Implement Custom atoi function in C Language

Custom atoi function in C Program Description:

Write a Program to implement custom atoi function in c programming language. We are going to write our own atoi function in c. The function should take an ASCII string from the user and convert it into an Integer Number.

If the user enters an invalid ASCII string, Then the function should return Zero(0). If the ASCII string is valid, Then it should convert the string into an integer number.

The program should also take the sign into the consideration.

Here are the expected input and output of the program.

Excepted Input and Output:

Input:

Enter a string : -8434

Output:

Input String: '-8434'

Atoi Converted Integer Value: -8434

The input string contains the negative sign and the output integer is also a negative number.

Prerequisites:

It is recommended to know the basics of the C-Strings, C-Arrays, and Functions in C to better understand the following program.

Algorithm to Implement custom atoi function in C Language:

We have defined a function called myatoi() in the following program. Here is the syntax of the myatoi() function.

int myatoi(char *str);

This function takes a string as the input and returns the integer equivalent of it.

Let’s look at the Algorithm to implement the custom atoi function in c:

Create the necessary variables, The result variable will store the converted string. Initialize it with Zero(0). The sign variable is used to update the resultant number sign, So initialize it with 1. Also, create the variable i which is used to traverse through the string. initialize it with Zero(0)
Check if the first character in the input string str is a sign i.e -. If so, Then update the sign variable with -1. Increment the i by 1.

Start traversing the string element by element using the while loop,
- At Each iteration, Check if the str[i] is a valid digit, If not, Then return Zero(0). use (str[i] < ‘0’ || str[i] > ‘9’) condition to check for digit.
- Update the result variable by removing the ASCII Value of the Zero(0) from the str[i]. – i.e result = result * 10 + str[i] – ‘0’;
- Finally, Increment the i.
Once the above Step-3 is completed, The resultwill contain the converted integer ( ASCII to Integer converted value). Finally, update the sign of the result by multiplying it with sign variable – i.e result = result * sign;
Return the result back to the caller.

Now, Call the myatoi() function from the main() function and pass the input string. Store the return value in the result variable.

int result = myatoi(str);

Finally, Check the return value of the myatoi() function, Do note that, The myatoi() function returns Zero(0), If the input string contains any invalid characters like non-digit characters.

So check if the result is equal to Zero(0). If it is true, Then the input string str is not valid, Display the error message on the console.

Otherwise, The myatoi() function is able to convert the given ASCII String into an Integer number. Display the resulton the console.

Program to Implement custom atoi function in c Language:

Here is the Program to convert the ASCII string into an integer number using the custom/own atoi() function.

program implement your own atoi function in c

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#include <stdio.h>

// Max Size of string

#define SIZE 100

/**

* @brief - Convert string to Integer

* @param str - Input string

* @return int - On success returns coverted string, 0 on failure

int myatoi(char *str)

{

int result = 0;

int sign = 1;

int i = 0;

// check first character

if(str[i] == '-')

{

// update the sign

sign = -1;

i++;

}

// Iterate over the string

while(str[i])

{

if(str[i] < '0' || str[i] > '9')

{

return 0;

}

// update the result

result = result * 10 + str[i] - '0';

// increment 'i'

i++;

}

// finally, update sign

result = result * sign;

return result;

}

int main()

{

// declare input strings with 'SIZE'

char str[SIZE];

// Take the string from the user

printf("Enter a string : ");

gets(str);

printf("Input String: '%s'\n", str);

// call 'myatoi' function

int result = myatoi(str);

// check results and print results

if(result == 0)

{

// Invalid input or Zero value

printf("Invalid String or Zero. Please enter valid string to convert\n");

}

else

{

// Success: Converted integer

printf("Atoi Converted Integer Value: %d\n", result);

}

return 0;

}

Program Output:

Let’s compile and Run the program using your favorite compiler. We are using the GCC compiler on Ubuntu Linux.

Compile the Program.

$ gcc atoi-custom.c

The above command generates the a.out executable file, Run the executable file.

Test Case 1: When the input string is valid:

$ ./a.out

Enter a string : 3992

Input String: '3992'

Atoi Converted Integer Value: 3992

implement-atoi-function-in-c-language-program-output

As we can see from the above output, The user provided the input ASCII string as "3992", and the program is able to convert the ASCII string to an Integer number and returned the result as 3992 ( Integer).

Let’s look at another example.

$ ./a.out

Enter a string : 123456

Input String: '123456'

Atoi Converted Integer Value: 123456

As we can see from the above output, The input string “ 123456” has been converted successfully to an integer 123456

Test Case 2: When the input string has a sign

// Test 2

$ ./a.out

Enter a string : -5679

Input String: '-5679'

Atoi Converted Integer Value: -5679

In this case, The input has the negative sign and the program is converted the string with the sign properly and returned the negative integer number ( -5679).

Test Case 3: Invalid string:

In this case, The user provided the invalid ASCII string, i.e the ASCII string has non-digit characters. So myatoi() function returned the Zero(0), which means the invalid input.

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