How to skip #ifdef part and run the #else part?

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I am using conditional compilation using #define.
I am iterating over a loop. Inside the loop, using #ifdef ... #else ... #endif
I want to skip the #ifdef part on the basis of some condition and want to run #else part.

What I have tried:

Example:

C++

#include <iostream>
using namespace std;

#define NOINPUT
int main ()
{
  for(int i =1 ; i <=10;i++)
  {
      int num = 0;
#ifdef NOINPUT
      //if(i > 5)
      //want to execute #else part for remaining iteration
      num = i;
#else
      cout<<"Enter a number:";
      cin>>num;
#endif
     cout<<"Number: "<<num<<"\n";
 }
}

Posted 28-Jan-22 5:23am

Member 14036158

Updated 28-Jan-22 8:50am

Rick York

v3

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Richard MacCutchan 28-Jan-22 11:38am

Remove the line that says

#define NOINPUT

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Greg Utas · Accepted Answer · 2022-01-28T05:46:00

I'm not sure I understand what you want, but here's a guess. If NOINPUT is defined, the following will execute the if statement, with num taking on the values 0, 0, 0, 0, 0, 6, 7, 8, 9, and 10 through the loop. If NOINPUT isn't defined, the user will be asked to input the number.

C++

for(int i = 1 ; i <= 10; i++)
{
   int num = 0;
#ifdef NOINPUT
   if(i > 5)
   {
      //want to execute #else part for remaining iteration
      num = i;
   }
#else
   {
      cout << "Enter a number:";
      cin >> num;
   }
#endif

k5054 · Accepted Answer · 2022-01-28T06:32:00

You should know that #define, #ifdef, #else etc are preprocessor directives. As such they modify the source code before it is passed into the compiler. Given this:

C++

#define NOINPUT
#ifdef NOINPUT
   cout << "foo\n";
#else
   cout << "bar\n";
#endif

by the time the compiler sees it, the source code has been modified to

C++

cout << "foo\n"

The section in the #else part has effectively been removed from the source.

At best guess, it looks like you are trying to execute different parts of your program depending on whether the user has entered a number or not. If that is so, then you can not do this with preprocessor tokens.

Patrice T · Accepted Answer · 2022-01-28T08:50:00

Solution 3

Quote:
I want to skip the #ifdef part on the basis of some condition and want to run #else part.

The slightest change is to replace

C++

#define NOINPUT

with

C++

// #define NOINPUT

Posted 28-Jan-22 8:50am

Patrice T

How to skip #ifdef part and run the #else part?

3 solutions

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