C library - atoi() function

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The C stdlib library atoi() function is used to convert a numeric string into an integer value.

An integer is a whole number that can be positive or negative, including zero. It can't be fraction, decimal, or percentage, For example, numbers like 1, 2, -5, and -10 are integers.

Syntax

Following is the syntax of the c atoi() function −

int atoi(const char *str)

Parameters

This function accepts a single parameter −

• str − It is a pointer to a null-terminated sting, which represent the a integer.

Return Value

This function returns an integral number, represents int value. If the input string is not a valid string's number, it returns 0.

Example 1

The following is the basic c example that converts the string value into the integer value using the atoi().

#include <stdlib.h>
#include <stdio.h>
int main(void)
{
   int val;
   char *str; 
   str = "1509.10E";
   val = atoi(str); 
   printf("integral number = %d", val);
}

Output

Following is the output −

integral number = 1509

Example 2

The below c example concatenate two string and then convert the resulting string into a integral number using the atoi().

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {	
   // Define two strings to concatenate
   char str1[] = "2023";
   char str2[] = "2024";
   
   //calculate the length of string first + second
   int length = strlen(str1) + strlen(str2) + 1;

   // Allocate memory for the concatenated string
   char *concatenated = malloc(length);

   // check memory allocation if null return 1.
   if(concatenated == NULL) {
       printf("Memory allocation failed\n");
       return 1;
   }

   // Concatenate str1 and str2
   strcpy(concatenated, str1);
   strcat(concatenated, str2);

   // Convert concatenated string into a integral number.
   // use the atoi() function
   int number = atoi(concatenated);

   printf("The concatenated string is: %s\n", concatenated);
   printf("The integral number is: %d\n", number);

   // at the last free the alocated memory
   free(concatenated);
   return 0;
}

Output

Following is the output −

The concatenated string is: 20232024
The integral number is: 20232024

Example 3

Following is another example of atoi(), here we convert the character string into the integral number.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main () {
   int res;
   char str[25];
   
   //define a string
   strcpy(str, "tutorialspoint India");
   
   //use atoi() function
   res = atoi(str);
   printf("String Value = %s\n", str);
   printf("Integral Number = %d\n", res);
   
   return(0);
}

Output

Following is the output −

String Value = tutorialspoint India
Integral Number = 0