atol(), atoll() and atof() functions in C/C++

1. atol(): This function converts a C-type string, passed as an argument to function call, to a long integer. It parses the C-string str interpreting its content as an integral number, which is returned as a value of type long int. The function discards the whitespace characters present at the beginning of the string until a non-whitespace character is found. If the sequence of non-whitespace characters in C-string str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed and zero is returned.

   Syntax:

   long int atol ( const char * str )
   

   Parameters: The function accepts one mandatory parameter str which is the representation of an integral number.

   Return Value: The function returns the converted integral number as a long int. If no valid conversion can be performed, it returns zero.

   // CPP program to illustrate
   // working of atol() function.
   #include <bits/stdc++.h>
   using namespace std;
   
   int main()
   {
       // char array of numbers
       char str1[] = "5672345";
   
       // Function calling to convert to a long int
       long int num1 = atol(str1);
   
       cout << "Number is " << num1 << "\n";
   
       // char array of numbers of spaces
       char str2[] = "10000002  0";
   
       // Function calling to convert to a long int
       long int num2 = atol(str2);
   
       cout << "Number is " << num2 << "\n";
       return 0;
   }
   Output:

   Number is 5672345
   Number is 10000002
   

2. atoll(): This function converts a C-type string, passed as an argument to function call, to a long long integer. It parses the C-string str interpreting its content as an integral number, which is returned as a value of type long long int. The function discards the whitespace characters present at the beginning of the string until a non-whitespace character is found.
   If the sequence of non-whitespace characters in C-string str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed and zero is returned.

   Syntax:

   long long int atoll ( const char * str )
   

   Parameters: The function accepts a mandatory parameter str which is the representation of an integral number.

   Return Value: The function returns the converted integral number as a long long int. If no valid conversion can be performed, it returns zero.

   // CPP program to illustrate
   // working of atol() function.
   #include <bits/stdc++.h>
   using namespace std;
   
   int main()
   {
       // char array of numbers
       char big_num1[] = "8239206483232728";
   
       // Function calling to convert to a long int
       long long int num1 = atoll(big_num1);
   
       cout << "Number is " << num1 << "\n";
   
       // char array of numbers of spaces
       char big_num2[] = "100000 9 1324100";
   
       // Function calling to convert to a long int
       long long int num2 = atoll(big_num2);
   
       cout << "Number is " << num2 << "\n";
       return 0;
   }
   Output:

   Number is 8239206483232728
   Number is 100000
   

3. atof() function: This function converts a C-type string, passed as an argument to function call, to double. It parses the C-string str interpreting its content as a floating point number, which is returned as a value of type double. The function discards the whitespace characters present at the beginning of the string until a non-whitespace character is found. If the sequence of non-whitespace characters in C-string str is not a valid floating point number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed and 0.0 is returned.

   Syntax:

   double atof ( const char * str )
   

   Parameters: The function accepts a single mandatory parameter str which is the representation of a floating point number.

   Return Value:The function returns the converted floating point number as a double value. If no valid conversion can be performed, the function returns zero (0.0).

   Return Value:

   // CPP program to illustrate
   // working of atol() function.
   #include <bits/stdc++.h>
   using namespace std;
   
   int main()
   {
       // char array
       char pi[] = "3.1415926535";
   
       // Calling function to convert to a double
       double pi_val = atof(pi);
   
       // prints the double value
       cout << "Value of pi = " << pi_val << "\n";
   
       // char array
       char acc_g[] = "9.8";
   
       // Calling function to convert to a double
       double acc_g_val = atof(acc_g);
   
       // prints the double value
       cout << "Value of acceleration due to gravity = "
            << acc_g_val << "\n";
       return 0;
   }
   Output:

   Value of pi = 3.14159
   Value of acceleration due to gravity = 9.8