_Location_t* max_in_row(int *row, size_t num_rows) {
Int_Location_t *loc = Int_Location_new();
/* Start with the first element as the max */
loc->num = row[0];
loc->idx = 0;
/* Compare starting with the second element */
for( size_t i = 1; i < num_rows; i++ ) {
if( row[i] > loc->num ) {
loc->num = row[i];
loc->idx = i;
}
}
return loc;
}
Rather than starting with some arbitrary max or min, I've used an alternative technique where I set the max to be the first element and then start checking from the second one.
Now that I have a function to find the max in a row, I can now loop over it, get the max of each row, and compare it with the minimum for the whole table.
int main() {
int grid[3][3] = {
{10, 12, 15},
{-50, -15, -10},
{1,2,3}
};
int min = INT_MAX;
size_t row = 0;
size_t col = 0;
for( size_t i = 0; i < 3; i++ ) {
Int_Location_t *max = max_in_row(grid[i], 3);
printf("max for row %zu is %d at %zu\n", i, max->num, max->idx);
if( max->num < min ) {
min = max->num;
col = max->idx;
row = i;
}
Int_Location_destroy(max);
}
printf("min for the grid is %d at row %zu, col %zu\n", min, row, col);
}
I used a different technique for initializing the minimum location, because getting the first maximum would require repeating some code in the loop. Instead I set min to the lowest possible integer, INT_MAX from limits.h which is highest possible integers. This allows the code to be used with any range of integers, there are no restrictions. This is a very common technique when working with min/max algorithms.
this is my first post, so sorry if it doesn't meet the standards in which I should post, if there are any issues with how I've placed the code just say and Ill take that into account for future posts. I tried to comment the code as much as possible, maybe a little too much.
So the idea of the code is to take a normal matrix in and then store it in reduced form, this works fine, but when I wish to also multiply this with with a vector it fails.
The code fails, since when I do the double for loop with an if statement it segments and I have no idea why.
It works fine if I specify the elements seperatly but then fails when I try to increment what elements it should select for Matrix_R when inside the if statement.
Any help would be greatly appreciated. Thanks !!
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
int i;
int j;
int n;
int m;
int k=0;
double * Vin;
double * Vout_M;
double * Vout_MR;
double ** Matrix;
double ** Matrix_R;
//Vout_M and Vout_MR are output vectors for the two possible inputted matricies
//Matrix_R is the reduced form of Matrix
n=4;
Vin = malloc(n*sizeof(double));
Vout_M = malloc(n*sizeof(double));
Vout_MR = malloc(n*sizeof(double));
Matrix = malloc(n*sizeof(double*));
for(i=0;i<n;i++)
Matrix[i] = malloc(n*sizeof(double));
//Allocates memory for the arrays
for(i=0;i<n;i++){
Vin[i]=1;
Vout_M[i]=0;
Vout_MR[i]=0;
}
//Initiates the vector arrays
for(i=0;i<n;i++)
for(j=0;j<n;j++)
Matrix[i][j]=0;
//Initiates the Matrix
Matrix[2][3]=5;
Matrix[0][2]=10;
for(i=0;i<n;i++)
for(j=0;j<n;j++)
if(Matrix[i][j]!=0)
m++;
//Scans through to determine what size the reduced array should be
//In Future, this could be set up to read from a text file, and so RAM isn't used to store the array to find elements
Matrix_R = malloc(m*sizeof(double*));
for(i=0;i<m;i++)
Matrix_R[i] = malloc(3*sizeof(double));
for(i=0;i<m;i++)
for(j=0;j<3;j++)
Matrix_R[i][j]=0;
//Produces the reduced array, and initiates it
printf("\n");
for(i=0;i<n;i++)
for(j=0;j<n;j++)
if(Matrix[i][j]!=0){
Matrix_R[k][0]=Matrix[i][j];
Matrix_R[k][1]=i;
Matrix_R[k][2]=j;
k++;
}
//Scans through the Matrix array and outputs to the Matrix_R array
for(i=0;i<k;i++)
printf("Matrix_R[%d][0] = %lf, Matrix_R[%d][1] = %lf, Matrix_R[%d][2] = %lf\n",i,Matrix_R[i][0],i,Matrix_R[i][1],i,Matrix_R[i][2]);
//Checks if it is storred correctly -- this outputs fine
//Now first do the multiplication between the normal matrix and vector --- Matrix * Vector
for(i=0;i<n;i++)
for(j=0;j<n;j++)
Vout_M[i] += Matrix[i][j]*Vin[j];
for(i=0;i<n;i++)
printf("\nVout_M[%d] = %lf",i,Vout_M[i]);
printf("\n");
//Prints the output from a standard Matrix * Vin in the form of Vout_M
k=0;
for(i=0;i<n;i++)
for(j=0;j<n;j++){
if(i == Matrix_R[k][1] && j == Matrix_R[k][2]){
Vout_MR[i] += Matrix_R[k][0]*Vin[j];
k++;
}}
//Goes through standard matrix-vector multiplication, using reduced matrix
//SEGMENTS :: When trying to go through an if statement and incrementing k.
//i.e. If I set k=int, and then not increase it in the if statement it works.
for(i=0;i<n;i++)
printf("\nVout_MR[%d] = %lf",i,Vout_MR[i]);
printf("\n");
//Outputs the Vout_MR which is from Matrix_R * VIN
free(Vin);
free(Vout_M);
free(Vout_MR);
for(i=0;i<n;i++)
free(Matrix[i]);
for(i=0;i<m;i++)
free(Matrix_R[i]);
return 0;
}
In your code, you never initialized
int m;
and you're using the value uninitialized in
if(Matrix[i][j]!=0)
m++;
and
Matrix_R = malloc(m*sizeof(double*));
It produces a read-before-write scenario. The behaviour is not defined.
Also, always check for the return value of malloc() to ensure success.
You can [and should] use %f format specifier to print double values. Change
printf("\nVout_M[%d] = %lf",i,Vout_M[i]);
to
printf("\nVout_M[%d] = %f",i,Vout_M[i]);
Next, you've free()d all the Matrix[i] and Matrix_R[i] but you forgot to free() Matrix and Matrix_R themselves.
EDIT
In your code, there is no boundary check for the value k in
if(i == Matrix_R[k][1] && j == Matrix_R[k][2])
Once your k >= m, you'll be accessing out-of-bound memory invoking undefined behaviour. A side effect is segmentation fault.
Most likely, in your case, m is having a value 2, and when k becomes 2 via k++, the next access to i == Matrix_R[k][1] generates out-of-bound memory access.
for(i=0;i<n;i++)
for(j=0;j<n;j++){
if(i == Matrix_R[k][1] && j == Matrix_R[k][2]){
Vout_MR[i] += Matrix_R[k][0]*Vin[j];
k++;
}}
Here you keep incrementing k and there is no guarantee k<m so as you keep incrementing k there is a access Matrix_R[k][1] where k>m leading to UB
I am trying to create this program that takes an int number from the user then randomizes a pair of numbers, based on the input, inside the an array of struct. Then it sorts this array based on the sum of the number pair the program randomized.
However my program won´t sort the array of struct. It doesn´t do the sorting properly and Im not sure why. Here is the code.
#define MAX 10
struct NumPair{
int n,m;
};
int main()
{
int i, j, amount=0;
NumPair NumPair[MAX];
srand(time(NULL));
printf("How many pair of numbers? (max 10): ");
scanf("%d", &amount);
for (i=0; i<amount; i++)
{
NumPair[i].n = rand() % 11;
NumPair[i].m = rand() % 11;
}
for (i=0; i<amount; i++)
{
for(j=1; j<amount; j++)
{
if( (NumPair[i].n+NumPair[i].m) > (NumPair[j].n+NumPair[j].m) )
{
int tmp;
tmp = NumPair[i].n;
NumPair[i].n = NumPair[j].n;
NumPair[j].n = tmp;
tmp = NumPair[i].m;
NumPair[i].m = NumPair[j].m;
NumPair[j].m = tmp;
}
}
}
for (i=0; i<amount; i++)
{
printf(" NumPair %d: (%d,%d)\n", i+1, NumPair[i].n, NumPair[i].m);
}
return 0;
}
What am I missing? It's probably something very silly.
Thanks in advance.
Your algorithm is incorrect. This little snippet:
for (i=0; i<amount; i++) {
for(j=1; j<amount; j++) {
will result in situations where i is greater than j and then you comparison/swap operation is faulty (it swaps if the i element is greater than the j one which, if i > j, is the wrong comparison).
I should mention that (unless this is homework or some other education) C has a perfectly adequate qsort() function that will do the heavy lifting for you. You'd be well advised to learn that.
If it is homework/education, I think I've given you enough to nut it out. You should find the particular algorithm you're trying to implement and revisit your code for it.
You are comparing iterators i with j. Bubble sort should compare jth iterator with the next one
for (i=0; i<amount; i++) //pseudo code
{
for(j=0; j<amount-1; j++)
{
if( NumPair[j] > NumPair[j+1] ) //compare your elements
{
//swap
}
}
}
Note that the second loop will go only until amount-1 since you don't want to step out of bounds of the array.
change to
for (i=0; i<amount-1; i++){
for(j=i+1; j<amount; j++){
The program should create a 2D table 8*8 which consists o random number<3
it should print that table.
Another task is to translate this table into another
For Example
120
210
111
The number in the center should be changed to the sum of all numbers around it 1+2+0+2+0+1+1+1=8
and that should be done for everything;
then the program should be printed
if there are any numbers larger than 9 it shoul be translated to hexadecimal.....
I didn't do the hexadecimal yet. but it is still not working ....
#include <stdio.h>
#include <stdlib.h>
#define cols 8
#define rows 8
void printA(int A[][cols]);
void printC(char C[][cols]);
void SumThemUp(int A[][cols], char C[][cols]);
int main()
{
srand(time(NULL));
int A[rows][cols];
char C[rows][cols];
int i, j;
for(i=0; i<rows; i++)
for(j=0; j<cols; j++)
A[i][j]=rand()%3;
printA(A);
SumThemUp(A,C);
printC(C);
return 0;
}
void printA(int A[][cols])
{ int i, j;
for(i=0;i<rows;i++)
{for(j=0;j<cols; j++)
{printf("%d ", A[i][j]);}
printf("\n");}
return ;
}
void printC(char C[][cols])
{
int i, j;
for(i=0;i<rows;i++)
{for(j=0;j<cols; j++)
{printf("%ch ", C[i][j]);}
printf("\n");}
return ;
}
void SumThemUp(int A[][cols], char C[][cols])
{
int i,j;
for(i=0;i<rows;i++)
{for(j=0;j<cols; j++)
C[i][j]=0;}
for(i=0;i<rows;i++)
{for(j=0;j<cols; j++)
A[i][j]=C[i++][j];
}
for(j=0;j<cols; j++)
{for(i=0;i<rows;i++)
C[i][j]+=A[i][j++];
}return;
}
So - I'm not entirely sure I know what you want the output to be -- but there are several problems with what you have:
0: For your arrays, the names should describe what the array actually holds, A and C are quite ambiguous.
1: Use { } for scoping, and put the { } on their own lines. (Maybe it just pasted poorly in Stack Overflow)
2: You have a set of loops which basically sets everything in C to 0:
for(i=0;i<rows;i++)
{
for(j=0;j<cols; j++)
{
C[i][j]=0;
}
}
Then immediately after that you have:
for(i=0;i<rows;i++)
{
for(j=0;j<cols; j++)
{
A[i][j]=C[i++][j]; // <--- problem here
}
}
So after that, both A and C are full of all 0s. On top of that, you have i++ inline when accessing columns in C. This actually changes the value that your for loop is using, so i is getting incremented for every row and every column. Presumably you want:
A[i][j]=C[i+1][j];
3: You have a similar problem here:
for(j=0;j<cols; j++)
{
for(i=0;i<rows;i++)
{
C[i][j]+=A[i][j++]; // Presumably you want j+1
}
}
4: Why are you using a char array for C? If it's holding the sum of integers it should probably be declared int. If this was your idea of printing the ints as hex (or just plain ints), it would be easier to simply use printf to output the ints as hex:
// use %d to print the integer "normally" (base 10)
// use %x if you want a hex value with lowercase letters
// use %X if you want a hex value with capital letters
printf("125 as hex is: 0x%x", 125); // 0x7d
I hope that points you in the right direction.
-- Dan
Do I understand correctly, that given matrix A, you want to get matrix C in SumThemUp, where each cell in C is a sum of its adjacent cells? In that case, these lines look suspicious as you modify the loop counters
A[i][j]=C[i++][j];
and
C[i][j]+=A[i][j++];
.
Anyway, a simple example, how I would do the summing part.
NB! Note that I use int type for matrix C. Given that you want to convert it to hex and you happend to have values 3 in all adjacent cells somewhere, you get decimal value of 3 * 8 = 24, which requires more than one character to represent. Thus, you should convert to hex during printing. (I understand that char can contain intergral values up to 255 also, but for the sake of consistency)
void SumThemUp(int A[][cols], int C[][cols]) {
int i, j, di, dj, i2, j2;
// iterate through all the rows
for (i=0 ; i<rows ; ++i) {
for (j=0 ; j<cols ; ++j) {
// initialize the cell to zero
C[i][j] = 0;
// iterate over nearby cells
for (di=-1 ; di<=1 ; ++di) {
for (dj=-1 ; dj<=1 ; ++dj) {
// do not count in the center
if (di == 0 && dj == 0) {
continue;
}
// make sure, we do not try to count in cells
// outside the matrix
i2 = i + di;
j2 = j + di;
if (i2 < 0 || j2 < 0 || i2 >= rows || j2 >= cols) {
continue;
}
// append the score here
C[i][j] += A[i2][j2];
}
}
}
}
}
Also, I did not test this code, so it may contain mistakes, but maybe it helps you finishing your summing part.
NB! And take note of comments of #Dan.