C Library Function - Atoi() - Tutorialspoint

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Home C_standard_library C Standard Library atoi Function C library - atoi() function Previous Quiz Next

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 Print Page Previous Next Advertisements