Printf() function acting strange

Question

Apologies if this question is stupid, I'm new to coding, especially in C++. I'm trying to read in data points from a space-separated text and then use the printf function to get it output them to the console. When I try this;

#include<iostream>
#include<fstream>
#include<string>
using namespace std;
int main() {
    double x[1000];
    int i = 0;
    int j;
    double y[1000];
    double sigmay[1000];

    ifstream dataFile("xys_test.txt");

    if (dataFile.is_open()) {
        while (!dataFile.eof())
        dataFile >> x[i] >> y[i] >> sigmay[i];
        printf("x = %5.2f, y = %5.2f, sigmay = %5.2f\n", x[i], y[i], sigmay[i]);
    i++;
}
}

All the console output gives me is the last data point, when I was hoping that all the points would be output into the console. How can I resolve this?

Why are you not using `cout` instead of `printf` in C++ code? — Ed Heal, Nov 22 '15 at 09:42
Also missing braces. Add then after the line that starts `while` — Ed Heal, Nov 22 '15 at 09:43
lesson #1 in C++ file i/o: [never use `eof` in a loop condition](http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong) — M.M, Nov 22 '15 at 09:43
Make it a habit to use braces whenever possible. It prevents one from getting caught with your trousers down :-) — Ed Heal, Nov 22 '15 at 09:47
If I replace `printf` directly with ` cout << ` I only get the last value of the sigmay array — R L W, Nov 22 '15 at 09:48
.. And replace the commas with << - Please read the text book on iostreams — Ed Heal, Nov 22 '15 at 09:49

score 2 · Accepted Answer · answered Nov 22 '15 at 09:43

You are missing a pair of { ... } for the contents of your while block. Currently, only the dataFile >> x[i] >> y[i] >> sigmay[i]; line is inside the while block.

if (dataFile.is_open()) {
    while (!dataFile.eof()) {
        dataFile >> x[i] >> y[i] >> sigmay[i];
        printf("x = %5.2f, y = %5.2f, sigmay = %5.2f\n", x[i], y[i], sigmay[i]);
        i++;
    }
}

score 2 · Answer 2 · answered Nov 22 '15 at 09:44

2

you lost your curly bracket,

while (!dataFile.eof()) {
  dataFile >> x[i] >> y[i] >> sigmay[i];
  printf("x = %5.2f, y = %5.2f, sigmay = %5.2f\n", x[i], y[i], sigmay[i]);
  i++;
}

answered Nov 22 '15 at 09:44

ibrohimislam

717
7
21

Upvoted and removed my answer as this was more clear than mine. – Daniel Nov 22 '15 at 09:48

score 2 · Answer 3 · edited May 23 '17 at 11:33

The code should be:

while ( i < 1000 && (dataFile >> x[i] >> y[i] >> sigmay[i]) )
{
    printf("x = %5.2f, y = %5.2f, sigmay = %5.2f\n", x[i], y[i], sigmay[i]);
    ++i;
}

You should test whether or not the read operation succeeded, in order to decide whether to proceed with printing and committing the data. Testing eof is irrelevant and a mistake.

Also #include <cstdio> should be used for printf. Possibly your compiler/library setup involves cstdio being included by iostream but in general that is not the case.

Printf() function acting strange

3 Answers3