[英]string subscript out of range error when assigning variable to function
為了更加熟悉 cpp,我開始編寫一個程序,該程序使用冪規則對簡單多項式進行導數。 到目前為止,它適用於多項式,例如5x^2+4x 。 但是,如果多項式包含一個常數(如5x + 3 ),我會得到一個字符串下標超出范圍錯誤。 我使用調試器並在第 33 行( std::string term = differentiateTerm(*iter);
)上找到了錯誤觸發器。 我不確定我在這里做錯了什么,我將不勝感激。
完整代碼:
#include <iostream>
#include <string>
#include <vector>
std::vector<std::string> terms;
std::vector<std::string>::const_iterator iter;
std::string takeDerivative(std::string expression);
void separateTerms(std::string expression);
std::string differentiateTerm(std::string inputTerm);
int main()
{
std::string userExpression;
std::cout << "Enter a polynomial.\n";
std::cin >> userExpression;
std::string outputExpression = takeDerivative(userExpression);
std::cout << "The derivative of your expression is: " + outputExpression;
return 0;
}
std::string takeDerivative(std::string expression)
{
std::string derivative;
separateTerms(expression);
for (iter = terms.begin(); iter != terms.end(); iter++)
{
std::string term = differentiateTerm(*iter);
if (iter - terms.begin() == 0)
{
derivative = term;
}
else
{
derivative += "+" + term;
}
}
return derivative;
}
void separateTerms(std::string expression)
{
int previousSign = 0;
bool firstTerm = true;
for (int i = 0; i < expression.size() + 1; i++)
{
if (expression[i] == '+' || expression[i] == '-')
{
if (firstTerm)
{
terms.push_back(expression.substr(0, i));
firstTerm = false;
previousSign = i;
}
else
{
terms.push_back(expression.substr(previousSign + 1, i - previousSign - 1));
previousSign = i;
}
}
else if (i == expression.size())
{
if (firstTerm)
{
terms.push_back(expression.substr(previousSign, i));
}
else
{
terms.push_back(expression.substr(previousSign + 1, i - previousSign));
}
}
}
}
std::string differentiateTerm(std::string inputTerm)
{
std::string outputTerm;
int coefficient = 1;
int exponent = 1;
int varPos = inputTerm.find('x');
if (inputTerm[varPos] == std::string::npos)
{
outputTerm = "0";
return outputTerm;
}
else {
if (inputTerm[varPos - 1] != std::string::npos)
{
coefficient = std::stoi(inputTerm.substr(0, varPos));
}
if (inputTerm[varPos + 1] == '^')
{
if (inputTerm[varPos + 2] != std::string::npos)
{
exponent = std::stoi(std::string(1, inputTerm[varPos + 2]));
}
}
}
coefficient = coefficient * exponent;
exponent--;
if (exponent <= 0)
{
outputTerm = std::to_string(coefficient);
}
else if (exponent == 1)
{
outputTerm = std::to_string(coefficient) + "x";
}
else
{
outputTerm = std::to_string(coefficient) + "x^" + std::to_string(exponent);
}
return outputTerm;
}
您沒有在 distinctTerm 中正確檢查find
的返回differentiateTerm
。 這會導致inputTerm[varPos]
訪問越界。
正確的檢查是
if (varPos == std::string::npos)
在隨后的if
語句中,由於您在varPos
中有一個有效的下標,您應該根據字符串的大小檢查它。 所以你會有if (varPos > 0)
, if (varPos < inputTerm.size() - 1)
等。
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