C atoi function – atoi C Library Function

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C atoi function: The function atoi converts a string to a integer(int). The function skips all white-space characters at the beginning of the string until the first non-whitespace character is found. Then it converts the subsequent characters into a integer number until it finds a character which is not a digit. It also takes an optional initial plus or minus sign before first base-10 digit in string.

Function prototype of atoi

int atoi(const char *str);
  • str : A pointer to a string beginning with the representation of an integer number.

Return value of atoi

atoi in c: This function returns the converted integer number as an int value otherwise it returns zero(0), If no valid conversion could be performed.

C program using atoi function

atoi c: The following program shows the use of atoi function for string to integer conversion.

 

#include <stdio.h>
#include <stdlib.h>
 
int main(){
    char string[100];
    int value;
    printf("Enter a string\n");
    scanf("%s", &string);
     
    /* String to Integer conversion */
    value = atoi(string);
    printf("Integer value %d\n", value);
      
    return 0;
}

Output

Enter a string
1234
Integer value 1234

Enter a string
1234.555
Integer value 1234

Enter a string
1234ABCD
Integer value 1234

Enter a string
ABCD
Integer value 0