将字符串另存为浮点数数组C ++

Question

I am receiving from terminal the following string:
"4 4 0.2 0.5 0.3 0.0 0.1 0.4 0.4 0.1 0.2 0.0 0.4 0.4 0.2 0.3 0.0 0.5"
My objective is to save this string as an array of floats like arr=[4,4,0.2,...]. I do not know the size of the array in advance so depends on what the user writes. The values are always separated by a space.

I have tried using std::stof(as in https://www.geeksforgeeks.org/stdstof-in-cpp/ ) , stringstream (as stated in https://www.geeksforgeeks.org/converting-strings-numbers-cc/ ) but none of them works.

Trials:

cout << "Introduce the transition matrix \n";
getline (cin, trans_matrix);
std::vector<float> arr(trans_matrix.size(), 0);
int j = 0, i;
// Traverse the string
for (i = 0; trans_matrix[i] != '\0'; i++) {
    // if str[i] is ' ' then split
    if (trans_matrix[i] == ' ') {
        j++;
    }
    else {
        arr[j] = std::stof(trans_matrix[i]) // string to float
    }
}

But the compiler says:

No matching function for call to 'stof'

Answer 1

Your code is quite mixed up. Half your code treats a string as a sequence of characters (which is correct) but the other half treats it as a sequence of floats which is not really true. For instance

std::vector<float> arr(trans_matrix.size(), 0);

this creates a vector the same size as the string. But the string size is the number of characters which is not the same as the number of floats in the string. Also

arr[j] = std::stof(trans_matrix[i]);

trans_matrix[i] is a character, it's not a string, so you can't use a function on it which converts a string to a float.

I'm trying to make it clear that you can't program by writing code that's approximately right. You have to think carefully about what you are doing and write code that is exactly right. You have to be completely clear and precise about the concepts.

How would you do this if you were reading from std::cout ? Well it's exactly the same way if you are reading from a string except you use a std::istringstream instead of std::cout . Here's one straightforward way.

#include <sstream>

std::vector<float> arr;
std::istringstream input(trans_matrix);
float f;
while (input >> f)
    arr.pusk_back(f);

Simple, create a string stream, read the floats one at a time, add them to the vector.