Thread: Give Error Message If User Inputs Wrong

Code:

#include<stdio.h>
void main 
{
int i;
int marks[10];


for( i=0; i<10; i++)
{
printf("\n %d",i+1);
scanf("%d",&marks[1]
}

}

how to give error and ask user to re enter if user enter other then a number?

You need to check the return code of scanf. It returns the number of items successfully read, or EOF if there was a problem. You can check the return and then use a loop, but if you do so you'll probably run into trouble due to the scanf operates. Basically the easiest way I find is the following

1. read a line
2. read the input from that line
3. if there is a problem, print an error and go to 1.

Here is an example

Code:

#define SSIZE (10000)
char s[SSIZE];

int main()
{
    int mark;
    while ((fgets(s, SSIZE, stdin)) != NULL) {
        if ((sscanf(s, "%d", &mark) != 1)) {
            printf("That is not a number. Try again.\n");
            continue;
        }
        printf("Got a number: %d\n", mark);
        break;
    }
    
    return 0;
}

You need to check the returned value of scanf, it normally returns the amount of successful inputs.

Looks like somebody needs a diaper change!

Start reading some FAQ FAQ > Why fflush(stdin) is wrong - Cprogramming.com

It looks like you have several problems, mostly having to do with a lack of understanding of the language.

First, you should be trying to compile and execute the code you write before posting it here. With respect to post #4, it's clear this was not done. You're missing a semi-colon after declaring "quit" and you wrote "scant" instead of "scanf". These are basic errors that, while permissible to make in the course of writing code, should easily be detected and corrected before posting.

Secondly, you're trying to cram too much functionality into your code. I can virtually guarantee that your "if()" statement isn't doing what you think it is doing.

Based on these observations, I feel it's a safe assumption that you're banging out code with little thought and wondering why it isn't working. You'll never get a decent program written that way.

Writing a program should not begin with code. It should begin with a clarification of requirements and planning on how to achieve those requirements.

Step 1: Write down, in words, what your program needs to do. Based on what you've posted, I would say it's something along the lines of: "Allow the user to enter up to ten numbers, and store them in an array. If the user does not enter a valid number, print an "invalid" message and ask again. The user can enter a specific ("sentinel") value to indicate they are finished." If you do not have clear instructions yourself for what you're trying to accomplish, you will not be able to program a computer to perform that task.

Step 2: Plan your program. You will need an array to store the input. You will need a loop that will keep asking for numbers until ten numbers have been entered, or the "sentinel" value has been entered (indicating the user is done entering numbers). If the user enters an invalid value, you do not want that loop to continue until they enter a valid number, so it looks like you will need a validation loop within your main loop.

Also note that trying to read for integers, and a character ('Q' or 'q') is, while possible, probably more difficult than you're ready to handle right now. Find a way to make this simpler. For instance, your array is called "marks", so if we assume that you're reading grades, it's probably a safe bet that negative numbers are not to be expected. Therefore, you can probably make your sentinel value -1 (if the user enters this number, it signifies they are finished entering values).

Step 3: Now that you have the basic requirements and a rough plan, you can start to design your logic (you still shouldn't be coding yet at this point). Write down, on paper, a step by step algorithm for how to accomplish your task. Based on this, you can create a flow chart or pseudo-code for the logic of your program.

Step 4: Now that you have a step by step approach planned out, you can begin coding. Don't bang out the code all at once - break it down into steps and just do one step at a time. (See "A Development Process" for a full example.) Compile and test often, each step of the way.

For instance:

1. Run a loop to read inputs from the user, store them in the array, the print out the array. Compile, test, and verify your results.
2. Add code to handle the sentinel value; i.e. break out of the loop early if the user enters the predetermined value. Compile, test, and verify your results.
3. Add code to validate each user input, and print an error message if an input is invalid. Compile, test, and verify your results.
4. Add code to re-prompt the user for an input if the input was invalid. Compile, test, and verify your results.

Approach your program with an informed, structural mindset and the whole process will go a lot smoother.

Also, your main function should be like this:

Code:

int main(void)
{
    // ...

    return 0;
}

FAQ > main() / void main() / int main() / int main(void) / int main(int argc, char *argv[]) - Cprogramming.com