C++ string to integer conversion

fatman · 04-07-2005, 05:11 PM

Is there a function, accessible in a common library, to convert from a string (of numbers) into an integer?
I mean an string library string, not an array of char's (so atoi is not an option)
I can do it manually, but that is kinda hacky as im sure i dont do proper error checking
I am just trying to find somethig universal and simple so i can go:

#include<magiclibrary>
int i;
string s = "123";
i = magicFunction(s);

Does such a creature exist in a librarry common to all compliers?

sirclif · 04-07-2005, 05:27 PM

the magicfunction is atoi()

prototype:

int atoi(string str)

i = atoi(s);

its in stdlib.h,but i think c++ compilers will want you to use cstdlib

http://www.cplusplus.com/ref/cstdlib/

here is a complete list of the function in stdlib.h

fatman · 04-07-2005, 09:32 PM

the problem with atoi is that it takes a "const char*" , where the string type is a "char*"

I think this may work:

string str = "123";
int i;
i = atoi(str.c_str());

FishBoy · 04-08-2005, 12:49 AM

well you could write your own code to do that too i guess
maybe something like

Code:

//atoi.c
#include<stdio.h>

int main(){
        char st[]="123";
        int i=0;

        for(;i<(int)strlen(st)+1;i++)
                printf("%d ",st[i]); //you could replace %d with whatever you want
        printf("\n");
        return 0;
}

that code should do it it'll convert any character into a number giving you it's ascii value

sirclif · 04-08-2005, 07:44 AM

fatman is right, i forgot about that.

keefaz · 04-08-2005, 08:17 AM

In C++ you could use string stream input capabilities :

Code:

#include <iostream>
#include <sstream>
using namespace std;

int main() {
    string s = "123";
    int i;
    istringstream input;

    input.str(s);
    input >> i;
    cout << "i = " << i << endl;

    return 0;
}

Christmas · 06-18-2006, 08:17 AM

What about this one I made? I'm still kind of newbie and learning so I hope it's not too long or buggy.

Code:

/*
	This function converts a string to an int.
	If the string contains not only numbers, but other
	signs (including "+" or "-"), the function will
	return 0.
*/

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

int string_to_int (char s[4])
{
	int n, i;
	n = 0;
	i = 0;
	while (i < strlen (s))
	{
		if (s[i] == 48)
			n = n * 10 + 0;
		if (s[i] == 49)
			n = n * 10 + 1;
		if (s[i] == 50)
			n = n * 10 + 2;
		if (s[i] == 51)
			n = n * 10 + 3;
		if (s[i] == 52)
			n = n * 10 + 4;
		if (s[i] == 53)
			n = n * 10 + 5;
		if (s[i] == 54)
			n = n * 10 + 6;
		if (s[i] == 55)
			n = n * 10 + 7;
		if (s[i] == 56)
			n = n * 10 + 8;
		if (s[i] == 57)
			n = n * 10 + 9;
		i = i + 1;
	}
	return n;
}

main ()
{
	char s[4];
	strcpy (s, "1453");
	printf ("The string is %s\n", s);
	printf ("The number is %i\n", string_to_int (s));
}