Validate User input(How to make sure it's not a character)

**ashley1nonly** · 02-28-2016

I got the program to ensure that it only accepts the numbers one and two. What do i put to make sure the user can't put in a character.

Code:

// Fig. 3.10: fig03_10.c
// Analysis of examination results.
#include<stdio.h>

// Function main begins program execution
int main(void)
{
        // intialize variable in definitions
        unsigned int passes = 0; // number of passes
        unsigned int failures = 0; // number of failures
        unsigned int student = 1; // student counter
        int result; // one exam result

// process 10 students using counter-controlled loop
        while(student <= 10) {

        // prompt user for input and obtain value from user
        printf("%s", "Enter result(1=pass, 2=fail):");
        scanf("%d", &result);
        if(result<1){
        printf("Invalid input\n");
        scanf("%d", &result);
        }
        if(result>2){
        printf("invalid input\n");
        scanf("%d", &result);
        }
        // if result 1, increment passes
        if (result ==1) {
        passes = passes + 1;
        }// end if

        else { // otherwise, increment failures
        failures = failures + 1;
        } // end else

        student = student + 1; // increment student counter
        }// end while

        // termination phase; display number of passes and failures
        printf("Passed %u\n", passes );
        printf("Failed %u\n", failures );

        // if more than eight passed, print "Bonus to instructor!"
        if (passes > 8) {
        puts("Bonus to instructor!");

        } // end if
        } // end function main

**ashley1nonly** · 02-28-2016

The program will scan in a total of 10 students, you will input a 1 for pass and 2 for fail, the program will then show you how many students passed and how many failed. If more then 8 students passed it will put bonus to the instructor.

I have already got it to accept only the numbers 1 and 2, how do i make sure a letter can't be input

**www.infysim.org** · 02-28-2016

Originally Posted by ashley1nonly

What do i put to make sure the user can't put in a character.

You can check the return value of scanf() to make sure everything went well.

If scanf() has returned 0 then there may be a problem (may be you have asked for int but user has entered a char) then you should print something to convey the user of the misuse and clean the buffer by calling fpurge() or fflush() something else before going to accept the next input.

The code may be something like this:

Code:

while (scanf("%d", &result) == 0) {
    printf ("Please enter an int.\n");
    fpurge();
}

**ashley1nonly** · 02-28-2016

can i also use that statement to help compress my code for example

Code:

while(scanf("%d", &results) ==0 || <1 || >2){
printf("please enter a valid input(1 or 2)\n");
fpurge();
}

**laserlight** · 02-28-2016

Originally Posted by www.infysim.org

If scanf() has returned 0 then there may be a problem (may be you have asked for int but user has entered a char) then you should print something to convey the user of the misuse

scanf might also return EOF if there is a read error before the first conversion was completed, so merely checking if it does or does not return 0 is insufficient. As such, it tends to be better to check for the number of expected conversions rather than for 0 or EOF.

Originally Posted by www.infysim.org

clean the buffer by calling fpurge() or fflush() something else before going to accept the next input.

Yes, you should discard the invalid input that is in the input buffer. However, you should not use fpurge or fflush unless you are absolutely sure that you are willing to be tied to a non-standard implementation: fpurge is non-standard, not even POSIX standard, and fflush is undefined for input streams.

**ashley1nonly** · 02-28-2016

Code:

#include<stdio.h>

// Function main begins program execution
int main(void)
{
        // intialize variable in definitions
        unsigned int passes = 0; // number of passes
        unsigned int failures = 0; // number of failures
        unsigned int student = 1; // student counter
        int result, temp; // one exam result

// process 10 students using counter-controlled loop
        while(student <= 10) {

        // prompt user for input and obtain value from user
        printf("%s", "Enter result(1=pass, 2=fail):");
        scanf("%d", &result);
        if(result<1){
        printf("Invalid input...please enter a 1 or 2\n");
        scanf("%d", &result);
        }
        if(result>2){
        printf("invalid input...please enter a 1 or 2\n");
        scanf("%d", &result);
        }
if(result=0){
printf("invalid input...please enter a 1 or 2\n");
scanf("%d", &result);
}
        // if result 1, increment passes
        if (result ==1) {
        passes = passes + 1;
        }// end if

        else { // otherwise, increment failures
        failures = failures + 1;
        } // end else

        student = student + 1; // increment student counter
        }// end while

        // termination phase; display number of passes and failures
        printf("Passed %u\n", passes );
        pr
        // if more than eight passed, print "Bonus to instructor!"
        if (passes > 8) {
        puts("Bonus to instructor!");

        } // end if
        } // end function main
intf("Failed %u\n", failures );

when i do it this way it say invalid input but then it goes directly to the EOF
is there a simple one line while loop that i can use.
I'm just trying to make sure the user can only put in a 1 or 2

thanks to all in advance

**ashley1nonly** · 02-28-2016

I can't use fpruge because i haven't got that far in my c programming class

**laserlight** · 02-28-2016

Originally Posted by ashley1nonly

can i also use that statement to help compress my code for example

Possible, but you need to write your comparisons correctly, e.g.,

Code:

while (scanf("%d", &results) != 1 || results < 1 || results > 2) {
    int c;
    printf("please enter a valid input(1 or 2)\n");
    while ((c = getchar()) != '\n' && c != EOF);
}

EDIT:
I noticed this line:

Code:

if(result=0){

The above is a mistake because instead of comparing for 0, i.e., result == 0, you assigned 0.

In general, you need to indent your code properly.

**rstanley** · 02-28-2016

Originally Posted by www.infysim.org

Code:

while (scanf("%d", &result) == 0) {
    printf ("Please enter an int.\n");
    fpurge();
}

It is recommended that instead of using fflush() or the non -standard fpurge() functions on input strings, it s better to use the following:

Code:

int ch = 0;
while ((ch = getchar()) != '\n' && ch != EOF);

EDIT:

I said "input strings", but meant "input files".

**ashley1nonly** · 02-28-2016

Code:



        // prompt user for input and obtain value from user
        printf("%s", "Enter result(1=pass, 2=fail):");
        while(!(scanf("%d", &result) <1 || >2 || ch==getchar() ))
        {
        printf("Please enter valid int\n");
        scanf("%d", &result);
        }

what about this while loop
i state while scanf cannot be less than 1, greater than 2, or (==) to getchar

**ashley1nonly** · 02-28-2016

Originally Posted by rstanley

It is recommended that instead of using fflush() or the non -standard fpurge() functions on input strings, it s better to use the following:

Code:

int ch = 0;
while ((ch = getchar()) != '\n' && ch != EOF);

you program is saying
while ch (which is an integer cannot equal getchar)
cannot equal new line
and cannot equal end of file

is this correct

so

Code:

while(!(scanf("%d", &result) <1 || >2){
       printf("please enter valid number\n");
       scanf("%d", &result)'
 while((ch = getchar()) != '\n' && ch != EOF){
    
 printf("please enter valid number\n");
       scanf("%d", &result);
}

**laserlight** · 02-28-2016

Originally Posted by ashley1nonly

what about this while loop
i state while scanf cannot be less than 1, greater than 2, or (==) to getchar

You have wrong logic and invalid syntax:

scanf does not return the value that you are trying to read: it returns the number of conversions made or EOF, i.e., with "%d" as your format string, you expect one conversion to be made. Hence, !(scanf("%d", &result) is wrong: !EOF would be 1. This is why I wrote: scanf("%d", &results) != 1 || results < 1. The subexpression before the || compares with 1 because 1 conversion is expected; the subexpression after the || compares with 1 because that is the lower limit of the valid values of results.
>2 is simply invalid syntax. Notice that I wrote: results > 2
ch==getchar() is simply out of place. The logic does not make sense at all. The use of getchar() is to discard the invalid input, hence it should be in a loop within the body of the loop that reads with scanf. Your attempt to put it all in a single loop is misguided.
Since scanf is called within the loop condition, it should not also be called within the loop body to read into results.

EDIT:

Originally Posted by ashley1nonly

you program is saying
while ch (which is an integer cannot equal getchar)
cannot equal new line
and cannot equal end of file

Not quite. ch is an int because EOF is an int, and getchar can return EOF. We would read it more like: "assign the return value of getchar to ch, and if after the assignment ch is not equal to '\n', and if ch is not equal to EOF, keep looping".

**ashley1nonly** · 02-28-2016

scanf("%d", &results) != 1

so you wrote this because using %d i can only make one conversion(meaning that it will only check to see if the result is right once) and if it is not right it will go to the EOF

trying to break down what you wrote step by step so i can get a full understanding.

**ashley1nonly** · 02-28-2016

also when i use result=0
i am storing the number zero at the integer location results
vs.
when i say result==0
i am saying if result is equal to 0(meaning that i am comparing what is already stored in the integer zero location then comparing it to 0

**laserlight** · 02-28-2016

Originally Posted by ashley1nonly

scanf("%d", &results) != 1

so you wrote this because using %d i can only make one conversion(meaning that it will only check to see if the result is right once) and if it is not right it will go to the EOF

No. Contrast with this:

Code:

int x, y;
while (scanf("%d%d", &x, &y) != 2) {
    /* ... */
}

Now, there are two %d conversion specifiers in the format string, hence two conversions are expected. If there is a read error, EOF would be returned; if there is a conversion error, 0 or 1 might be returned. If both conversions are successful, 2 will be returned, upon which the loop will terminate.

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