Here is a little young tyro's problem with C code trying simply to prevent the user from typing a character or an integer less than 0 or more than 23.
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
const char *input;
char *iPtr;
int count = 0;
int rows;
printf("Enter an integer: ");
scanf("%s", input);
rows = strtol(input, &iPtr, 0);
while( *iPtr != '\0') // Check if any character has been inserted
{
printf("Enter an integer between 1 and 23: ");
scanf("%s", input);
}
while(0 < rows && rows < 24) // check if the user input is within the boundaries
{
printf("Select an integer from 1 to 23: ");
scanf("%s", input);
}
while (count != rows)
{
/* Do some stuff */
}
return 0;
}
I made it halfway through and a small push up will be appreciated.
Use scanf("%d",&rows) instead of scanf("%s",input)
This allow you to get direcly the integer value from stdin without need to convert to int.
If the user enter a string containing a non numeric characters then you have to clean your stdin before the next scanf("%d",&rows).
your code could look like this:
#include <stdio.h>
#include <stdlib.h>
int clean_stdin()
{
while (getchar()!='\n');
return 1;
}
int main(void)
{
int rows =0;
char c;
do
{
printf("\nEnter an integer from 1 to 23: ");
} while (((scanf("%d%c", &rows, &c)!=2 || c!='\n') && clean_stdin()) || rows<1 || rows>23);
return 0;
}
Explanation
1)
scanf("%d%c", &rows, &c)
This means expecting from the user input an integer and close to it a non numeric character.
Example1: If the user enter aaddk and then ENTER, the scanf will return 0. Nothing capted
Example2: If the user enter 45 and then ENTER, the scanf will return 2 (2 elements are capted). Here %d is capting 45 and %c is capting \n
Example3: If the user enter 45aaadd and then ENTER, the scanf will return 2 (2 elements are capted). Here %d is capting 45 and %c is capting a
2)
(scanf("%d%c", &rows, &c)!=2 || c!='\n')
In the example1: this condition is TRUE because scanf return 0 (!=2)
In the example2: this condition is FALSE because scanf return 2 and c == '\n'
In the example3: this condition is TRUE because scanf return 2 and c == 'a' (!='\n')
3)
((scanf("%d%c", &rows, &c)!=2 || c!='\n') && clean_stdin())
clean_stdin() is always TRUE because the function return always 1
In the example1: The (scanf("%d%c", &rows, &c)!=2 || c!='\n') is TRUE so the condition after the && should be checked so the clean_stdin() will be executed and the whole condition is TRUE
In the example2: The (scanf("%d%c", &rows, &c)!=2 || c!='\n') is FALSE so the condition after the && will not checked (because what ever its result is the whole condition will be FALSE ) so the clean_stdin() will not be executed and the whole condition is FALSE
In the example3: The (scanf("%d%c", &rows, &c)!=2 || c!='\n') is TRUE so the condition after the && should be checked so the clean_stdin() will be executed and the whole condition is TRUE
So you can remark that clean_stdin() will be executed only if the user enter a string containing non numeric character.
And this condition ((scanf("%d%c", &rows, &c)!=2 || c!='\n') && clean_stdin()) will return FALSE only if the user enter an integer and nothing else
And if the condition ((scanf("%d%c", &rows, &c)!=2 || c!='\n') && clean_stdin()) is FALSE and the integer is between and 1 and 23 then the while loop will break else the while loop will continue
#include <stdio.h>
main()
{
char str[100];
int num;
while(1) {
printf("Enter a number: ");
scanf("%[^0-9]%d",str,&num);
printf("You entered the number %d\n",num);
}
return 0;
}
%[^0-9] in scanf() gobbles up all that is not between 0 and 9. Basically it cleans the input stream of non-digits and puts it in str. Well, the length of non-digit sequence is limited to 100. The following %d selects only integers in the input stream and places it in num.
You could create a function that reads an integer between 1 and 23 or returns 0 if non-int
e.g.
int getInt()
{
int n = 0;
char buffer[128];
fgets(buffer,sizeof(buffer),stdin);
n = atoi(buffer);
return ( n > 23 || n < 1 ) ? 0 : n;
}
char check1[10], check2[10];
int foo;
do{
printf(">> ");
scanf(" %s", check1);
foo = strtol(check1, NULL, 10); // convert the string to decimal number
sprintf(check2, "%d", foo); // re-convert "foo" to string for comparison
} while (!(strcmp(check1, check2) == 0 && 0 < foo && foo < 24)); // repeat if the input is not number
If the input is number, you can use foo as your input.
You will need to repeat your call to strtol inside your loops where you are asking the user to try again. In fact, if you make the loop a do { ... } while(...); instead of while, you don't get a the same sort of repeat things twice behaviour.
You should also format your code so that it's possible to see where the code is inside a loop and not.
MOHAMED, your answer is great and it really helped me. Here I have posted a code, that I think it is a little bit simpler:
#include <stdio.h>
int getPositive(void);
void clean_input(void);
int main(void) {
printf("%d\n",getPositive());
return 0;
}
int getPositive(void) {
int number;
char buffer; // Holds last character from user input.
// (i.e '\n' or any other character besides numbers)
int flag; // Holds scanf return value
do{
flag = scanf("%d%c", &number, &buffer); // Gets input from user
// While scanf did not read 2 objects (i.e 1 int & 1 char)
// or the user inputed a number and then a character (eg. 12te)
// ask user to type a valid value
while (flag !=2 || buffer!='\n') {
clean_input();
printf("%s","You have typed non numeric characters.\n"
"Please type an integer\n?");
flag = scanf("%d%c", &number, &buffer);
}
if(number<0) {
printf("%s","You have typed a non positive integer\n"
"Please type a positive integer\n?");
} else { // If user typed a non negative value, exit do-while.
break;
}
}while(1);
}
void clean_input(void) {
while (getchar()!='\n');
return;
}
In my case I want the number to be just positive. If you want your number to be between 1 and 23, you replace the number<0 with number<1 || number>23 in the if statement. Also you will have to change the printf to print an appropriate message.
来源：https://stackoverflow.com/questions/14099473/how-to-scanf-only-integer-and-repeat-reading-if-the-user-enter-non-numeric-chara