字符串与数字转换

1. itoa，非标准的C语言扩展函数，头文件为 #include<stdlib.h>

char* itoa (int value, char* string, int radix);

#include<algorithm>
#include<cstdlib>
#include<cstring>
using namespace std;

int main(){
	char str[10];
	int n=12345;
	itoa(n,str,10);
	cout<<str[2]<<endl;
	string newStr(str);
	cout<<newStr[3]<<endl;
	

	return 0;
}

2. springf，格式化数据到字符串缓冲区，头文件为 #include<stdlib.h>

int sprintf( char *buffer, const char *format, [ argument] … );

#include<stdio.h>
#include<stdlib.h>
int main()
{
	char buffer[50];
	int n,a=5,b=3;

	n = sprintf(buffer,"%dplus%dis%d",a,b,a+b);

	printf("[ %s ] is a string %d chars long
",buffer,n);/*“格式输出”*/
	 
	return 0;
}

3.  atoi，C库函数， 将字符串变为数字，头文件为 #include<stdlib.h>

int atoi(const char *nptr);

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

int main(void)
{
	double n;
	char*str="12345.67";
	n=atoi(str);
	printf("string=%s integer=%d
",str,(int)n);
	return 0;
}

4. 写一个atoi的实现函数

#include <iostream>
using namespace std;

void main(void){
	char str[20];      
	int i,n=0;	

	cout<<"Enter the string:";         
	cin.getline(str,20,'
');
	
    for (i=0; str[i] != ''; i++) 
		n = n*10+(str[i]-'0');

	cout<<"Corresponding number is "<<n<<endl;

    cout<<"The digits of the number from low to high is ";
	while(n){
		cout<<n%10<<',';   
		n/=10;
	}
	cout<<endl;
}