Sum of n natural numbers

Numbers 1, 2, 3, …., n are known as natural numbers. This program takes the value of n and finds the sum of first n natural numbers. For example, If user enters 5 as the value of n then the sum of first n(n=5) natural numbers would be:

1+2+3+4+5 = 15

Example: Program to calculate the sum of n natural numbers

To understand this program you should have the knowledge of C++ while loop.

 
#include  
using namespace std;  
int main(){     
int n, sum=0;     
cout<<"Enter the value of n(should be a positive integer): ";     
 //Storing the input integer value into the variable n     
cin>>n;       
/* If user enters value of n as negative then display an       
* error message to user that the value of n is invalid       */     
if(n<=0){        
cout<<"Invalid value of n";     
}     
else{        
// We are using while loop to calculate the sum       
int i=1;       
while(i<=n){  	 
sum=sum+i;  	
 i++;      
 }       
cout<<"Sum of first n natural numbers is: "<<sum;    
 }       
return 0; 
 }

Output

Enter the value of n(should be a positive integer): 5

Sum of first n natural numbers is: 15

You can also use for loop for this program. You just need to replace this part:

int i=1;    
while(i<=n){         
sum=sum+i;          
i++;     
}

With this

for(int i = 1; i <= n; i++) {     
sum = sum+i;  
}

Using For Loop

 
#include  
using namespace std;  
int main(){     
int n, sum=0;     
cout<<"Enter the value of n(should be a positive integer): ";     
 //Storing the input integer value into the variable n     
cin>>n;       
/* If user enters value of n as negative then display an       
* error message to user that the value of n is invalid       */     
if(n<=0){        
cout<<"Invalid value of n";     
}     
else{        
     
for(int i = 1; i <= n; i++) {     
sum = sum+i;  
}      
cout<<"Sum of first n natural numbers is: "<<sum;    
 }       
return 0; 
 }

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