数组与字符串 1.4

编写一个方法，将字符串中的空格全部替换为"%20"。假定该字符串尾部有足够的空间存放新增字符，并且知道字符串的"真实"长度。

示例

输入："Mr John Smith"

输出："Mr%20John%20Smith"

分析：先确定替换后的字符串的长度，然后从后往前处理输入字符串。

#include <iostream>
#include <fstream>
#include <cstring>
#include <assert.h>

using namespace std;

void replace( char *s );

int main( int argc, char *argv[] ) {
    string data_file = "./1.4.txt";
    ifstream ifile( data_file.c_str(), ios::in );
    if( !ifile.is_open() ) {
        fprintf( stderr, "cannot open file: %s\n", data_file.c_str() );
        return -1;
    }
    const int BUFFER_SIZE = 1024;
    char buffer[3*BUFFER_SIZE];
    while( ifile.getline( buffer, BUFFER_SIZE ) ) {
        cout <<buffer <<": ";
        replace( buffer );
        cout <<buffer <<endl;
    }
    ifile.close();
    return 0;
}

void replace( char *s ) {
    int space = 0, slen = strlen(s);
    for( int i = 0; i < slen; ++i ) {
        if( s[i] == ' ' ) { ++space; }
    }
    int i = slen+1, j = slen + 2*space;
    while( --i >= 0 ){
        if( s[i] == ' ' ) {
            s[j] = '0'; s[j-1] = '2'; s[j-2] = '%';
            j -= 3;
        } else {
            s[j--] = s[i];
        }
    }
    assert( j == -1 );
    return;
}

测试文件

aa aa aa
nfsdfafds sdfsdf sdfsdf 
a a a a a a           
                asdfdsfs sdfdfs dfsdfsdfs
    aa dfs asdf asdf  sdf sdf   sdfs
Mr John Smith