In this C program, we will convert a string to integer using atoi function and using a user defined function. To convert string to integer, we first take a string as input from user using gets function. Input string should consist of digits('0' to '9') and minus sign('-') for negative numbers. It may contains some non-numerical characters like alphabet, but as soon as we see any non-numerical character we stop conversion and return converted integer till now.
For ExampleInput String : "12345"
Output Integer : 12345
Input String : "-123abcd"
Output Integer : -123
C program to convert a string to integer using atoi function
atoi function is defined inside stdlib.h header file. Function atio converts the string parameter to an integer. If no valid conversion exist for that string then it returns zero. Here is the declaration for atoi() function.
#include <stdio.h>
#include <stdlib.h>
int main(){
char inputString[20];
printf("Enter a String\n");
gets(inputString);
printf("Integer: %d \n", atoi(inputString));
return 0;
}
Output
Enter a String 2014 Integer: 2014
Enter a String -2000abcd Integer: -2000
C program to convert a string to integer without using atoi function
In this program we convert a string to integer without using atoi function. We first check that inputString[0] is '-' or not to identify negative numbers. Then we convert each numerical character('0' to '9') to equivalent digit and appends it to converted integer. Then we multiply converted integer with -1 or 1 based upon whether input string contains negative or positive number. Finally, it prints the integer on screen using printf function.
#include <stdio.h>
int main(){
char inputString[20];
int sign = 1, number = 0, index = 0;
printf("Enter a String\n");
gets(inputString);
if(inputString[0] == '-'){
sign = -1;
index = 1;
}
while(inputString[index] != '\0'){
if(inputString[index] >= '0' &&
inputString[index] <= '9'){
number = number*10 + inputString[index] - '0';
} else {
break;
}
index++;
}
number = number * sign;
printf("String : %s \n", inputString);
printf("Integer: %d \n", number);
return 0;
}
Output
Enter a String for Integer conversion -24356 String : -24356 Integer: -24356
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