How do I convert multiple elements of a character array into an integer?

Question

Say I have,

char x[0] = '1';
char x[1] = '2';

I need to 'concatenate' these two into an integer variable,

int y = 12;

How do I do this?

Many will disagree so I just put it into a comment: http://www.cplusplus.com/reference/cstdlib/atoi/ — Bence Kaulics, Oct 18 '16 at 08:12
Using simple math: `int y = ((x[0] - '0') * 10) + (x[1] - '0');` (Note `'0'` means ASCII of digit `0`) — David Ranieri, Oct 18 '16 at 08:13
Possible duplicate of [Convert char array to a int number in C](http://stackoverflow.com/questions/10204471/convert-char-array-to-a-int-number-in-c) — Julien Lopez, Oct 18 '16 at 08:21
@BenceKaulics because [`atoi` doesn't return any error message](http://stackoverflow.com/q/17710018/995714), [`strtol`](http://stackoverflow.com/q/7021725/995714) is better — phuclv, Oct 18 '16 at 08:54

Hatted Rooster · Answer 1 · 2016-10-18T08:20:50.190

1

As long as you terminate x with a NULL-terminator you can use atoi.

Example:

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

int main() {
    char x[3];
    x[0] = '1';
    x[1] = '2';
    x[2] = '\0';
    int x_int = atoi(x);
    printf("%i", x_int);
    return 0;
}

edited Oct 18 '16 at 08:20

answered Oct 18 '16 at 08:16

Hatted Rooster

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This is C++, the question is about C. – unwind Oct 18 '16 at 08:18

unwind · Accepted Answer · 2016-10-18T08:38:14.807

The simplest way, if it's really only two digits and you don't want to "go up" (in complexity) to a string in order to use string-conversion functions, is to just compute it directly using the basic structure of a number. Each digit is worth 10 times more than the one to its right.

const char x[2] = { '1', '2' };
const int value = 10 * (x[0] - '0') + (x[1] - '0');

This will set value to 12 by computing 10 * 1 + 2.

The subtraction of 0, meaning "the numerical value of the digit zero in the target's character encoding", works since C requires the decimal digits to be encoded in sequence.

How do I convert multiple elements of a character array into an integer?

2 Answers2