# atan2() function with example in C++

C++ atan2() function: Here, we are going to learn about the atan2() function with example of cmath header in C++ programming language?
Submitted by IncludeHelp, on April 28, 2019

## C++ atan2() function

atan2() function is a library function of cmath header, it is used to find the principal value of the arc tangent of y/x, where y is the proportion of the y-coordinate and x is the proportion of the x-coordinate , it accepts two arguments (y, x) and returns arc tangent of y/x in radians.

## Syntax

Syntax of atan2() function:

```atan2(y, x);
```

## Parameter(s)

y, x – are the numbers (proportions of y-coordinate and x-coordinate) to calculate the principal value of the arc tangent of y/x.

## Return value

double – it returns double type value that is the principal value of the arc tangent of y/x.

## Sample Input and Output

```Input:
float x = -1.0;
float y = -2.5;

Function call:
atan2(y, x);

Output:
-1.9513
```

## Example

```// C++ code to demonstrate the example of
// atan2() function

#include <iostream>
#include <cmath>
using namespace std;

// main() section
int main()
{
float x;
float y;

x = -1.0;
y = -2.5;
cout<<"atan2("<<y<<","<<x<<"): "<<atan2(y,x)<<endl;

x = 11.0;
y = 22.5;
cout<<"atan2("<<y<<","<<x<<"): "<<atan2(y,x)<<endl;

x = -1.0;
y = 0;
cout<<"atan2("<<y<<","<<x<<"): "<<atan2(y,x)<<endl;

return 0;
}
```

### Output

```atan2(-2.5,-1): -1.9513
atan2(22.5,11): 1.11608
atan2(0,-1): 3.14159
```

Reference: C++ atan2() function