Read complex numbers (a+bi) from text file in C++

Question

i want to read an array of complex numbers (in a+bi form). There are several suggestions I found on the internet, however those methods only result the real part, while the imaginary part is always 0. And infact the real part of the next number is the imaginary of the previous number.

For example, I have an text file as follow:

2+4i
1+5i
7+6i

And here is a suggestion for reading complex data

int nRow = 3;
std::complex<int> c;
std::ifstream fin("test.txt");
std::string line;
std::vector<std::complex<int> > vec;
vec.reserve(nRow);
while (std::getline(fin, line))
{
    std::stringstream stream(line);
    while (stream >> c)
    {
        vec.push_back(c);
    }
}

for (int i = 0; i < nRow; i++){
    cout << vec[i].real() << "\t" << vec[i].imag() << endl;
}

while (1);
return 0;

And the output result is:

2   0
4   0
1   0

Is there a proper way to read a+bi complex numbers from text file? Or do I have to read the data as a string, and then process the string to extract and convert it back to complex numer?

Thanks !

Note that if you have control over the text format, `` already provides an `operator>>` and `operator<<` to read/write in the format `(re,im)`. — Jerry Coffin, Jan 05 '16 at 22:48

vsoftco · Accepted Answer · 2016-01-05T22:48:36.737

4

One option is to read separately the real and imaginary part in 2 ints, and the sign into a char, then emplace_back into the complex vector, like

int re, im;
char sign;
while (stream >> re >> sign >> im)
{
    vec.emplace_back(re, (sign == '-') ? -im : im);
}

Here sign is a char variable that "eats" up the sign.

edited Jan 05 '16 at 22:48

answered Jan 05 '16 at 22:34

vsoftco

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what if the `junk` is minus? – Ziezi Jan 05 '16 at 22:35
@simplicisveritatis Then you need to process it ;) This was an over simplified example. Will edit the answer though, thanks. – vsoftco Jan 05 '16 at 22:35
that's a whole new line including an `if` statement and a comparison :) – Ziezi Jan 05 '16 at 22:36
1

@simplicisveritatis Wrote it into one line so I deserve the upvote ;) – vsoftco Jan 05 '16 at 22:44
I think I'd reverse the logic, so it treats *only* `-` as meaning negation, and anything as meaning `+` (e.g., so `1, 2i` would be read correctly). – Jerry Coffin Jan 05 '16 at 22:47
@JerryCoffin You're right, edited. That's how it was originally (with the `if` statement before edit). – vsoftco Jan 05 '16 at 22:48

score 2 · Answer 2 · answered Jan 05 '16 at 22:31

FILE *fp;
char line[80];
int a, b;

if ((fp = fopen("filename", "r") != NULL)
    while (fgets(line, 80, fp) != NULL) {
        if (sscanf(line, "%d + %di", &a, &b) == 2)
            /* do something with a and b */
        else
            fprintf(stderr, "Not in a+bi form");
    }

Peter · Answer 3 · 2016-01-05T23:49:59.663

2

Your requirement (the format of input you seek) does not correspond to the method you are using to read it. The default streaming of a complex is the real and the imaginary part separated by spaces and in brackets - the sign between is not mandatory, and the trailing i is not required.

The obvious way to parse for input you seek is to read the real part, read a character, check if the character is a signed (+ or -) and ignore spaces, then read the imaginary part. That is trivial with istreams.

edited Jan 05 '16 at 23:49

answered Jan 05 '16 at 23:31

Peter

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thank @Peter. You're right, the format was not suitable for the method I used. I'll look at {istream} function. By the way, I think the above answer from vsoftco may use a similar method like your suggestion. – BL_ Jan 07 '16 at 12:59

score 1 · Answer 4 · answered Jan 05 '16 at 23:30

int real[10] = { 0 };
int imaginary[10] = { 0 };
FILE *lpFile = fopen("filename" "rt"); // Open the file
for (int i = 0; i < 10; i++) {
    fscanf(lpFile, "%d+%di", &real[i], &imaginary[i]); // Read data 10 times
}
fclose(lpFile); // Close the file

This example code will read 10 complex numbers from a file.

Read complex numbers (a+bi) from text file in C++

4 Answers4

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