Reverse the elements in an array - c

I've been trying to write a void function called printReverse that takes a double array named d and an int called n as arguments, where n is the number of elements in d. I want printReverse to print the n array values in reverse order. This is what I have so far, but I'm having difficulty declaring arguments.
#include <stdio.h>
int n
double d[n]
void printReverse(d[], double newArray){
int i;
int j;
for (i=0; j=n-1; i<n; i++; j--;){
newArray[j]=d[i];
}
printf("%lf\n",newArray);
}
Any help in what I'm doing wrong would be very much appreciated.
REVISION: Would this code work
void printReverse(double [] d, int n){
int i;
for(i=n-1;i>=0;i--){
printf("%f\n",d[i]);
}

If you want both print and store the array in reverse order you could do something like this:
#include <stdio.h>
void printReverse(double *d, double *newArray, int n){
int i;
int j;
for (i=0, j=n-1; i<n; i++, j--){
newArray[i]=d[j];
printf("%lf\n",newArray[i]);
}
}
int main(){
int n = 4;
double array[] = {1, 2, 3, 4};
double newArr[n];
printReverse(array,newArr,n);
}

//take pointer to array, and size of array as parameters
void printReverse(double* arr,int size){
for(int i =0;i<size;i++){
printf("%f\n",arr[(size-1)-i]);
//(size-1)-i will compute the proper index of the array to read backwards.
//remeber arrays are 0 based meaning that the first member in the array is index 0
}
}
this should do the trick.
EDIT: or if you are trying to reverse the array into a new array and then print...
void printReverse(double* arr,double* arr2,int size){
for(int i =0;i<size;i++){
arr2[(size-1)-i]=arr[i];
}
for (int i=0; i<size; i++) {
printf("%f\n",arr2[i]);
}
}
you will need to have the arrays in the code that calls this function.
so...
double arr[5] = {1,2,3,4,5};
double arr2[5];
printReverse(arr,arr2, 5);

Related

How to print a 2D array?

I have two functions. One that creates a multiplication table of a given number and the other function prints the array out. Below is my code:
Here's the error (Line 18):
expression must be a pointer to a complete object type
How do I fix this error and print the array? Also, I don't know how to print a new line after every row.
#include "multiplication.h"
#include <stdio.h>
int arr[][];
void mulitpication(int num){
/* initialize array and build*/
int arr[num][num];
for(int i=0; i<num;i++){
for(int j=0;j<num;j++){
arr[i][j]= (i+1)*(j+1);
}
}
}
void print(int arr[][]){
/* print the arr*/
int i;
for(i=0;i<sizeof(arr);i++){
for(int j=0;j<sizeof(arr);j++){
printf("%d ",arr[i][j])**(line 18)**;
}
}
}
If using C99 or later with VLA support, pass into the print function the dimensions needed.
// void print(int arr[][]){
void print(size_t rows, size_t cols, int arr[row][col]){
size_t r,c;
for (size_t r = 0; r < rows; r++) {
for (size_t c = 0; c < cols; c++) {
printf("%d ",arr[r][c]);
}
printf("\n");
}
}
You need to declare the array in main(), so it can be passed to both functions.
When an array is passed as a function parameter, it just passes a pointer. You need to pass the array dimensions, they can't be determined using sizeof.
To get each row of the table on a new line, put printf("\n"); after the loop that prints a row.
#include <stdio.h>
void multiplication(int num, arr[num][num]){
/* initialize array and build*/
for(int i=0; i<num;i++){
for(int j=0;j<num;j++){
arr[i][j]= (i+1)*(j+1);
}
}
}
void print(int num, int arr[num][num]){
/* print the arr*/
int i;
for(i=0;i<num;i++){
for(int j=0;j<num;j++){
printf("%d ",arr[i][j]);
}
printf("\n");
}
}
int main(void) {
int size;
printf("How big is the multiplication table? ");
scanf("%d", &size);
int arr[size][size];
multiplication(size, arr);
print(size, arr);
return 0;
}

Can somone please explain this to me

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.

Function that calculates memory reserved in different function

I have two functions, one that creates a double array,fill it with zeros and reserve memory for it. The other function 'noarr' is supposed to calculate the memory I have reserved in the 'myarr' function for the array. I'm pretty sure I have successfully filled the array with zeros,
but I am not sure how I can access the array from another function? Am I on the right track of doing it?
#include <stdio.h>
#include <stdio.h>
#include <stdlib.h>
void noarr(double *** myarr(int n, int m));
double** myarr(int n, int m);
double** myarr(int n, int m){
int i, j;
double ** A;
A = malloc(sizeof(double*)*n*m);
for(i = 0; i < n; i++){
A[i] = malloc(m*sizeof(double));
for(j = 0; j < m; j++){
A[i][j] = 0;
}
}
return A;
}
void noarr(double *** myarr(int n, int m)){
printf("%d\n", sizeof(myarr));
}
int main(int argc, const char * argv[]){
int i,j;
int n,m;
double (*A)[n];
A = myarr(n,m);
printf("%.1f %.1f \n",A[0][0],A[2][9]);
noarr(A);
return 0;
}
You simply cannot derive the size of memory (m)alloced somewhere when solely having a pointer to that memory. You need to pass the dimension(s) separately.

Use void pointer to store float numbers into array

I'm new to pointers and this may be a silly question.
Not able to store float numbers into float array using void pointer. Below is the code and the output:
Code:
int main()
{
int size=5;
float f_array[size];
populate(f_array, size);
// printing array below
}
void populate(void *p, int size)
{
int i;
for(i=0; i<size; i++)
{
scanf("%f", (((float *)p)+i));
}
}
Output:
//Entering five float numbers to be stored in array
1.2 // not able to enter other numbers and gives the below output
a[0] = 1
a[1] = garbage value
a[2] = garbage value
a[3] = garbage value
a[4] = 0
#include <stdio.h>
#define SIZE 5
void populate(void *p, int size)
{
int i;
float *array = (float*)p;
for (i = 0; i < size; i++)
{
scanf("%f", &array[i]);
}
}
int main()
{
int i;
float f_array[SIZE];
populate(f_array, SIZE);
//print array
}

Using pointers as a dynamically allocated array

I'm trying to write a program that dynamically allocates memory for an array, which the user then fills with integer values and the program sorts said integer values. However, it seems that my array isn't working as intended. I have managed to get the program working with a static array, but the dynamic allocation is causing me a lot of problems with incorrect values and whatnot. Here's what I have so far for the dynamically allocated version (if it would help you guys, I can also provide the version that uses a static array):
#include <stdio.h>
#include "genlib.h"
#include "simpio.h"
void sortArray (int *numbers, int i2);
int indexMax (int *numbers, int low, int high);
void swap (int *num1, int *num2);
int getArray (int *numbers);
void displayArray (int *numbers, int i2);
main()
{
int *numbers, i2;
i2=getArray(numbers);
sortArray(numbers, i2);
displayArray (numbers, i2);
}
int getArray (int *numbers)
{
int i, i2;
printf("Please enter the amount of elements you wish to sort: ");
i2=GetInteger();
numbers=(int *)malloc(i2*sizeof(int));
for(i=0;i<i2;i++, numbers++)
{
printf("Enter next integer: ");
*numbers=GetInteger();
printf("\n");
}
return(i2);
}
void displayArray (int *numbers, int i2)
{
int i;
printf ("\nThe sorted list is: \n\n");
for (i=0;i<i2;i++, numbers++)printf ("%d\n", *numbers);
}
void sortArray (int *numbers, int i2)
{
int i, minInd;
for(i=0;i<i2;i++)
{
minInd=indexMax(numbers, i, i2-1);
swap(&numbers[i], &numbers[minInd]);
}
}
int indexMax (int *numbers, int low, int high)
{
int i, maxInd;
maxInd=high;
for (i=high;i>=low;i--)
{
if(*(numbers+i)>*(numbers+maxInd)) maxInd=i;
}
return (maxInd);
}
void swap (int *num1, int *num2)
{
int temp;
temp=*num1;
*num1=*num2;
*num2=temp;
}
Here is a working solution:
#include <stdio.h>
#include <stdlib.h>
void sortArray (int *numbers, int i2);
int indexMax (int *numbers, int low, int high);
void swap (int *num1, int *num2);
int getArray (int **numbers);
void displayArray (int *numbers, int i2);
main()
{
int *numbers, i2;
i2=getArray(&numbers);
sortArray(numbers, i2);
displayArray (numbers, i2);
}
int getArray (int **numbers)
{
int i, i2;
printf("Please enter the amount of elements you wish to sort: ");
scanf("%d", &i2);
(*numbers) = malloc(i2 * sizeof(int));
int *temp = *numbers;
for(i = 0; i < i2; i++)
{
printf("Enter next integer: ");
scanf("%d", &temp[i]);
printf("\n");
}
return(i2);
}
void displayArray (int *numbers, int i2)
{
int i;
printf ("\nThe sorted list is: \n\n");
for (i=0;i<i2;i++, numbers++)printf ("%d\n", *numbers);
}
void sortArray (int *numbers, int i2)
{
int i, minInd;
for(i=0;i<i2;i++)
{
minInd=indexMax(numbers, i, i2-1);
swap(&numbers[i], &numbers[minInd]);
}
}
int indexMax (int *numbers, int low, int high)
{
int i, maxInd;
maxInd=high;
for (i=high;i>=low;i--)
{
if(*(numbers+i)>*(numbers+maxInd)) maxInd=i;
}
return (maxInd);
}
void swap (int *num1, int *num2)
{
int temp;
temp=*num1;
*num1=*num2;
*num2=temp;
}
The thing is in your main when you are declaring int *numbers , numbers pointer is pointing to some junk memory location as local variables can have any garbage value, so while you are passing this numbers pointer to getArray() function you are passing its value, Suppose numbers is pointing to some random value = 1234 and suppose address of numbers is = 9999. Now when you call getArray(numbers) you tell taht whatever is there in numbers pass it to getArray's numbers variable that we are letting is 1234.
Then when you allocate memory to numbers that is local variable of getArray() function not main's and it might have address suppose = 0x8888. Then malloc allocates some address space as specified and stores the start address of that allocated address space(suppose i.e. = 0x7777) into location 0x8888 not 0x9999 which is the adress of main's numbers variable.
Thus when the getArray function ends and next time you call sortArray you pass it value present in numbers variable of main which is still junk 1234. and the actual value you should be passing is present at address 0x8888.
Try this:
int main(void)
{
int *numbers, i2;
i2 = getArray(&numbers);
sortArray(numbers, i2);
displayArray (numbers, i2);
return 0;
}
int getArray (int **numbers)
{
int i, i2;
printf("Please enter the amount of elements you wish to sort: ");
i2 = GetInteger();
*numbers = malloc(i2 * sizeof int);
etc...
You simply need one more level of indirection to make int getArray(int**) return a pointer to an allocated array.
As an aside, you can use the C library function qsort() to do the sorting work for you. Here is an example:
int main(void)
{
int a[]={4,7,9,1,34,90,66,12};
qsort(a, sizeof(a)/sizeof(a[0]), sizeof(int), compare);
return 0;
}
int compare(const void *a, const void *b){
const int *x = a, *y = b;
if(*x > *y)
return 1;
else
return(*x < *y) ? -1: 0;
}
Works just as well with dynamically allocated int arrays, or char arrays

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