The atoi() function in C takes a string (which represents an integer) as an argument and returns its value of type int. So basically the function is used to convert a string argument to an integer.
Syntax of atoi()
int atoi(const char strn);
Parameters
- The function accepts one parameter strn which refers to the string argument that is needed to be converted into its integer equivalent.
Return Value
- If strn is a valid input, then the function returns the equivalent integer number for the passed string number.
- If no valid conversion takes place, then the function returns zero.
Example
C++
#include <bits/stdc++.h>using namespace std;
int main()
{ int val;
char strn1[] = "12546";
val = atoi(strn1);
cout << "String value = " << strn1 << endl;
cout << "Integer value = " << val << endl;
char strn2[] = "GeeksforGeeks";
val = atoi(strn2);
cout << "String value = " << strn2 << endl;
cout << "Integer value = " << val << endl;
return (0);
}// This code is contributed by shivanisinghss2110 |
C
#include <stdio.h>#include <stdlib.h>#include <string.h>int main()
{ int val;
char strn1[] = "12546";
val = atoi(strn1);
printf("String value = %s\n", strn1);
printf("Integer value = %d\n", val);
char strn2[] = "GeeksforGeeks";
val = atoi(strn2);
printf("String value = %s\n", strn2);
printf("Integer value = %d\n", val);
return (0);
} |
Java
import java.util.*;
public class Main {
public static void main(String[] args)
{
int val;
String strn1 = "12546";
val = Integer.parseInt(strn1);
System.out.println("String value = " + strn1);
System.out.println("Integer value = " + val);
String strn2 = "GeeksforGeeks";
try {
val = Integer.parseInt(strn2);
System.out.println("String value = " + strn2);
System.out.println("Integer value = " + val);
}
catch (NumberFormatException e) {
val = 0;
System.out.println("String value = " + strn2);
System.out.println("Integer value = " + val);
}
}
} |
Python3
def main():
strn1 = "12546"
val = int(strn1)
print("String value = ", strn1)
print("Integer value = ", val)
strn2 = "GeeksforGeeks"
try:
val = int(strn2)
except ValueError:
val = 0
print("String value = ", strn2)
print("Integer value = ", val)
if __name__ == "__main__":
main()
|
C#
using System;
namespace GeeksforGeeks {
class Program {
static void Main(string[] args)
{
String strn1 = "12546";
int val = int.Parse(strn1);
Console.WriteLine("String value = " + strn1);
Console.WriteLine("Integer value = " + val);
String strn2 = "GeeksforGeeks";
try {
val = int.Parse(strn2);
}
catch (FormatException e) {
val = 0;
Console.WriteLine("String value = " + strn2);
Console.WriteLine("Integer value = " + val);
}
}
}} |
Javascript
// Javascript code to convert string to integerlet val;let strn1 = "12546";
val = parseInt(strn1);console.log("String value = " + strn1);
console.log("Integer value = " + val);
let strn2 = "GeeksforGeeks";
val = parseInt(strn2);console.log("String value = " + strn2);
console.log("Integer value = " + val);
// This code is contributed by prasad264 |
Output
String value = 12546 Integer value = 12546 String value = GeeksforGeeks Integer value = 0
Complexity Analysis:
- Time Complexity: O(n), Only one traversal of the string is needed.
- Space Complexity: O(1), As no extra space is required.
Now let’s understand various ways in which one can create their own atoi() function supported by various conditions:
Approach 1
The following is a simple implementation of conversion without considering any special case.
- Initialize the result as 0.
- Start from the first character and update the result for every character.
- For every character update the answer as result = result * 10 + (s[i] – ‘0’)
Below is the implementation of the above approach:
C++
// A simple C++ program for// implementation of atoi#include <bits/stdc++.h>using namespace std;
// A simple atoi() functionint myAtoi(char* str)
{ // Initialize result
int res = 0;
// Iterate through all characters
// of input string and update result
// take ASCII character of corresponding digit and
// subtract the code from '0' to get numerical
// value and multiply res by 10 to shuffle
// digits left to update running total
for (int i = 0; str[i] != '\0'; ++i)
res = res * 10 + str[i] - '0';
// return result.
return res;
}// Driver codeint main()
{ char str[] = "89789";
// Function call
int val = myAtoi(str);
cout << val;
return 0;
}// This is code is contributed by rathbhupendra |
C
// Program to implement atoi() in C#include <stdio.h>// A simple atoi() functionint myAtoi(char* str)
{ // Initialize result
int res = 0;
// Iterate through all characters
// of input string and update result
// take ASCII character of corresponding digit and
// subtract the code from '0' to get numerical
// value and multiply res by 10 to shuffle
// digits left to update running total
for (int i = 0; str[i] != '\0'; ++i)
res = res * 10 + str[i] - '0';
// return result.
return res;
}// Driver Codeint main()
{ char str[] = "89789";
// Function call
int val = myAtoi(str);
printf("%d ", val);
return 0;
} |
Java
// A simple Java program for// implementation of atoiclass GFG {
// A simple atoi() function
static int myAtoi(String str)
{
// Initialize result
int res = 0;
// Iterate through all characters
// of input string and update result
// take ASCII character of corresponding digit and
// subtract the code from '0' to get numerical
// value and multiply res by 10 to shuffle
// digits left to update running total
for (int i = 0; i < str.length(); ++i)
res = res * 10 + str.charAt(i) - '0';
// return result.
return res;
}
// Driver code
public static void main(String[] args)
{
String str = "89789";
// Function call
int val = myAtoi(str);
System.out.println(val);
}
}// This code is contributed by PrinciRaj1992 |
Python3
# Python program for implementation of atoi# A simple atoi() functiondef myAtoi(string):
res = 0
# Iterate through all characters of
# input string and update result
for i in range(len(string)):
res = res * 10 + (ord(string[i]) - ord('0'))
return res
# Driver programstring = "89789"
# Function callprint(myAtoi(string))
# This code is contributed by BHAVYA JAIN |
C#
// A simple C# program for implementation// of atoiusing System;
class GFG {
// A simple atoi() function
static int myAtoi(string str)
{
int res = 0; // Initialize result
// Iterate through all characters
// of input string and update result
// take ASCII character of corresponding digit and
// subtract the code from '0' to get numerical
// value and multiply res by 10 to shuffle
// digits left to update running total
for (int i = 0; i < str.Length; ++i)
res = res * 10 + str[i] - '0';
// return result.
return res;
}
// Driver code
public static void Main()
{
string str = "89789";
// Function call
int val = myAtoi(str);
Console.Write(val);
}
}// This code is contributed by Sam007. |
Javascript
<script>// A simple Javascript program for// implementation of atoi// A simple atoi() functionfunction myAtoi(str)
{ // Initialize result
let res = 0;
// Iterate through all characters
// of input string and update result
// take ASCII character of corresponding digit and
// subtract the code from '0' to get numerical
// value and multiply res by 10 to shuffle
// digits left to update running total
for (let i = 0; i < str.length; ++i)
res = res * 10 + str[i].charCodeAt(0) - '0'.charCodeAt(0);
// return result.
return res;
}// Driver codelet str = "89789";
// Function calllet val = myAtoi(str);document.write(val);// This code is contributed by rag2127</script> |
Output
89789
Complexity Analysis:
- Time Complexity: O(n), Only one traversal of the string is needed.
- Space Complexity: O(1), As no extra space is required.
Approach 2
This implementation handles the negative numbers.
- If the first character is ‘-‘ then store the sign as negative and then convert the rest of the string to number using the previous approach while multiplying the sign with it.
Below is the implementation of the above approach:
C++
// A C++ program for// implementation of atoi#include <bits/stdc++.h>using namespace std;
// A simple atoi() functionint myAtoi(char* str)
{ // Initialize result
int res = 0;
// Initialize sign as positive
int sign = 1;
// Initialize index of first digit
int i = 0;
// If number is negative,
// then update sign
if (str[0] == '-') {
sign = -1;
// Also update index of first digit
i++;
}
// Iterate through all digits
// and update the result
for (; str[i] != '\0'; i++)
res = res * 10 + str[i] - '0';
// Return result with sign
return sign * res;
}// Driver codeint main()
{ char str[] = "-123";
// Function call
int val = myAtoi(str);
cout << val;
return 0;
}// This is code is contributed by rathbhupendra |
C
// A C program for// implementation of atoi#include <stdio.h>// A simple atoi() functionint myAtoi(char* str)
{ // Initialize result
int res = 0;
// Initialize sign as positive
int sign = 1;
// Initialize index of first digit
int i = 0;
// If number is negative,
// then update sign
if (str[0] == '-') {
sign = -1;
// Also update index of first digit
i++;
}
// Iterate through all digits
// and update the result
for (; str[i] != '\0'; ++i)
res = res * 10 + str[i] - '0';
// Return result with sign
return sign * res;
}// Driver codeint main()
{ char str[] = "-123";
// Function call
int val = myAtoi(str);
printf("%d ", val);
return 0;
} |
Java
// Java program for// implementation of atoiclass GFG {
// A simple atoi() function
static int myAtoi(char[] str)
{
// Initialize result
int res = 0;
// Initialize sign as positive
int sign = 1;
// Initialize index of first digit
int i = 0;
// If number is negative, then
// update sign
if (str[0] == '-') {
sign = -1;
// Also update index of first
// digit
i++;
}
// Iterate through all digits
// and update the result
for (; i < str.length; ++i)
res = res * 10 + str[i] - '0';
// Return result with sign
return sign * res;
}
// Driver code
public static void main(String[] args)
{
char[] str = "-123".toCharArray();
// Function call
int val = myAtoi(str);
System.out.println(val);
}
}// This code is contributed by 29AjayKumar |
Python3
# Python program for implementation of atoi# A simple atoi() functiondef myAtoi(string):
res = 0
# initialize sign as positive
sign = 1
i = 0
# if number is negative then update sign
if string[0] == '-':
sign = -1
i += 1
# Iterate through all characters
# of input string and update result
for j in range(i, len(string)):
res = res*10+(ord(string[j])-ord('0'))
return sign * res
# Driver codestring = "-123"
# Function callprint(myAtoi(string))
# This code is contributed by BHAVYA JAIN |
C#
// C# program for implementation of atoiusing System;
class GFG {
// A simple atoi() function
static int myAtoi(string str)
{
// Initialize result
int res = 0;
// Initialize sign as positive
int sign = 1;
// Initialize index of first digit
int i = 0;
// If number is negative, then
// update sign
if (str[0] == '-') {
sign = -1;
// Also update index of first
// digit
i++;
}
// Iterate through all digits
// and update the result
for (; i < str.Length; ++i)
res = res * 10 + str[i] - '0';
// Return result with sign
return sign * res;
}
// Driver code
public static void Main()
{
string str = "-123";
// Function call
int val = myAtoi(str);
Console.Write(val);
}
}// This code is contributed by Sam007. |
Javascript
<script> // JavaScript program for implementation of atoi
// A simple atoi() function
function myAtoi(str)
{
// Initialize result
var res = 0;
// Initialize sign as positive
var sign = 1;
// Initialize index of first digit
var i = 0;
// If number is negative, then
// update sign
if (str[0] == '-') {
sign = -1;
// Also update index of first
// digit
i++;
}
// Iterate through all digits
// and update the result
for (; i < str.length; ++i)
res = res * 10 + str[i].charCodeAt(0) - '0'.charCodeAt(0);
// Return result with sign
return sign * res;
}
// Driver code
var str = "-129";
var val=myAtoi(str);
document.write(val);
</script> <! --This code is contributed by nirajgusain5 -->
|
Output
-123
Complexity Analysis:
- Time Complexity: O(n), Only one traversal of the string is needed.
- Space Complexity: O(1), As no extra space is required.
Approach 3
Four corner cases need to be handled:
- Discard all leading whitespaces
- Sign of the number
- Overflow
- Invalid Input
Below are the steps for the above approach:
- To remove the leading whitespaces, run a loop and ignore the whitespaces until a character of the digit is reached.
- It keeps a sign variable to keep track of the sign of the number.
- It checks for valid input characters if all characters are from 0 to 9 and converts them into integers.
- If an overflow occurs and if the number is greater than or equal to INT_MAX/10, return INT_MAX if the sign is positive and return INT_MIN if the sign is negative.
The other cases are handled in previous approaches.
Dry Run:
Below is the implementation of the above approach:
C++
// A simple C++ program for// implementation of atoi#include <bits/stdc++.h>using namespace std;
int myAtoi(const char* str)
{ int sign = 1, base = 0, i = 0;
// if whitespaces then ignore.
while (str[i] == ' ') {
i++;
}
// sign of number
if (str[i] == '-' || str[i] == '+') {
sign = 1 - 2 * (str[i++] == '-');
}
// checking for valid input
while (str[i] >= '0' && str[i] <= '9') {
// handling overflow test case
if (base > INT_MAX / 10
|| (base == INT_MAX / 10 && str[i] - '0' > 7)) {
if (sign == 1)
return INT_MAX;
else
return INT_MIN;
}
base = 10 * base + (str[i++] - '0');
}
return base * sign;
}// Driver Codeint main()
{ char str[] = " -123";
// Functional Code
int val = myAtoi(str);
cout << " " << val;
return 0;
}// This code is contributed by shivanisinghss2110 |
C
// A simple C++ program for// implementation of atoi#include <limits.h>#include <stdio.h>int myAtoi(const char* str)
{ int sign = 1, base = 0, i = 0;
// if whitespaces then ignore.
while (str[i] == ' ') {
i++;
}
// sign of number
if (str[i] == '-' || str[i] == '+') {
sign = 1 - 2 * (str[i++] == '-');
}
// checking for valid input
while (str[i] >= '0' && str[i] <= '9') {
// handling overflow test case
if (base > INT_MAX / 10
|| (base == INT_MAX / 10 && str[i] - '0' > 7)) {
if (sign == 1)
return INT_MAX;
else
return INT_MIN;
}
base = 10 * base + (str[i++] - '0');
}
return base * sign;
}// Driver Codeint main()
{ char str[] = " -123";
// Functional Code
int val = myAtoi(str);
printf("%d ", val);
return 0;
}// This code is contributed by Yogesh shukla. |
Java
// A simple Java program for// implementation of atoiclass GFG {
static int myAtoi(char[] str)
{
int sign = 1, base = 0, i = 0;
// if whitespaces then ignore.
while (str[i] == ' ') {
i++;
}
// sign of number
if (str[i] == '-' || str[i] == '+') {
sign = 1 - 2 * (str[i++] == '-' ? 1 : 0);
}
// checking for valid input
while (i < str.length && str[i] >= '0'
&& str[i] <= '9') {
// handling overflow test case
if (base > Integer.MAX_VALUE / 10
|| (base == Integer.MAX_VALUE / 10
&& str[i] - '0' > 7)) {
if (sign == 1)
return Integer.MAX_VALUE;
else
return Integer.MIN_VALUE;
}
base = 10 * base + (str[i++] - '0');
}
return base * sign;
}
// Driver code
public static void main(String[] args)
{
char str[] = " -123".toCharArray();
// Function call
int val = myAtoi(str);
System.out.printf("%d ", val);
}
}// This code is contributed by 29AjayKumar |
Python3
# A simple Python3 program for# implementation of atoiimport sys
def myAtoi(Str):
sign, base, i = 1, 0, 0
# If whitespaces then ignore.
while (Str[i] == ' '):
i += 1
# Sign of number
if (Str[i] == '-' or Str[i] == '+'):
sign = 1 - 2 * (Str[i] == '-')
i += 1
# Checking for valid input
while (i < len(Str) and
Str[i] >= '0' and Str[i] <= '9'):
# Handling overflow test case
if (base > (sys.maxsize // 10) or
(base == (sys.maxsize // 10) and
(Str[i] - '0') > 7)):
if (sign == 1):
return sys.maxsize
else:
return -(sys.maxsize)
base = 10 * base + (ord(Str[i]) - ord('0'))
i += 1
return base * sign
# Driver CodeStr = list(" -123")
# Functional Codeval = myAtoi(Str)
print(val)
# This code is contributed by divyeshrabadiya07 |
C#
// A simple C# program for implementation of atoiusing System;
class GFG {
static int myAtoi(char[] str)
{
int sign = 1, Base = 0, i = 0;
// if whitespaces then ignore.
while (str[i] == ' ') {
i++;
}
// sign of number
if (str[i] == '-' || str[i] == '+') {
sign = 1 - 2 * (str[i++] == '-' ? 1 : 0);
}
// checking for valid input
while (i < str.Length && str[i] >= '0'
&& str[i] <= '9') {
// handling overflow test case
if (Base > int.MaxValue / 10
|| (Base == int.MaxValue / 10
&& str[i] - '0' > 7)) {
if (sign == 1)
return int.MaxValue;
else
return int.MinValue;
}
Base = 10 * Base + (str[i++] - '0');
}
return Base * sign;
}
// Driver code
public static void Main(String[] args)
{
char[] str = " -123".ToCharArray();
int val = myAtoi(str);
Console.Write("{0} ", val);
}
}// This code is contributed by 29AjayKumar |
Javascript
<script>// A simple JavaScript program for// implementation of atoi function myAtoi(str)
{
var sign = 1, base = 0, i = 0;
// if whitespaces then ignore.
while (str[i] == ' ')
{
i++;
}
// sign of number
if (str[i] == '-' || str[i] == '+')
{
sign = 1 - 2 * (str[i++] == '-');
}
// checking for valid input
while (str[i] >= '0' && str[i] <= '9')
{
// handling overflow test case
if (base > Number.MAX_VALUE/ 10
|| (base == Number.MAX_VALUE / 10
&& str[i] - '0' > 7))
{
if (sign == 1)
return Number.MAX_VALUE;
else
return Number.MAX_VALUE;
}
base = 10 * base + (str[i++] - '0');
}
return base * sign;
} // Driver code
var str = " -123";
// Function call
var val = myAtoi(str);
document.write(" ", val);
// This code is contributed by shivanisinghss2110</script> |
Output
-123
Complexity Analysis:
- Time Complexity: O(n), Only one traversal of the string is needed.
- Space Complexity: O(1), As no extra space is required.
Related Articles:
Exercise:
Write your won atof() that takes a string (which represents a floating point value) as an argument and returns its value as double.
This article is compiled by Abhay Rathi.