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Help with string processing in C

In the program, to find out if a given number is an Armstrong number , I saved the no (3 digits) input as a string as follows.

char input[10];
scanf("%s",&input);

Now I have to calculate the cube of each digit using the pow math.h method as follows.

int a;
a = pow(input[0],3);

By coding this way I was unable to get the correct result. If I type the value "a" it shows some irrelevant answer. My doubt is how to convert from string value to integer value?

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7 replies


You are doing your calculation on the ASCII value of the digit. You will need to convert it to a numeric value like this:



int digit = input[0] - '0';

int a; a = pow(digit, 3);
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There are two problems that are already detailed. First, your scan requires a char *, not a char **. Correct it with what Jeremy said:

scanf("%s", input);


Then calculate the cardinality correctly, as Adam said:

a = pow(input[0]-'0',3);
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You don't need to get the address of the array input

using &input

. Just passing input

will pass a pointer to your scanf () string. Your scanf () call should look like this:

scanf("%s", input);

Another way to do it, with the operator's address:

scanf("%s", &input[0]);
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Armstrong numbers are numbers that represent the armstron property at any given base, not just base 10.

int isArmstrong(int n, int b)
{
    int sum = 0;
    int n2 = n;
    int nDigits = 0;
    while(n2 != 0)
    {
        nDigits++;
        n2 /= b;
    }
    n2 = n;
    for(int i = 0; i < nDigits; i+++)
    {
        sum += pow(n2 % b, nDigits);
        n2 /= b;
    }

     return sum == n;
}
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On the other hand, you may need to replace the power with a more general one like power = strlen (input)

so the code should look like this.

char input[10];
int power, sum = 0;

scanf("%s", input);
power = strlen(input);
sum += pow(input[0] - '0', power);

/* you need to compare in here */
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Doh, I could spend too much time in C because I think most of the answers here will miss the end goal ... Undo advice and of course of course, but there are still a few problems with scanf.
First, it should be OK for a code / homework dump, but it's dangerous: enter 11 characters and get a buffer overflow.
Second, IIRC, since you need a number, you must use an int variable and get it. Untested code from a rusty mind:

int inputNumber;
scanf("%d", &inputNumber);

You will also need it here because you no longer have a pointer.
C should do the conversion for you and you have safer input.

Another problematic code:

int a = pow(input[0], 3);

You are not doing math with the first digit, but with the Ascii value of the first digit! I.e. 49 by 1, 50 by 2 ...
Since you have a number from my previous correction, divide it by the correct power of 10 to get a digit.

If you prefer the string route, use input[0] - '0'

as a minimum.

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Freehand coding based on @PhiLho's idea for using actual integer input:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
  int a, sum, i;

  printf("Enter an Armstrong number, an integer in the range 100..999:\n");
  if(scanf("%d", &a) != 1)
    return EXIT_FAILURE;
  if(a < 100 || a > 999)
    return EXIT_FAILURE;

  /* Now extract digits, and compute sum of cubes. */
  for(sum = i = 0; i < 3; i++)
  {
    sum += pow(a % 10, 3);
    a /= 10;
  }
  printf("sum is: %d\n", sum);
  return EXIT_SUCCESS;
}
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