Problem with iteration on array in For loop - c++

I'm having a trouble for a long time in a problem related to array and loops.
Assume an array like:
int arr[] = {2, 3, 5, 3, 6, 8, 10, 1, 9};
The program should print the expected column as shown (note that the table is an idea for clarification of what I actually want to achieve):
max: expected arr[index] for max
------------------------------------
2 : 3 5 value of: arr[0]
3 : 6, 8, 10 value of: arr[3]
1 : 9 value of: arr[7]
This is what I've tried so far:
#include <iostream>
int main(void) {
int arr[] = {2, 3, 5, 3, 6, 8, 10, 1, 9};
int max = arr[0];
int i = 1, it = i;
for (; i <= max; i++) {
if (i == max) {
std::cout << arr[i] << std::endl;
max = arr[it + 1]; // when loop end has come, increment max to next element value
it = i + 2; // incrementing iterator by 2 next element position (after max)
} else {
std::cout << arr[i] << ' '; // when loop is executing
}
}
return 0;
}
What it actually prints which is unexpected V/S what it should print:
3 5 | 3, 5
3 6 8 | 6, 8, 10
10 1 9 | 1, 9
10 | <nothing>
The program is about to get max value and print the next elements until max value reaches the number of element position.
Where the problem's occurring and how to fix it? Please let me know.

So I rewrote your code, frankly I couldn't follow it, it seems a bit confused with variable names that seem a bit off.
Here's some working code. The two key concepts in the code are the place where each block starts (the variable start) and the size of each block (the variable count). Plus I use the size of the whole array sizeof(arr)/sizeof(arr[0]) to terminate the outer loop. Something that wasn't present in your code. If you are using C++17, you can use std::size(arr) instead of sizeof.
int main(void) {
int arr[] = {2, 3, 5, 3, 6, 8, 10, 1, 9};
size_t start = 0;
while (start < sizeof(arr)/sizeof(arr[0]))
{
int count = arr[start];
for (int i = 0; i < count; ++i)
{
std::cout << arr[start + i + 1] << ' ';
}
std::cout << std::endl;
start += count + 1;
}
}
The output is
3 5
6 8 10
9

Related

How can i remove the duplicate element in output / arrays?

#include <iostream>
using namespace std;
int main()
{
const int ARRAY_SIZE = 10;
int value[ARRAY_SIZE] = { 1, 2, 3, 4, 3, 4, 2, 3, 5, 6};
int value2[100];
for (int i = 0; i < ARRAY_SIZE; i++)
{
for (int j = i + 1; j <= ARRAY_SIZE; j++)
{
if (value[i] == value[j])
{
cout << value[i] << " ";
}
}
}
return 0;
}
The output is
2 3 3 4 3
How can I make the output become 2 3 4 ?
Edit: I'm trying to print all numbers appearing more than once in the value array
I think I should create one more array to store value, but I stuck with it and don't know how to do it.
It helps considerably to sort your array. Then you only need two indices:
a write index, starting at 0
a read index, starting at 1
Loop over your array using the read index. For every array[read] that has a duplicate value at array[read-1], IFF that value also does not exist at array[write], then copy it over and increment your write index.
Finally, the new length of your data is equal to your write index.
The basic idea is this: if I have a sorted list of values:
1 3 3 4 5 5 5 7 7 9
Then I can easily see if a value is a duplicate or not by comparing the current (read) with the previous.
┌────┐
1 3 3 │4 5│ 5 5 7 7 9 -- not duplicates
└────┘
↑
┌────┐
1 3 3 4 │5 5│ 5 7 7 9 -- duplicate values
└────┘
↑
The only remaining trick is to make just a single copy of that value to the write index, which we can do by simply looking at the last thing we wrote.
You can use a std::map to count the number of times a value is in the array. If the number appears 2 or more times, then print it.
#include <iostream>
#include <map>
int main()
{
const int ARRAY_SIZE = 10;
int value[ARRAY_SIZE] = { 1, 2, 3, 4, 3, 4, 2, 3, 5, 6};
std::map <int, int> mp;
for(int i : value)
++mp[i];
for(const auto& p : mp)
if(p.second > 1)
std::cout << p.first << ' ';
}
Link.

Code for counting frequency of element in array not working

I'm trying to find the frequency of elements in an array. I have found lots of programs on Google that can do just this, but I can't get them to count right for my array.
This is the code I use:
#include <iostrem>
#include <cmath>
const int nmax = 50;
int main() {
int afk[nmax], count;
const int N = 40;
int dn[N] = { 6, 1, 2, 1, 5, 3, 1, 0, 3, 1, 2, 3, 6, 2, 1, 1, 5, 5, 1, 5, 0, 1, 3, 0, 0, 1, 0, 2, 1, 0, 0, 3, 1, 5, 2, 1, 4, 1, 0, 3 };
for (int i = 0; i < N; i++) {
afk[i] = -1;
}
for (int i = 0; i < N; i++) {
count = 1;
for (int j = i + 1; j < N; j++) {
if (dn[i] == dn[j]) {
count++;
afk[j] = 0;
}
}
if (afk[i] != 0) afk[i] = count;
}
for (int i = 0; i <= 6; i++) {
if (afk[i] != 0) {
cout << endl << i << " | " << afk[i];
}
}
}
This gives the output:
0 | 2
1 | 13
2 | 5
4 | 5
5 | 6
But I know this is worng and the right answer is
0 | 8
1 | 13
2 | 5
3 | 6
4 | 1
5 | 5
6 | 2
So the code only gets the frequency of elemnet 1 and 2 right.
Can anyone tell what's wrong with this code?
You are counting the frequencies just fine (well, not as efficiently as you could be), but you are not displaying them correctly.
The afk[] array is storing each number's frequency at the same index as its first occurrence in the dn[] array.
In your final display loop, you are printing the loop counter i itself instead of printing the number at the i'th index of the dn[] array. So, if you change this:
cout << endl << i << " | " << afk[i];
To this:
cout << endl << dn[i] << " | " << afk[i];
Then you will get a more meaningful result:
6 | 2
1 | 13
2 | 5
5 | 5
3 | 6
Note that the frequencies of 0 and 4 are missing, though. The first occurrence of 0 in the dn[] array is at index 7, and the first occurrence of 4 is at index 36. But your display loop stops after index 6, which is why it does not display the frequencies of 0 and 4. You would need to loop over the entire afk[] array to fix that, then you will see the missing entries:
6 | 2
1 | 13
2 | 5
5 | 5
3 | 6
0 | 8
4 | 1
Demo
However, the results are not sorted by number, as you want. To fix that, you will have to either:
sort the dn[] array before counting its frequencies (which, incidentally, will help make counting easier).
change the afk[] array to hold a struct type that contains int number and int frequency members, and then you can sort afk[] on number after counting the frequencies.
if nmax is greater than the largest number in the dn[] array (ie, > 6 in this case), then your counting loop can simply use the number stored at dn[i] as the index into afk[] rather than using i as the index. Then your display loop can simply display the afk[] array as-is (I suspect this is what you originally meant to do). Demo
In which case, it would be far easier to count the frequencies using a std::map instead, and let it handle the counting and sorting for you, eg:
#include <iostream>
#include <map>
using namespace std;
int main() {
map<int, int> afk;
int dn[40] = {6, 1, 2, 1, 5, 3, 1, 0, 3, 1, 2, 3, 6, 2, 1, 1, 5, 5, 1, 5, 0, 1, 3, 0, 0, 1, 0, 2, 1, 0, 0, 3, 1, 5, 2, 1, 4, 1, 0, 3};
for(auto number : dn){
afk[number]++;
}
for(auto &p : afk){
cout << p.first << " | " << p.second << endl;
}
}
0 | 8
1 | 13
2 | 5
3 | 6
4 | 1
5 | 5
6 | 2
Demo

Product modulo random numbers

A prime p is fixed. A sequence of n numbers is given, each from 1 to p - 1. It is known that the numbers in the sequence are chosen randomly, equally likely and independently from each other. Choose some numbers from the sequence so that their product, taken modulo p, is equal to the given number x. If no numbers are selected, the product is considered equal to one.
Input:
The first line contains three integers separated by spaces: the length of the sequence n, the prime number p and the desired value x
(n=100, 2<=p<=10^9, 0<x<p)
Next, n integers are written, separated by spaces or line breaks: the sequence a1, a2,. . ., an
(0 <ai <p)
Output:
Print the numbers from the sequence whose product modulo p is equal to x. The order in which numbers are displayed is not important. If there are several possible answers, print any of them
Example:
INPUT:
100 11 4
9 6 1 1 10 4 9 10 3 1 10 1 6 8 3 3 9 8
10 3 7 7 1 3 3 1 5 2 10 4 1 5 6 7 2 6
2 8 3 3 6 7 6 3 1 5 10 2 2 10 9 6 8 6
2 10 3 2 7 4 3 2 8 6 4 1 7 2 10 8 4 9
7 9 8 7 4 7 3 2 8 2 3 7 1 5 2 10 7 1 8
6 4 10 10 3 6 10 2 1
OUTPUT:
4 6 10 9
My solution:
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main()
{
int n,p,x,y,m,k,tmp;
vector<int> v;
cin >> n >> p >> x;
for (int i = 0; i<n; i++){
cin >> tmp;
v.push_back(tmp);
}
sort(v.begin(), v.end());
v.erase(v.begin(), upper_bound(v.begin(), v.end(), 1));
k=-1;
while(1){
k++;
m = 1;
y = x+p*k;;
vector<int> res;
for (int i = 0; i<n; i++){
if (y == 1) break;
if ( y%v[i] == 0){
res.push_back(v[i]);
m*=v[i];
m%=p;
y = y/v[i];
}
}
if (m==x) {
for (int i = 0; i<res.size(); i++){
cout << res[i] << " ";
}
break;
}
}
return 0;
}
In my solution, I used condition (y=x+k*p, where y is the product of numbers in the answer, and k is some kind of natural number). And also iterated over the value k.
This solution sometimes goes beyond the allotted time. Please tell me a more correct algorithm.
I would consider a backtracking routine over the hashed multiset of the input list. Since p is a prime, at any point we can consider if the current multiple, m, has (multiplicative_inverse(m, p) * x) % p in our multiset (https://en.wikipedia.org/wiki/Multiplicative_inverse). If it exists, we're done. Otherwise, try multiplying either by the same number we are currently visiting in the multiset, or by the next one (keep the result of the multiplication modulo p).
Please see comment below for a link to example code in Python. The example you gave has trivial solutions so it would be helpful to have some non-trivial, as well as challenging examples to test and refine on. Please also clarify if more than one number is expected in the output.
You can use dynamic programming approach. It requires O(p) memory cells and O(p*n) loop iterations. There is possible several optimization (to exclude processing input duplicates, or print longest/shortest selection chain). Following is simplest and basic DP-program, demonstrating this approach.
#include <stdio.h>
#include <stdlib.h>
int data[] = {
9, 6, 1, 1, 10, 4, 9, 10, 3, 1, 10, 1, 6, 8, 3, 3, 9, 8,
10, 3, 7, 7, 1, 3, 3, 1, 5, 2, 10, 4, 1, 5, 6, 7, 2, 6,
2, 8, 3, 3, 6, 7, 6, 3, 1, 5, 10, 2, 2, 10, 9, 6, 8, 6,
2, 10, 3, 2, 7, 4, 3, 2, 8, 6, 4, 1, 7, 2, 10, 8, 4, 9,
7, 9, 8, 7, 4, 7, 3, 2, 8, 2, 3, 7, 1, 5, 2, 10, 7, 1, 8,
6, 4, 10, 10, 3, 6, 10, 2, 1
};
struct elm {
int val; // Value
int prev; // from which elemet we come to this
int n; // add loop cound for prevent multiple use same val
};
void printsol(int n, int p, int x, const int *in) {
struct elm *dp = (struct elm *)calloc(p, sizeof(struct elm));
int i, j;
for(i = 0; i < n; i++) // add initial elements into DP array
dp[in[i]].val = in[i];
for(i = 0; i < n; i++) { // add elements, one by one, to DP array
if(dp[in[i]].val <= 1) // skip secondary "1" multipliers
continue;
for(j = 1; j < p; j++)
if(dp[j].val != 0 && dp[j].n < i) {
int y = ((long)j * in[i]) % p;
dp[y].val = in[i]; // current value, for printout
dp[y].prev = j; // reference to prev element
dp[y].n = n; // loop num, for prevent double reuse
if(x == y && dp[x].n > 0) {
// targed reached - print result, by iterate linklist
int mul = 1;
while(x != 0) {
printf(" %d ", dp[x].val);
mul *= dp[x].val; mul %= p;
x = dp[x].prev;
}
printf("; mul=%d\n", mul);
free(dp);
return;
}
} // for+if
} // for i
free(dp);
}
int main(int argc, char **argv) {
printsol(100, 11, 4, data);
return 0;
}

c++ array copy gives weird result

I have known java for a while and I was trying to translate a java program i wrote to c++ but the copy function gives an odd result:
#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
long gcd2(long a, long b) {
if ( a == 0 )
return b;
return gcd2(b%a,a);
}
long gcd(long nums[]) {
long ans = nums[0];
int len = sizeof(nums);
for (int i = 1; i < len; i++)
ans = gcd2( nums[i] , ans );
return ans;
}
string com(string s) {
s = s+",";
return (","+s);
}
void printa(long array[]) {
for (int i = 0 ; i < sizeof(array); i++)
cout << array[i] << ", ";
cout << "\n";
}
int main()
{
int length;
cin >> length;
long input[length];
for (int i = 0; i < length; i++)
cin >> input[i];
string possible = "";
int ans = 0;
for (int a = 0; a < length; a++) {
for (int b = length; b > a; b--) {
long arr[b-a];
std::copy(input+a,input+b,arr);
printa(arr);
long gcdans = gcd(arr);
if (possible.find( com(gcdans+"") ) == -1 ) {
possible += com(gcdans+"");
ans++;
}
}
}
cout << (ans);
return 0;
}
I give it the input of:
4
9 6 2 4
and it returns:
9, 6, 2, 4, 140725969483488, 4197851, 9, 6,
9, 6, 2, 4197851, 9, 6, 2, 4,
9, 6, 2, 4197851, 9, 6, 2, 4,
9, 4197851, 9, 6, 2, 4, 140725969483488, 4197766,
6, 2, 4, 4197851, 9, 6, 2, 4,
6, 2, 4, 4197851, 9, 6, 2, 4,
6, 4197851, 9, 6, 2, 4, 140725969483488, 4197766,
2, 4, 6, 4197851, 9, 6, 2, 4,
2, 4197851, 9, 6, 2, 4, 140725969483488, 4197766,
4, 4197851, 9, 6, 2, 4, 140725969483488, 4197766,
1
the number at the very end is what i want the program to output at the end, all the numbers above are me test printing the array to see its contents. Basically I am trying to copy a range of the array(for example (2,3,4) from (1,2,3,4,5,6)) But it gives weird numbers like 140725969483488 and 4197766 when the only numbers I input are 9 6 2 4
Variable length arrays is a C++ extension, not standard C++. If your compiler will allow them, then OK. However standard C++ would use an std::vector container which is dynamically sized at runtime, meaning you can initialise them with any size or numbers at runtime, and add anything you want at runtime.
Also note when passing an array in C++ to functions which take an array argument always (with the exception of explicitly declared sized reference to an array) gets passed as a pointer, so you can't know the size of the array once passed as an argument. So this:
void printa(long array[])
{
for (int i = 0 ; i < sizeof(array); i++) {}
// At this point of the code the sizeof(array) will return the size of
// a pointer, usually 4 or 8 bytes.
// It's a quirk that this happens, and is a holdover from C.
}
By taking an argument of std::vector you can know the size of the array. You can take the argument by value or by reference or pointer.
void printa(const std::vector<long>& array)
{
for (int i = 0 ; i < array.size(); i++)
{
cout << array[i] << ", ";
cout << "\n";
}
}
This is the better way to do it. If you want to use a C array or raw array the way you did, you will have to pass both the array and the size of the array as separate arguments.
Also, about the variable length array extension feature, I'm not sure whether it is reliable or not because I've never used the extension. Again, standard C++ requires that size of arrays are constant values, (known at compile time). Edit: actually (known at compile-time) is a bad description because:
int main()
{
int num = 6;
int myarray[num]; // In standard C++ this won't compile
//but
const int num = 6;
int myarray[num]; // Will
}
And one last thing, as SolutionMill pointed out, even if the sizeof(array) does give the right size and not the size of a pointer, it is the size given in bytes, not the number of elements, which was not you were wanting in:
for (int i = 0 ; i < sizeof(array); i++)
If the array is of 2 elements of 32 bit int, then the sizeof() operator will return size 8. A common but by no means pretty way to get the number of elements in an array is something like sizeof(array) / sizeof(array[0])

Adding to Individual Array Elements - C++

I need to add either +0.25 or -0.25 to all elements within an array. Here is what I have so far. Any help would be appreciated.
int main() {
double i;
// Arrays
double x[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
double x2[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
for(i=1; i<=10; i++) {
const double pM[2] = {-1, 1};
int randoid = rand() % 2;
for(i=1; i<=10; i++){
x2[i] = x[i] + pM[randoid]*0.25; //Error Line
}
}
cout << x;
cout << x2;
}
I get this error at the marked line: "invalid types 'double[10][double] for array subscript"
The problem is that i is a double. Then you write x2[i].
It's not a very good error message; however with the [] operator, one of the operands must be a pointer and the other must be an integer. There is no implicit conversion of floating-point to integer when using this operator.
To fix this change double i; to int i;
Another issue is that your code accesses out of bounds of the arrays. double x2[10] means that there are 10 elements whose indices are 0 through 9. But your loop tries to write to x2[10]. This causes undefined behaviour, which could explain your strange output.
There is also a potential logic error. Maybe you meant to use a different variable for the inner loop than the outer loop. As it stands, the inner loop will take i to 11 (or 10 if you fix the code) and then the outer loop will be complete and not execute any more iterations.
Based on your description though, perhaps you only meant to have one loop in the first place. If so, remove the outer loop and just leave the contents there.
Also you do not need two separate arrays, you could just perform the addition in-place.
Regarding the output, cout << x and cout << x2 will output the number of the memory address at which the array is located. To output the contents of the array instead you will need to write another loop, or use a standard library algorithm that iterates over containers.
I see 3 issues -
Change the type of i to int.
x and x2 are arrays of size 10. You need to loop from i =
0 to i = 9. But you are looping from i = 1 to i = 10. x[10]
is out of bounds since arrays are 0 indexed.
cout << x - This is a wrong way to print an array. You need
to loop through the array and print - e.g. -
for(i = 0; i < 10; i++)
cout << x[i] << " ";
Try this, it works, I converted to C
int main( )
{
int i = 0;
// Arrays
double x[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
double x2[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
for ( i = 1; i < 10; i++ )
{
const double pM[2] = { -1, 1 };
int randoid = rand( ) % 2;
for ( i = 1; i <= 10; i++ )
{
x2[i] = x[i] + pM[randoid] * 0.25; //Error Line
printf( "\nx[%d]==%2.2f", i, x[i] );
printf( "\nx2[%d]==%2.2f", i, x2[i] );
}
}
}