Reading data from file into string

Question

I am trying to read a file and put data into string. However the compiler is outputting this.

012 345 678 ����012345678

with the new lines. Can you explain what is happening?

#include <iostream> using namespace std; #include <fstream> #include <cstring> int main() { ofstream output("transform.out"); ifstream input("transform.in"); int num = 0; input >> num; char tmp[num+1]; char data[num * num +1]; while(input >> tmp){ cout << tmp << '\n'; strcat(data, tmp); } cout << data; }

transform.in has this data

3 012 345 678

char tmp[num+1]; is not valid C++. Use a vector or better yet std::string. — user2100815
– user2100815, Commented Oct 20, 2018 at 22:43
Does the code compile? When you say "the compiler is outputting" do you mean "the program is outputting"? What do you mean "with the newlines". Can you provide a full snippet of the output, that would help us understand the situation better. — ChilliDoughnuts
– ChilliDoughnuts, Commented Oct 20, 2018 at 22:50
The exercise requires use of c strings. Can you explain why that is not valid? — Adrian5404
– Adrian5404, Commented Oct 20, 2018 at 22:51
This char tmp[num+1] and this char data[num * num +1] are invalid syntaxes, the array size defined inside the square brackets must be a constant value. — Ahmed Aboumalek
– Ahmed Aboumalek, Commented Oct 20, 2018 at 22:54

Stephan Lechner · Accepted Answer · 2018-10-20 22:51:48Z

Note that standard C++ does not support variable length arrays like char data[num * num +1] (num is not a constexpr). Your code compiles because you probably use a compiler with an extension supporting VLAs. For portable code, however, you'd need to use some dynamic data structures, e.g. a vector.

Anyway, you do not initialize data, such that your very first strcat might append a (valid) content of tmp to data that starts with garbage. Your output �� is not a result of a newline but just that garbage to which the very first strcat appends your file contents.

char data[num * num +1] = { 0 }; should solve this problem.

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