为什么在scanf（）之后此指针的默认值为10？

Question

This small test has helped in pinpointing issues with my program. Now a weird thing is happening: The third and final scanf() is getting skipped over and the test line immediately following prints a value of 10 for cBoard[*tile] .

void takeTurn(int *iap, int *tile, char *cap) {
    printf("\n\tPRINT TEST Take Turn()\n");

    if (*iap == 1) *cap == 'X';
    if (*iap == 2) *cap == 'O';

    printf("\nWhich tile would you like?");
    printf("\n\tTEST the value of cBoard[0] is %c. At [3] is %c.\n1  ::", cBoard[0], cBoard[3]);
    scanf("%c", &cBoard[0]);

        //user inputs 'h'. 

    printf("\n\tTEST the value of cBoard[0] is now %c\n2  ::", cBoard[0]);
    scanf("%d", tile);

        //it prints
        //TEST the value of cBoard[0] is now h
        //user inputs 6. It prints

    printf("\n\tTEST the value of cBoard[%d] is %d.\n3  ::", *tile, cBoard[*tile]+1);

        //it prints
        //TEST the value of cBoard[6] is 1. 

        scanf("%c", &cBoard[*tile]);

        //This scanf() does not run. 

    printf("\n\tTEST the value of cBoard[%d] is %d.\n", *tile, cBoard[*tile]);

        //it prints
        //TEST the value of cBoard[6] is 10.

    ...
}

The final print returns cBoard[6] as 10 regardless of whatever its previous assignment was. Something about the improperly functioning scanf() must be setting the new value to 10.

What is going on?

Answer 1

scanf("%d", tile); reads a number but leaves the '\\n' in the standard input buffer. The next read operation scanf("%c", &cBoard[*tile]); reads this character, which happens to have the value 10 in ASCII. You can fix the behavior by ignoring white space before the character with:

scanf(" %c", &cBoard[*tile]);  // notice the space before the %c

Note also that you should check the return value of scanf to verify that correct input was provided.