CODEWITHSUNDEEP
c
Applik
Applik

Reputation: 431

How to make scanf() work inside while test condition?

If user enters a character instead of a number, I want to give him another option to try once more but below code prints out "Invalid. Pls enter a number." forever if user enters a character instead of a number. Why doesn't it wait for the user to enter again? (scanf part is for that I assume) 

#include <stdio.h>
long get_long(void);

int main(void) {
long start;

printf("Enter a number: ");
start = get_long();

return 0;
}

long get_long(void)
{
long num = 0;   
while (scanf("%ld", &num) != 1)
{       
    printf("Invalid. Pls enter a number.");
}

return num;
}

Upvotes: 0

Views: 347

Answers (3)

jasinth premkumar
jasinth premkumar

Reputation: 1413

like this. it executes while 2 times

    int i=1;
    while (i!= 2)
    {      scanf("%ld", &num) ; 
         if(num==1)
           break;
        printf("Invalid. Pls enter a number.");
    i++;
    }

Upvotes: -1

Steve Summit
Steve Summit

Reputation: 47925

This is a common problem with scanf. (The question has been asked many times; there are probably duplicate answers.) The problem is that when you type simething that isn't a valid number, scanf fails (and returns 0), but it leaves the unmatched input on the input stream.

You have to flush the unread input somehow. One way is like this. Write the function

void flush_one_line()
{
    int c;
    while((c = getchar()) != EOF && c != '\n')
        { /* ignore */ }
}

This function reads and discards one line of input, up to a newline. (That is, it throws away anything and everything that the previous scanf call didn't read.)

Then modify your original program like this:

while (scanf("%ld", &num) != 1)
    {
    printf("Invalid. Please enter a number.");
    flush_one_line();
    }

Upvotes: 0

Nothing Nothing
Nothing Nothing

Reputation: 122

Your logic behind the code was flawed. What you essentially did with your code is ask the while loop to work as long as correct input isn’t made. 

while(incorrect input is made) {
    print
}

Proper solution would be along the lines of

while(number of tries > 0) {
    do operations
    check for right input, if it is correct break the loop, else keep on going
    decrement number of tries by one
}

Upvotes: -1

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