在C中执行if语句后提示用户？

Question

I'm new to C and trying to write a simple program. I ask the user to enter a month. I know how to check if it's a valid month, but unsure how to prompt the user to enter another if it ISNT a valid month. Here is what I came up with: EDIT: updated code

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    int mon;

    do {
        printf("Enter a month(1=Jan, ..., 12=Dec: ");
        scanf("%d", &mon);
    } while (mon < 1 || mon > 12);
        printf("Not a valid month.");
}

But now my while-block is only getting printed if I enter a number between 1-12. If I enter an invalid month I am still prompted to enter another, but the program terminates if I enter a correct number.

Answer 1

You can use do-while loop as follows:

int mon;
do{
    printf("Enter a month(1=Jan, ..., 12=Dec): \n");
    scanf("%d", &mon);
} while(mon < 1 || mon > 12);

It will continue, if the input is correct and loop again when false input.

Based on your updated code, you will print the message when the users input is valid.

You probably wanted something like this:

for (;;)//infinite loop (no condition)
{
    printf("Enter a month(1=Jan, ..., 12=Dec): ");
    scanf("%d", &mon);

    if (mon < 12 && mon > 0)    // if inside the range (1-12) then break the loop and continue
        break;
    else // otherwise show the error message and repeat again
        printf("Error message \n");
}

Answer 2

scanf() is not friendly to bad input. I would suggest using fgets() and then parse.

int main(void) {
  int mon;
  for (;;) {
    char buf[100];
    printf("Enter a month(1=Jan, ..., 12=Dec): ");
    fflush(stdout);
    if (fgets(buf, sizeof buf, stdin) == NULL) 
      return 1;  // no more input
    if (sscanf(buf, "%d", &mon) == 1 && mon >= 1 && mon <= 12) {
      break;  // Success!
    }
    printf("Not a valid month ");
  } 
  return 0;
}

Additional testing could to used to detect overflow or data after the integer.

Answer 3

If you like kind of oneliners, here it is:

while(printf("Enter Month\n") && scanf("%d",&mon) && (mon < 1 || mon > 12));

code:

main()
{
    int mon;
    while(printf("Enter Month\n") && scanf("%d",&mon) && (mon < 1 || mon > 12));
}

Output:

$ ./a.out 
Enter Month
0
Enter Month
99
Enter Month
98
Enter Month
44
Enter Month
13
Enter Month
12
$

Answer 4

#include<stdio.h>
int main()
{
    int month, condition = 0;
    do
    {
        printf("enter month");
        scanf("%d", &month);
        if (month < 1 || month>12)
            condition = 1;
        else
            condition = 0;
    } while (condition == 1);
    return 0;
}