CODEWITHSUNDEEP
cfor-loop
QWERTY
QWERTY

Reputation: 2315

C programming for loop start from right hand side

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.

Upvotes: 1

Views: 2569

Answers (4)

bin
bin

Reputation: 724

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--) 
{ 

}

Upvotes: 0

Tejas Patel
Tejas Patel

Reputation: 870

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

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

Upvotes: 0

Mogsdad
Mogsdad

Reputation: 45720

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 kth-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 kth-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';

Upvotes: 2

3442
3442

Reputation: 8576

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.

Upvotes: 0

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