c/c++再学习：常用字符串转数字操作

能实现字符串转数字有三种方法，atof函数，sscanf函数和stringstream类。

具体demo代码和运行结果

#include "stdio.h"

#include <iostream>

#include <>

int main()

{

	printf("字符串转数字：stof()函数 string转单精度浮点数\n");

	std::string stof_str("686.123456789123456");

	float stof_val = std::stof(stof_str);

	printf("stof_val=%.9f\n", stof_val);

	printf("字符串转数字：stod()函数 string转双精度浮点数\n");

	std::string stod_str("686.123456789123456");

	double stod_val = std::stod(stod_str);

	printf("stod_val=%.9f\n", stod_val);

	printf("字符串转数字：atof()函数\n");

	char* atof_str = "123.123456789123456";

	double atof_val = atof(atof_str);

	printf("atof_val=%.9f\n", atof_val);

	printf("字符串转数字：sscanf()函数\n");

	char* strInt = "123456789";

	int   strInt_num = 0;

	sscanf(strInt, "%d", &strInt_num);

	printf("strInt_num=%d\n", strInt_num);

	char* strLong = "1234567890123456789";

	long long  strLong_num = 0;

	sscanf(strLong, "%lld", &strLong_num);

	printf("strLong_num=%lld\n", strLong_num);

	char* strFloat = "1.23456789";

	float strFloat_num = 0.0;

	sscanf(strFloat, "%f", &strFloat_num);

	printf("strFloat_num=%f\n", strFloat_num);

	char* strDouble = "1.23456789";

	double strDouble_num = 0.0;

	sscanf(strDouble, "%lf", &strDouble_num);

	printf("strDouble_num=%.8lf\n", strDouble_num);

	printf("字符串转数字：stringstream类\n");

	stringstream ss;

	ss.clear();

	printf("stringstream precision=%d\n", ss.precision());

	string string_val1 = "1234567890";

	int val1 = 0;

	ss.str(string_val1);

	ss >> val1;

	printf("val1=%d\n", val1);

	ss.clear();

	string string_val2 = "1.234567890123456789";

	double val2 = 0;

	ss.str(string_val2);

	ss >> val2;

	printf("val2=%.9f\n", val2);

}

运行结果

字符串转数字：stof()函数 string转单精度浮点数

stof_val=686.123474121

字符串转数字：stod()函数 string转双精度浮点数

stod_val=686.123456789

字符串转数字：atof()函数

atof_val=123.123456789

字符串转数字：sscanf()函数

strInt_num=123456789

strLong_num=1234567890123456789

strFloat_num=1.234568

strDouble_num=1.23456789

字符串转数字：stringstream类

stringstream precision=6

val1=1234567890

val2=1.234567890