[英]how to extract integer from a string? c++
我從txt文件中提取了這一行(文件的第一行):
#operation=1(create circle and add to picture) name X Y radius.
為什么此代碼不采用整數1並將其放入k?
Circle Circle::CreateCirc(Circle c){
int k;
ifstream myfile("cmd.txt");
if (!myfile.is_open())
cout<<"Unable to open the requested file "<<endl;
string line,line2="create circle";
for (int i=1;i<countrows();i++)
{
getline(myfile,line);
if (line.find(line2)!=string::npos)
{
istringstream ss(line);
ss>>k;
cout<<k<<endl;
}
}
return c;
}
相反,我正在獲取地址記憶...幫助PLZ
因為該行不是以數字開頭。 在提取數字之前,您需要跳過#operation #operation=
部分。
您應該檢查提取結果和getline
,以幫助確定失敗的原因。
另外,如果countrows()
返回文件中的預期行數,則循環將錯過最后一行。 從零開始循環,或者當i <= countrows()
; 或者,如果您要處理文件中的每一行,則可以簡單地循環while (getline(myfile,line))
。
如果您嘗試讀取的文件中的實際文本以"#operation=1"
開頭,並且您希望從中開始使用數字1
,則不能使用簡單的輸入運算符。 它將首先讀取字符'#'
,因為它不是數字,所以解析將失敗,並且不會初始化k
。 如果k
未初始化,它將是不確定的值,讀取該值將導致不確定的行為和看似隨機的輸出。
您需要檢查提取是否有效:
if (ss >> k)
std::cout << k << '\n';
但是,這不能解決您的問題,就像我上面說的那樣,您不能在此處使用簡單的輸入運算符。 您需要使用其他方法來解析字符串。 一種方法可能是找到相等的字符'='
然后獲取一個子字符串以嘗試提取數字。
編輯:一種方法來做它有點接近您嘗試使用atoi()而不是流來做的方式。
#include <iostream>
#include <cstdlib> // for atoi()
int main(){
std::string str = "#operation=1(create circle and add to picture) name X Y radius.";
int k;
std::string line=str, line2="(create circle";
std::size_t fnd = line.find(line2);
if (fnd!=std::string::npos)
{
k = atoi(&str[fnd-1]); // int atoi(const char *str) == argument to integer
std::cout<< k << " " << str[fnd-1] << str[fnd] << " ";
}
}
有幾種方法可以從字符串中提取整數,但我喜歡從字符串中過濾出數字;
#include <iostream>
int main(){
std::string str = "#operation=1(create circle and add to picture) name X Y radius.";
int k = 0;
// an array of our base10 digits to filter through and compare
const char digit[] = {'0','1','2','3','4','5','6','7','8','9'};
for(int s_filter = 0; s_filter<str.size(); ++s_filter){
for(int d_filter = 0; d_filter<10; ++d_filter){
// filter through each char in string and
// also filter through each digit before the next char
if(digit[d_filter] == str[s_filter]) {
// if so the char is equal to one of our digits
k = d_filter;// and d_filter is equal to our digit
break;
} else continue;
}
}
switch(k) {
case 1:
std::cout<< "k == 1";
// do stuff for operation 1..
return 0;
case 2:
std::cout<< "k != 1";
// do more stuff
break;
//case 3: ..etc.. etc..
default:
std::cout<< "not a digit";
return 1;
}
}
嘗試這個:
Circle Circle::CreateCirc(Circle c){
const std::streamsize ALL = std::numeric_limits< std::streamsize >::max(); // #include <limits> needed
int k;
ifstream myfile("cmd.txt");
if (!myfile.is_open())
cout<<"Unable to open the requested file "<<endl;
for (int i=1;i<countrows(); ++i, myfile.ignore(ALL,'\n') ) // skip rest of the line
{
if( myfile.ignore(ALL,'=') >> k )
{
cout<<k<<endl;
}
else
break; // read error
}
return c;
}
// find_num.cpp (cX) 2015 adolfo.dimare@gmail.com
// http://stackoverflow.com/questions/21115457/
#include <string> // std::string
#include <cctype> // isnum
/// Find the number in 'str' starting at position 'pos'.
/// Returns the position of the first digit of the number.
/// Returns std::string::npos when no further numbers appear within 'str'.
/// Returns std::string::npos when 'pos >= str.length()'.
size_t find_num( const std::string str, size_t pos ) {
size_t len = str.length();
bool isNegative = false;
while ( pos < len ) {
if ( isdigit(str[pos]) ) {
return ( isNegative ? pos-1 : pos );
}
else if ( str[pos]=='-' ) {
isNegative = true;
}
else {
isNegative = false;
}
++pos;
}
return std::string::npos;
}
#include <cassert> // assert()
#include <cstring> // strlen();
int main() {
std::string str;
str = "";
assert( std::string::npos == find_num( str, 0 ) );
assert( std::string::npos == find_num( str, 9 ) );
str = "#operation=1(create circle and add to picture) name X Y radius.";
assert( strlen("#operation=") == find_num( str, 0 ) );
str = "abcd 111 xyx 12.33 alpha 345.12e-23";
/// 0123456789.123456789.123456789.123456789.
assert( 5 == find_num( str, 0 ) );
assert( 13 == find_num( str, 5+3 ) );
assert( 25 == find_num( str, 20 ) );
str = "abcd-111 xyx-12.33 alpha-345.12e-23";
/// 0123456789.123456789.123456789.123456789.
assert( 4 == find_num( str, 0 ) );
assert( 12 == find_num( str, 5+3 ) );
assert( 24 == find_num( str, 20 ) );
str = "-1";
assert( 0 == find_num( str, 0 ) );
assert( 1 == find_num( str, 1 ) );
assert( std::string::npos == find_num( str, 2 ) );
assert( std::string::npos == find_num( str, strlen("-1") ) );
return 0;
}
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