I'm learning C++.
nav is an integer .
I want to ask user for typing a valid value, if he / she type an invalid value.
void main()
{
printf("Type an integer : ");
if(!scanf("%d", &nav))
{
system("cls");
printf("Invalid ! \n");
main();
}
}
But it's blinking after typing first value . It's blinking like reloading screen. I think it's infinite loop.
How can i do it in right way ? I want to ask a number from users, until it's typing a real number .
If the user types an invalid input, scanf() won't consume it, and you'll be left peeking the same offending input character forever. You need to first read whatever the user enters — I recommend using std::getline() — and then try to parse that with strtol(), sscanf() or std::istringstream. Don't use atoi() because it doesn't report failures.
int nav;
{
string line;
while (getline(cin, line))
if (istringstream(line) >> nav)
break;
}
EDIT: See the comments for a rather beautiful rendition of the above logic. I've left it out of the answer because: a) I don't want to steal someone else's idea, and b) I'm not sure I'd present a newcomer to C++ with that formulation — not in one go, at least.
P.S.: You can't call main() in C++.
for (std::string line; std::getline(std::cin, line) && !(std::istringstream(line) >> nav); ) { }.
main() is illegal, but the OP's symptoms suggest that the recursive call was, in fact, succeeding.Two things.
Scanf needs the 'enter' key pressed before it will process input, so the blinking could just be the cursor waiting for the next key.
Also, calling main from main is fairly non-standard. You should look into a 'while' loop.
Your program is not a good one, but nevertheless I will tell you what is happening.
In C/C++, when scanf cannot read an integer (%d) from input, it wouldn't read anything. That is, whatever prevented scanf from reading an int, will remain there. On the next scanf the same character is going to cause an error.
Let me demonstrate by an example. Imagine you are reading many integers from this input:
12 13 Shahbaz 15
Now if you call scanf with %d, you will read 12 and the input would be:
13 Shahbaz 15
Next, you call scanf with %d and you will read 13. now the input would be:
Shahbaz 15
Again, you call scanf with %d. Here, the input begins with an S (after the whitespace) which makes scanf return with a failure as it could not read an integer. The input is left untouched (save perhaps for the whitespace). That is, the input will be:
Shahbaz 15
As you can see, reading the input with %d will give you the exact same error and you are stuck in an infinite loop.
To solve this, you have many choices. This very much depends on how you want to handle the situation, but two methods would be to either read a character (with %c) or a string (with %s) right after printing printf("Invalid\n").
The first method is good for handling input like this:
12 13 q14 15
where q is a mistake that needs to be ignored. The second method is good for handling input like this:
12 13 Shahbaz 15
where the invalid data are meaningful words, but you just want to ignore them.
And the way I would write it, if I wanted to use scanf would be:
int main() // always write int main
{
int nav;
printf("Type an integer: ");
while (scanf("%d", &nav) != 1) // scanf returns number of successful %'s read
{
printf("Invalid number. Try again: ");
scanf("%*s"); // read a %s but ignore it
}
// The rest of the program, using nav
return 0;
}
void main()is illegal in C++. 2) callingmain()is illegal in C++.