Character input with %d causing infinite loop [closed] - c

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int valid=0, running=1;
printf("\n1. Generate\n2. Retrieve");
while(!valid){
printf("\n\nEnter choice> ");
scanf("%d", &c);
if(c==1){
valid=1;
generate();
while(running){
printf("\n\nPress Y to generate again. Press N to retrieve> ");
scanf(" %c", retry);
if(retry == 'Y' || retry == 'y'){
idx++;
generate();
}else if(retry == 'N' || retry == 'n')
running = 0;
else
printf("Invalid input. Try again.");
}
retrieve();
}else if(choice==2){
valid = 1;
retrieve();
}else
printf("Invalid input. Try again");
}
Here the user should enter either 1 or 2. If the user enters any other number or character then I want to ask the user to input again. The program works fine if the user enters any other number like 5/6/7 etc. But if the user enters a character the program goes into an infinite loop. I can break the loop with a scanf status check but then the program stops. Instead, I want to prompt the user to input again if he enters anything except 1 or 2.

Answer 1

scanf returns the number of successful input assignments, or EOF on end of file or error. You should get in the habit of checking this return value. In this case of scanf( "%d", &c ), you should expect a return value of 1 on a successful input.
The %d conversion specifier tells scanf to skip over any leading whitespace, then read characters up to the first character that isn't a decimal digit, leaving that character in the input stream.
Example - suppose you enter "12.3" as an input. scanf( "%d", &c ) will read, convert, and assign the "12" portion of the input to c and return 1. The ".3" portion of the input is left in the input stream.
If you call scanf( "%d", &c ) again, the first thing it sees is that '.' character, so it immediately stops reading (you have a matching failure).
Since no input was actually read, nothing gets assigned to c and scanf returns 0. This will keep happening until you remove that '.' character with some other input operation like getchar() or scanf( "%*c" ), etc.
You should always check the result of scanf to make sure you read as many items as you expect:
int r = 0;
do
{
r = scanf( "%d", &c );
if ( r == EOF )
{
// end of file or error signaled on the input stream; in this case we
// just exit the program
exit( 0 );
}
else if ( r == 0 )
{
// matching failure - there's a bad character in the input stream
// remove it with getchar and try again
getchar();
}
} while( r != 1 );
// at this point we either have good input or have already exited the program

Answer 2

Apparently, using scanf with %d, but input characters causes buffer issues. See code below for one way to avoid the problem:
#include <stdio.h>
#include <stdlib.h>
void generate(){
printf("generate\n");
}
void retrieve(){
printf("retrieve\n");
}
int main(){
int valid=0, running=1, idx=0, c=-100;
char retry;
char s[25];
printf("\n1. Generate\n2. Retrieve");
while(!valid){
printf("\n\nEnter choice> ");
scanf("%s", s);
c = atoi(s);
if(c==1){
valid=1;
generate();
while(running){
printf("\n\nPress Y to generate again. Press N to retrieve> ");
while(scanf(" %c", &retry)==0);
if(retry == 'Y' || retry == 'y'){
idx++;
generate();
}else if(retry == 'N' || retry == 'n')
running = 0;
else
printf("Invalid input. Try again.");
}
retrieve();
}else if(c==2){
valid = 1;
retrieve();
}else
printf("Invalid input. Try again");
}
return(0);
}
One possible execution is shown below:
1. Generate
2. Retrieve
Enter choice> Bad
Invalid input. Try again
Enter choice> Worse
Invalid input. Try again
Enter choice> 1
generate
Press Y to generate again. Press N to retrieve> Y
generate
Press Y to generate again. Press N to retrieve> n
retrieve