How to change variable mentioned in input string?

Question

I have a student's project for school, I'm mostly programming in PHP, so C++ is not a my strong side :) Please let me know, is it possible to change variable which I mention in cin string? Maybe my code below will be more understandable:

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

string A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, R, Q, S, T, U, V, W, X, Y, Z;
string instrukcja;
string krok[3];
int i = 0;

string MAKENULL()
{
    return " ";
}

void WRITE(string x)
{
    cout << x;
}

int main()
{   
    // get instruction Begin
    char s[10];
    scanf("%10[0-9a-zA-Z ]", s);
    istringstream iss(s);
    while(iss)
    {
        string sub;
        iss >> sub;
        krok[i] = sub;
        i++;
    }
    // get instruction End


    if (krok[0] == "MAKENULL")
    {
        "" Variable in krok[1] "" = MAKENULL();
    }

    if (krok[0] == "WRITE")
    {
        WRITE("" Variable in krok[1] "");
    }

    if (krok[0] == "OPPOSITE")
    {
        OPPOSITE("" Variable in krok[1] "", "" Variable in krok[2] "")
    }

}

In code above I will do something like ADD A B, and A should became A => A+B (one string stick to the other). Main question is, can I point a variable to change in scanf?

Its not really clear what you want to do. You want to concatenate two strings? and what is OPPOSITE() ? — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Apr 11, 2016 at 9:35
Maybe I will try to explatin whole project. There will be registers (A-Z), and this program will only get one input (in loop) with instruction (like ADD A, Z; OPPOSITE A, W; WRITE A etc...) and that I need now is can I point in scanf/cin a specific variable to change? — Karol Gasienica
– Karol Gasienica, Commented Apr 11, 2016 at 9:41
if I understand you correctly, you have to provide some mapping from the string representations ("A","B"...) to the variables holding the values (ie. the string A,B,...). You could use a std::map<std::string,std::string> where the key is the string that you get from scanf and the value (i.e. the second string in the map) is the value that you want to modify — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Apr 11, 2016 at 9:43
Example: if I write CREATE A qwerty, A should become qwerty (A = "qwerty"), if I write then OPPOSITE A, A should become ytrewq (A = "ytrewq") — Karol Gasienica
– Karol Gasienica, Commented Apr 11, 2016 at 9:57

marcinj · Accepted Answer · 2016-04-11 10:02:03Z

2

If I understood correctly then instead of keeping variables as now A,B,C,... you need a map container:

#include <map>
// ...
std::map<std::string, std::string> varMap;

where key is the variable name: A,B,C and value is a its value.

Then you can use your current way of reading, when you assign value to variable A, then you use:

varMap["A"] = newValue;

and when you read it you also use std::string value = varMap["A"], but be carefull here, if "A" does not exist in map it will be created and value initialized (default constructor is called - so that it will be empty string). To check if key exsists use if (varMap.find("A") == varMap.end()) {/*no such variable*/}

Another thing is that you istringstream usage is wrong you should check if something was read before using it, the proper idiom is:

istringstream iss(s);
string sub;
while(iss >> sub)
{
    krok[i] = sub;
    i++;
}

[edit]

What you probably had in mind was a possibility to use php like variable variables concept, or reflection from java and other languages. There is no such thing in c++, and the recomended way - as also in php is to use associative container like map.

edited Apr 11, 2016 at 10:02

answered Apr 11, 2016 at 9:56

marcinj

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3 Comments

Karol Gasienica Over a year ago

It might work... I will try and give feedback :) Thank you!

Karol Gasienica Over a year ago

If I understand correctly, it is some kind of array, but instead of numbers, there are chars, right?

marcinj Over a year ago

Associative container - but you might think of this as an array with std::string as indexes.

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