On the first run, it works fine. The Program runs the last chosen case when you input a char
int x;
do{
printf("Input a number:");
scanf("%d", &x);
switch(x){
case 1: printf("A\n\n");
break;
case 2: printf("B\n\n");
break;
case 3: printf("C\n\n");
break;
case 4: printf("End");
break;
default: printf("Invalid.\n");
}
}while(x!=4);
how do I prevent this?
Check the return value of scanf and if it doesn't return 1 then you will know that either user inputted something not desired by the format specifier or some error occured.
if(scanf("%d", &x) != 1){
fprintf(stderr,"Error occured\n");
exit(1);
}
Better solution is to use fgets and parse for number using strtol. It will give a much more error handling and control over the whole procedure than scanf would.
As I said, you can also go for using fgets to read a line and then use sscanf to parse the integer input. Play a bit with this bare minimum function you will understand what I mean:-
#define MAXLEN 50
int f() {
int v;
char buff[MAXLEN];
while (fgets(buff, sizeof buff, stdin)) {
if (sscanf(buff, "%d", &v) == 1 ) {
return v;
}
}
return -1;
}
The Program runs the last chosen case when you input a char.
This is because when scanf("%d", &x); failed it did not updated the variable x. xstill remembers the last chosen good number.
Check the standard 7.21.6.4 The scanf function.
You can read about behaviour of scanf
here.
On success, the scanf returns the number of items successfully read. This count can match the expected number of readings or fewer, even zero, if a matching failure happens. In the case of an input failure before any data could be successfully read, EOF is returned.
Knowing that you can check the return value of scanf and make appropriate decision. In the program below, bad scanf input is treated as case 4: and program ends:
#include <stdio.h>
int main(){
int x;
do{
printf("Input a number:\n");
if(scanf("%d", &x) != 1){
printf("Error - END.\n");
break;
}
switch(x){
case 1: printf("A\n\n");
break;
case 2: printf("B\n\n");
break;
case 3: printf("C\n\n");
break;
case 4: printf("End\n");
break;
default: printf("Invalid.\n");
}
}while(x!=4);
return 0;
}
Output:
Input a number:
0
Invalid.
Input a number:
1
A
Input a number:
2
B
Input a number:
3
C
Input a number:
N
Error - END.
Edit:
If OP requires program to continue to run after user bad input, that requires more work.
The naive approach is to replace the break with continue.
That will not work! Any character that does not match the format string causes scanf to stop scanning and leaves the invalid character still in the buffer!
To continue we have to flush the invalid character out of the buffer.
The modified program is given below:
#include <stdio.h>
#include <ctype.h>
int main(){
int x;
char c;
int error = 0;
do{
c = '0';
if(!error)
printf("Input a number:\n");
else
error = 0;
if(scanf("%d", &x) != 1)
{
printf("No letters! Input a number:\n");
do
{
c = getchar();
}
while (!isdigit(c));
ungetc(c, stdin);
error = 1;
continue;
}
switch(x){
case 1: printf("A\n\n");
break;
case 2: printf("B\n\n");
break;
case 3: printf("C\n\n");
break;
case 4: printf("End\n");
break;
default: printf("Invalid.\n");
}
}while(x!=4);
return 0;
}
Output:
Input a number:
1
A
Input a number:
X
No letters! Input a number:
1
A
Input a number:
4
End
