Why the "cout" is invoked twice?

Xiangbuilder · 09-21-2003, 09:46 PM

I think the "cout" will be invoked once when the loop is invoked once. However the "cout" is invoked twice when the loop is invoked once.

Code:

#include<iostream>
using namespace std;
//a macro to define dummy functions:
#define DF(N) void N() {cout<<"function"#N"called..."<<endl;}
DF(a); DF(b); DF(c);
void (*func_table[])()={a, b, c}; //define and initialize an array of pointers to functions
int main ()
{
   while(1) {
      cout<<"press a key from 'a' to 'b' or 'q' to quit"<<endl;
      char c;
      cin.get(c);
      if (c=='q')
         break; //...out of while (1)
      if (c<'a' || c>'g')
         continue;
      (*func_table[c-'a'])(); //dereferencing calls the function
   }
}

Here is the result:
[root@localhost abide]# g++ fun_poi_c.cpp -o fun_poi_c
[root@localhost abide]# ./fun_poi_c
press a key from 'a' to 'b' or 'q' to quit
a
functionacalled...
press a key from 'a' to 'b' or 'q' to quit
press a key from 'a' to 'b' or 'q' to quit
b
functionbcalled...
press a key from 'a' to 'b' or 'q' to quit
press a key from 'a' to 'b' or 'q' to quit
q
[root@localhost abide]#

What is the wrong? Thank you.

megaspaz · 09-22-2003, 12:26 AM

well i just played with it, and my guess is because cin.get () leaves garbage in the buffer. placing a cin.ignore (); after the cin.get (c); line makes it work as you expect it to work.

Code:

#include<iostream>
using namespace std;
//a macro to define dummy functions:
#define DF(N) void N() {cout<<"function"#N"called..."<<endl;}
DF(a); DF(b); DF(c);
void (*func_table[])()={a, b, c}; //define and 
                                 //initialize an array 
                                 //of pointers to functions
int main ()
{
   while(1) {
      cout<<"press a key from 'a' to 'b' or 'q' to quit"<<endl;
      char c;
      cin.get(c);
      cin.ignore ();
      if (c=='q')
         break; //...out of while (1)
      if (c<'a' || c>'g')
         continue;
      (*func_table[c-'a'])(); //dereferencing calls the function
   }
}

output:

Code:

[vjong@Tron temp]$ ./test.exe
press a key from 'a' to 'b' or 'q' to quit
a
functionacalled...
press a key from 'a' to 'b' or 'q' to quit
b
functionbcalled...
press a key from 'a' to 'b' or 'q' to quit
q
[vjong@Tron temp]$

Xiangbuilder · 09-22-2003, 01:08 AM

Thank you. It works well.
I don't know there is command, cin.ignore(), thank you tell me that. This below works well also:

Code:

#include<iostream>
using namespace std;
//a macro to define dummy functions:
#define DF(N) void N() {cout<<"function"#N"called..."<<endl;}
DF(a); DF(b); DF(c);
void (*func_table[])()={a, b, c}; //define and initialize an array of pointers to functions
int main ()
{
   while(1) {
      cout<<"press a key from 'a' to 'b' or 'q' to quit"<<endl;
      char c, cr;
      cin.get(c); 
      cin.get(cr); //second one for CR
      if (c=='q')
         break; //...out of while (1)
      if (c<'a' || c>'g')
         continue;
      (*func_table[c-'a'])(); //dereferencing calls the function
   }
}

The code is from <<thinking in c++>>,

megaspaz · 09-22-2003, 02:28 AM

well the only real time i've ever seen a need for cin.ignore was if you were mixing cin with cin.getline where cin.ignore () would be the line right after the last cin before any getline line.

ie:
char c;
char str[5];

cin >> c;
cin.ignore ();
cin.getline (str,4);