Basic C programming_while_runtime error

Question

As a beginner I learn programming currently with Sublime Text on OSX. The code attached is compiled via Sublime Text. The runtime error is, when I input any non-integer data such as 'a' or '9.3', the while-loop will not break.

Below is the instruction of work:
Test a program with only a main() function that repeatedly:
1. requests the user to enter an integer,
2. reads an integer,
3. processes the integer as follows: returns to terminate the program for 0, prints "one" for 1, prints "two" for 2, prints "I don't know that" for any other integer.

e.g. the following is a possible sequence that might appear at the terminal, where the integers are entered by the user:
Please enter an integer
2
two
Please enter an integer
33
I don't know that
Please enter an integer
1
one Please enter an integer
0

Below is my code.

Thanks for help.

#include <stdio.h>
int main(){

    int n;
    int a = 1;

    while (a!=0){
        printf("Please enter an integer\n");
        scanf("%d", &n);
        if (n == 1) printf("one\n");
        else if (n==2) printf("two\n");
        else if (n==0) return 0;
        else if (n<0 || n >2) printf("I don't know that\n");
    }

}

Hello World does work. I referred to this link the other day.http://stackoverflow.com/questions/10059511/how-to-compile-and-run-a-simple-c-program-with-sublime-text-2 — green_claws, Mar 02 '14 at 05:18

score 1 · Answer 1 · answered Mar 02 '14 at 22:08

Your loop never terminates because you never tell it to terminate. It continues to execute as long as a != 0. You initialize a to 1 and you never change it, so naturally you have an infinite loop.

You're also not checking whether scanf succeeded. If you enter some garbage value, it will leave n with its previous value and return a result indicating that it failed to read a new value, but your program doesn't distinguish between success and failure.

scanf returns an int result indicating how many items it was able to read (or EOF, a negative value, if there was an error). You should at least compare the value to 1 to see whether it succeeded.

It appears that you intend to use a to determine whether the loop should continue executing. You should give it a more meaningful name, like keep_going. And you can make it a bool (if you have #include <stdbool.h>, if your compiler supports it), but using an int with 0 for false and 1 for true is also ok.

int keep_going = 1;
while (keep_going) {
    /* ... */
    if (...) {
        keep_going = 0;
    }
}

Or you can write an infinite loop and use break to terminate it:

while (1) {
    /* ... */
    if (...) {
        break;
    }
}

Thank you. In C language, how to make scanf tell the type of data input, integer, float or char? — green_claws, Mar 04 '14 at 21:07
You need to read the documentation for `scanf`, either by running `man scanf` if your system has man pages) or by [finding it on the web](http://linux.die.net/man/3/scanf). `scanf` format strings are similar (but not identical) to `printf` format strings; they're effectively small programs in a tiny special-purpose language. — Keith Thompson, Mar 04 '14 at 21:25

score 0 · Answer 2 · edited Mar 02 '14 at 21:57

0

Instead of using if else, I would suggest using a switch statement

If you need to use only if else, then include another else statement mentioning.

else
    exit(0);

for your program to terminate in such a case.

edited Mar 02 '14 at 21:57

Keith Thompson

254,901
44
429
631

answered Mar 02 '14 at 04:53

user3256147

378
2
9

That is one solution. – green_claws Mar 02 '14 at 05:33
Please don't use silly abbreviations like "u" and "ur". Spelling out the words "you" and "your" makes your answer much easier to read. – Keith Thompson Mar 02 '14 at 21:58

user3503045 · Accepted Answer · 2014-08-11T01:15:31.777

0

#include <stdio.h>

int main(int argc, char **argv)
{
    int looper = 1;
    while(looper){
        printf("Please enter an integer:\n");
        int i;
        scanf("%d", &i);
        if(i==0){
            looper = 0;
        }
        else if(i==1){
            printf("one\n");

        }
        else if(i==2){
            printf("two\n");
        }
        else{
            printf("I don't know that\n");
        }
    }

    return 0; 
}

edited Aug 11 '14 at 01:15

answered Aug 07 '14 at 07:48

user3503045

20
5

`terminator` is a poor name for a variable that tells the loop *not* to terminate. `terminator = 0;` would be much clearer than `terminator -= 1;`. – Keith Thompson Aug 07 '14 at 14:55
Fair enough! Thumbs up and I've modified this piece. – user3503045 Aug 11 '14 at 01:14

Basic C programming_while_runtime error

Below is my code.

Thanks for help.

3 Answers3