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Haywood J. Ablome
Haywood J. Ablome

Reputation: 1

Assign a Variable in If statement [C++]

I am trying to assign a variable if one variable equals another value. Here is a simplified version of my problem:

#include <iostream>
using namespace std;

int main()
{
   int a = 5;
   int b;

   cout << "A is this value (before if):  " << a << endl;
   cout << "B is this value (before if):  " << b << endl;


   if (a==5)
   {
       cout << "A equals five" << endl;
       int b = 6;
       cout << "B is this value (if stmt):  " << b << endl;
   }

cout << "B is this value (outside):  " << b << endl;

   return 0;

}

It outputs the following:

A is this value (before if):  5
B is this value (before if):  0
A equals five
B is this value (if stmt):  6
B is this value (outside):  0

Why does variable "b" not stay assigned as 6 once it leaves the if statement? And is there a better way to assign it? In my actual code I have five variables I compare to a.

Upvotes: 0

Views: 355

Answers (4)

ΦXocę 웃 Пepe&#250;pa ツ
ΦXocę 웃 Пepe&#250;pa ツ

Reputation: 48258

int b = 6 inside of the if block is bringing a scope problem, the new variable is shadowing the variable you declared in main function

you need to do:

  if (a==5)
   {
       cout << "A equals five" << endl;
       b = 6;
       cout << "B is this value (if stmt):  " << b << endl;
   }

Upvotes: 0

victor
victor

Reputation: 812

int main()
{
   int a = 5;
   int b; // this instance of b has scope over the entire main() function

   cout << "A is this value (before if):  " << a << endl;
   cout << "B is this value (before if):  " << b << endl;


   if (a==5)
   {
       cout << "A equals five" << endl;
       int b = 6; // this instance of b only exists inside this if statement. As soon as the program leaves this if, this instance of b stops existing
       cout << "B is this value (if stmt):  " << b << endl;
   }

cout << "B is this value (outside):  " << b << endl;

   return 0;

}

Upvotes: 0

Hatted Rooster
Hatted Rooster

Reputation: 36463

Because int b = 6; introduces a new variable b that's initialized to 6. It doesn't set 6 to the b of the outer scope. To do that, you should remove the type specifier:

b = 6;

Right now because b is never initialized you have undefined behaviour.

Upvotes: 1

Barmar
Barmar

Reputation: 780724

You declared a new variable inside the if block. Replace the variable declaration with an assignment.

Also, you should initialize the original b variable. Printing its value without initializing it results in undefined behavior.

#include <iostream>
using namespace std;

int main()
{
   int a = 5;
   int b = 0;

   cout << "A is this value (before if):  " << a << endl;
   cout << "B is this value (before if):  " << b << endl;

   if (a==5)
   {
       cout << "A equals five" << endl;
       b = 6;
       cout << "B is this value (if stmt):  " << b << endl;
   }

   cout << "B is this value (outside):  " << b << endl;
   return 0;
}

Upvotes: 2

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