C++ Error C2451 conditional String is illegal - c++

Question

i'm fairly new to C++.
I want to write a Program/Function that checks a string input (from console or other source, not important here) if is already in an array. If it's not then it should be written into array. Otherwise do nothing.
My Problem is the for loop and if the if condition. What am I missing?
#include <iostream>
#include <string>
#include <fstream>
#include <stdio.h>
using namespace std;
typedef struct {
string id[10];
string foo1[10];
string type[10];
string func[10];
}Device;
int main() {
Device fooDevice;
string mystring;
int i = 0;
mystring = "foo";
ofstream temp;
temp.open("temp.txt", ios::out | ios::app);
for (fooDevice.id[i]; fooDevice.id[9]; i++) {
if (fooDevice.id[i] != mystring) {
fooDevice.id[i] = mystring;
temp << mystring << endl;
} else {
//do nothing
}
}
return 0;
}

Answer 1

The problem is the structure of your for loop. I'm not sure what you think your condition means, but here's how it should look:
for (std::size_t i = 0; i < 10; ++i) {
This increments the index value i from 0 to 9 (inclusive). You can then check the value of fooDevice[i].
At the moment, it seems you are trying to overwrite every element of the array with the new string. I'm not sure how you know how full the array is at any given time. Let's suppose you stop when you get to the first empty string:
for (std::size_t i = 0; i < 10; ++i) {
if (myString == fooDevice.id[i]) {
// already there, stop looping
break;
}
else if (fooDevice.id[i].empty()) {
// none of the currently set elements matches
fooDevice.id[i] = myString;
temp << myString << '\n';
break;
}
}
You could also replace this with a range-based-for:
for (auto& deviceId: fooDevice.id) {
if (myString == deviceId) {
// already there, stop looping
break;
}
else if (deviceId.empty()) {
// none of the currently set elements matches
deviceId = myString;
temp << myString << '\n';
break;
}
}
Better yet, use a std::vector with std::find from the <algorithm> header. (Warning: untested code):
struct Device {
// ...
std::vector<std::string> id;
// ...
};
// ...
auto foundId = std::find(fooDevice.id.begin(), fooDevice.id.end(), myString);
if (fooDevice.id.end() == foundId) {
// not already there
fooDevice.id.push_back(myString);
temp << myString << '\n';
}
It seems you are a bit confused about the difference between C and C++ as well:
The C++ version of <stdio.h> is <cstdio>, but you don't need it here at all (and you usually don't in a C++ program).
You don't need to typedef the name of a struct in C++, just do struct Device { ... };
And regarding C++ style, please reconsider your use of what are often considered bad practices: using namespace std; and endl.

Answer 2

for (fooDevice.id[i]; fooDevice.id[9]; i++) seems no scence. I think you want:
for(; i<9; ++i)

Answer 3

You can't use a string value as a loop condition. Only expressions that evaluate to booleans.
Also, you are also not initializing the device arrays before searching them.
Try something more like this:
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
struct Device {
string id[10];
string foo1[10];
string type[10];
string func[10];
};
int main() {
Device fooDevice;
string mystring;
// fill fooDevice as needed...
mystring = "foo";
ofstream temp;
temp.open("temp.txt", ios::out | ios::app);
bool found = false;
int idx_available = -1;
for (int i = 0; i < 10; ++i) {
if (fooDevice.id[i] == mystring) {
//do nothing
found = true;
break;
}
if ((idx_available == -1) && fooDevice.id[i].empty())
idx_available = i;
}
if ((!found) && (idx_available != -1)) {
fooDevice.id[idx_available] = mystring;
temp << mystring << endl;
}
return 0;
}
Which would be better handled with some rewriting:
#include <iostream>
#include <string>
#include <fstream>
#include <vector>
#include <algorithm>
struct Device {
std::string id;
std::string foo1;
std::string type;
std::string func;
};
struct isDeviceId {
const std::string &m_id;
isDeviceId(const std::string &id) : m_id(id) {
}
bool operator()(const Device &device) {
return (device.id == m_id);
}
};
int main() {
std::vector<Device> devices;
std::string mystring;
// fill devices as needed...
mystring = "foo";
if (std::find_if(devices.begin(), devices.end(), isDeviceId(mystring)) == devices.end()) {
Device device;
device.id = mystring;
devices.push_back(device);
std::ofstream temp;
temp.open("temp.txt", std::ios::out | std::ios::app);
temp << mystring << std::endl;
}
return 0;
}
Alternatively, in C++11 and later:
#include <iostream>
#include <string>
#include <fstream>
#include <vector>
#include <algorithm>
struct Device {
std::string id;
std::string foo1;
std::string type;
std::string func;
};
int main() {
std::vector<Device> devices;
std::string mystring;
// fill devices as needed...
mystring = "foo";
if (std::find_if(devices.begin(), devices.end(),
[mystring&](const Device &device) { return (device.id == mystring); }
) == devices.end()) {
devices.emplace_back();
devices.back().id = mystring;
// or, in C++17:
// devices.emplace_back().id = mystring;
std::ofstream temp;
temp.open("temp.txt", std::ios::out | std::ios::app);
temp << mystring << std::endl;
}
return 0;
}