c++将字符转换为数字

将字符数组中的字符‘1到9转换为整数数字

#include<iostream>
using namespace std;
int chnum(char a[]);

int main()
{
	char ar[5] = { '3','5','3','7','4'};
	cout << chnum(ar) << endl;

	return 0;
}

int chnum(char str[])
{
	int num = 0;
	for (int i = 0; str[i]!='\0'; i++)
	{
		if (str[i] >= '0' && str[i] <= '9')
		{
			num = num * 10 +(int)(str[i]-'0');
		}
	}
	return num;
}

将字符串转换为整数

#include<iostream>
#include<string>//头文件
using namespace std;
int chnum(string);

int main()
{
	string str = "234";
	cout << chnum(str) << endl;

	return 0;
}

int chnum(string str)
{
	int num = 0;
	for (int i = 0; str[i]!='\0'; i++)
	{
		if (str[i] >= '0' && str[i] <= '9')//1~9的字符编码以次49~57
		{
			num = num * 10 +(int)(str[i]-'0');//重点理解这里，为何要num*10;
		}
	}
	return num;
}

将字符串字母转换为数字 

#include<iostream>
#include<string>//头文件
using namespace std;
int chnum(string);

int main()
{
	string str = "wer";
	cout << chnum(str) << endl;

	return 0;
}

int chnum(string str)
{
	int num = 0;
	for (int i = 0; str[i]!='\0'; i++)
	{
		if (str[i] >= 'a' && str[i] <= 'z')//a~z对应字符编码97~122
		{
			num = num * 10 +(int)(str[i]-'0');//重点理解这里，为何要num*10;
		}
	}
	return num;
}