c/c++再學習：常用字符串轉數字操作

c/c++再學習：常用字符串轉數字操作

能實現字符串轉數字有三種方法，atof函數，sscanf函數和stringstream類。

具體demo代碼和運行結果

#include "stdio.h"
#include <iostream>
#include <>
int main()
{
	printf("字符串轉數字：stof()函數 string轉單精度浮點數\n");
	std::string stof_str("686.123456789123456");
	float stof_val = std::stof(stof_str);
	printf("stof_val=%.9f\n", stof_val);

	printf("字符串轉數字：stod()函數 string轉雙精度浮點數\n");
	std::string stod_str("686.123456789123456");
	double stod_val = std::stod(stod_str);
	printf("stod_val=%.9f\n", stod_val);

	printf("字符串轉數字：atof()函數\n");
	char* atof_str = "123.123456789123456";
	double atof_val = atof(atof_str);
	printf("atof_val=%.9f\n", atof_val);

	printf("字符串轉數字：sscanf()函數\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("字符串轉數字：stringstream類\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);

}

運行結果
字符串轉數字：stof()函數 string轉單精度浮點數
stof_val=686.123474121
字符串轉數字：stod()函數 string轉雙精度浮點數
stod_val=686.123456789
字符串轉數字：atof()函數
atof_val=123.123456789
字符串轉數字：sscanf()函數
strInt_num=123456789
strLong_num=1234567890123456789
strFloat_num=1.234568
strDouble_num=1.23456789
字符串轉數字：stringstream類
stringstream precision=6
val1=1234567890
val2=1.234567890