Check Number sequence - c++

Question

You are given S a sequence of n integers i.e. S = s1, s2, ..., sn. Compute if it is possible to split S into two parts : s1, s2, ..., si and si+1, si+2, ….., sn (0 <= i <= n) in such a way that the first part is strictly decreasing while the second is strictly increasing one.
Note : We say that x is strictly larger than y when x > y.
So, a strictly increasing sequence can be 1 4 8. However, 1 4 4 is NOT a strictly increasing sequence.
That is, in the sequence if numbers are decreasing, they can start increase at one point. And once number starts increasing, they cannot decrease at any point further.
Sequence made up of only increasing numbers or only decreasing numbers is a valid sequence. So in both the cases, print true.
You just need to print true/false. No need to split the sequence.
Input format :
Line 1 : Integer 'n'
Line 2 and Onwards : 'n' integers on 'n' lines(single integer on each line)
Output Format :
"true" or "false" (without quotes)
Constraints :
1 <= n <= 10^7
Sample Input 1 :
5
9
8
4
5
6
Sample Output 1 :
true
Sample Input 2 :
3
1
2
3
Sample Output 2 :
true
Sample Input 3 :
3
8
7
7
Sample Output 3 :
false
Explanation for Sample Format 3 :
8 7 7 is not strictly decreasing, so output is false.
Sample Input 4 :
6
8
7
6
5
8
2
Sample Output 4 :
false
Explanation for Sample Input 4 :
The series is :
8 7 6 5 8 2
It is strictly decreasing first (8 7 6 5). Then it's strictly increasing (5 8). But then it starts strictly decreasing again (8 2). Therefore, the output for this test case is 'false'
#include<iostream>
using namespace std;
int main() {
int n;
cin >> n;
int p;
cin >> p;
bool isDec = true;
int i = 1,c;
while(i <= n - 1){
cin >> c;
if(c > p){
if(isDec == true){
isDec = false;
}
isDec = true;
p = c;
} else if(c < p){
if(isDec == false){
isDec = false;
}
isDec = true;
p = c;
} else {
isDec = false;
break;
}
i++;
}
if(isDec){
cout << "true" << endl;
} else{
cout << "false" << endl;
}
}
What is wrong with this code? It failed for last test case.

Answer 1

#include<iostream>
using namespace std;
int main() {
int n;
cin>>n;
int current_term;
cin>>current_term;
bool isDecreasing = true,is_valid_sequence_yet=true;
int i=2;// first term of sequence has already been taken
while(i<=n){
int next_term;
cin>>next_term;
if(is_valid_sequence_yet && isDecreasing && current_term>next_term){
current_term=next_term;
isDecreasing=true;
}else if(is_valid_sequence_yet && current_term < next_term){
current_term=next_term;
isDecreasing = false;
}else{
is_valid_sequence_yet=false;
}
i++;
}
if(is_valid_sequence_yet){
cout << "true" << endl;
}else{
cout << "false" <<endl;
}
}

Answer 2

import java.util.*;
public class test{
public static void main(String args[]){
Scanner s=new Scanner(System.in);
int n=s.nextInt();
int arr[]=new int[n];
for(int i=0;i<n;i++){
arr[i]=s.nextInt();
}
boolean flag=true;
for(int i=0;i<n-2;i++){
if((arr[i]<arr[i+1])&& arr[i+1]>arr[i+2])){
flag=false;
break;
}
}
System.out.print(flag);
}
}

Answer 3

int num , temp = 1 , i =1 ;
cin>>num;
bool Isdec = true;
int Previes_num , current_num;
cin>>Previes_num;
while(i <= num-1){
cin>>current_num;
if(Previes_num > current_num){
if(Isdec){
Isdec = true ;
temp++;
}
else{
cout<<"false";
return 0;
}
}
else if(current_num > Previes_num){
if(Isdec){
Isdec = false ;
}
}
else{
cout<<"false";
return 0 ;
}
++i;
Previes_num=current_num;
}
if(temp==num){
cout<<"true";
}
else if(Isdec == 0){
cout<<"true" ;
}
return 0;
}

Answer 4

#include<iostream>
using namespace std;
int main() {
int n;
cin >> n;
cout << endl;
int t1,t2; // two consecutive terms
cin >> t1;
cout << endl;
bool changed=false; // this will turn true once the series has switched to increasing mode
bool result=true; // we will change it to flase if any condition is voilated
for(int i=2;i<=n;i++) { // i=2 since we already took one input
cin >> t2;
cout << endl;
if (t1>t2) {
if (changed == true) {//we are in the increasing part
result = false; //since now the terms are decreasing but we are in incresaing part the series is no longer valid
}
}
else if (t1<t2) {
if ( changed == false) {
changed = true; // the series has now changed to increasing series if it wasn't before
}
}
else {
result = false; // if tow terms are equal the result is automatically false
}
t1=t2; // replacing the terms to get new input
}
if (result) cout << "true"; // if we dont do this we get 0,1 as output
else cout << "false";
}
We need to set two bools as shown. Once our series changes from decreasing to increasing we will set change to true.
If we already have change set to true but our series changes again from decreasing to increasing we will set result bool to false.
In the end we can output result to get our required answer.
In case we get two consecutive terms equal we straightaway set result to false as it directly violates our condition of strictly increasing or strictly decreasing.
We will not use break anywhere because user is expected to give all the inputs and then result is displayed.
As for debugging your code, you need to change your approach. Also, what did you even intend to accomplish by this :
if(isDec == false){
isDec = false;
}