从右手边开始循环的C编程

Question

I was having some problem when trying to loop thru a string starting from right hand side based on the digit position entered by user. So basically this will be the output of my program:

Enter a number: 1234567
Enter the digit position: 3
digitValue1(): 5 

And here is my code:

int main()
{
    int num, digit, result;

    printf("Enter the number: ");
    scanf("%d", &num);
    printf("Enter the digit position: ");
    scanf("%d", &digit);
    printf("digitValue1(): %d\n", digitValue1(num, digit));
    return 0;
}

int digitValue1(int num, int k)
{
    int result, i;
    char str[15];
    sprintf(str, "%d", num);
    int len = strlen(str);
    for (i = 0; i < len; i++) {
        printf("%d ", i);
        if (i == k) {
            result = k;
        }
    }
    return result;
}

So from what I did in the digitValue1() is I take the int num entered by user and converted it into string and loop thru. However, it started from left hand side. So this is my output:

Enter a number: 1234567
Enter the digit position: 3
digitValue1(): 3

I was thinking how should I make the loop start from right hand side. I tried to reverse the for loop and it doesn't make sense.

Thanks in advance.

Answer 1

Initialize i to start at the end. Remember that strlen() is a Natural number, so the index of the last character is 1 less.

There are a few other details to note:

• This does not actually reverse the string being scanned; rather it starts scanning from the end (right-side) of the string.

• Since the loop is starting at the end of the string and working towards the front, the stopping condition needs to adjust to look for the k th-last character.

• We want the integer value of the result digit to be returned, so we must convert the char to an int . That's easiest done by subtracting the ascii value for '0' from the result digit.

Updated code:

int digitValue1(int num, int k)
{
    int result, i;
    char str[15];
    sprintf(str, "%d", num);
    int len = strlen(str);
    for (i = len-1; i >= 0; i--) { // Loop backwards from end of str
      printf("%d ", i);
      if (len-i == k) {           // Check if we are k from end
          result = str[i] - '0';  // Convert digit char to int
      }
    }
    return result;
}

This remains an exhaustive search... it will continue through the whole string even after finding the k th-last element. (Just as your original question did.) You'll want to think about how to optimize that, so it stops looking once the result is identified.

Instead of a loop though, it would be more effective to calculate the index you're looking for:

result = str[strlen(str)-k] - '0';

Answer 2

Well, I may note a few things in your code to let you avoid future bugs:

• You're using sprintf() instead of snprintf() . This is ( almost ) as dangerous as using the evil gets() .
• You don't do bounds checking (see previous note).
• You use fixed-size arrays for dynamically-sized input (see the two previous notes).
• strlen() returns a size_t , not an int (this can cause really serious issues)!

And finally, you appear to not understand the for loop. Just remember this: in C(-like) languages, a for loop is not exactly an iterator over a pseudo-list. In these languages, this:

for(initializer; condition; helper) {
    ...
}

Is completely equivalent and shorthand for :

{
    initializer;
    while(condition) {
        ...
        helper;
    }
}

Thus, taking all this stuff into account, digitValue1() could be reworded as something like this (note that the first position is zero):

int digitValue1(int num, int position) {
    do {
        int digit = num % 10;
        num /= 10;

        if(position-- == 0)
            return digit;
    } while(num != 0);

    // This shall be understood as an error
    return -1;
}

Hope this helps! Cheers, KemyLand.

Answer 3

You can change your for loop from your side as well like following

for (i = len-1; i >= 0; i--) 
 { 
    // your implementation code
 }

Answer 4

As you are starting from 0 position. If you want to print from right then start from right. For that we need to set initialize value as len and ending with 0.

for (i = len-1; i >= 0; i--) 
{ 

}