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Joseph Haugh

University of New Mexico

Int to Digits

How can we transform an int into an array of digits?

void intToDigits(int x, int digits[], int n) {
  int i = n - 1;
  while (x > 0) {
    ...
  }
}

Int to Digits

void intToDigits(int x, int digits[], int n) {
  int i = n - 1;
  while (x > 0) {
    digits[i] = x % 10; // 1s place
    x = x / 10;         // All but 1s place
    i--;
  }
}

Int to Digits

Calling the function:

int main() {
    int x = 123;
    int numDigits = floor(log10(x)) + 1;
    int digits[numDigits];
    intToDigits(x, digits, numDigits);
    for (int i = 0; i < numDigits; i++) {
        printf("digits[%d]=%d\n", i, digits[i]);
    }
    return 0;
}

Note: We need to include the math.h library to use log10 and floor which means we need to compile with -lm.

Digits to Int

How can we transform an array of digits into an int?

int digitsToInt(int digits[], int n) {
  int result = 0;
  for (int i = 0; i < n; i++) {
    result = result * 10 + digits[i];
  }
  return result;
}

Reading in Characters

How do you read in characters in C?

The getchar function reads in a single character from standard in.

What happens if we read in a 1 character and print it as an int?

int main() {
    int c = getchar(); // Read in '1'
    printf("%d\n", c); // Prints 49
    return 0;
}

Why does it print 49?

That is the ASCII value of the character 1.

Reading in Characters

How could we then read in characters from standard in and convert it to an int?

First we need to transform the character ‘1’ to the int 1 not 49.

Reading in Characters

Let’s look at part of the ASCII table:

Character	ASCII
‘0’	48
‘1’	49
‘2’	50
‘3’	51
‘4’	52

We need to subtract the ‘0’ character from the character we read in.

Reading in Characters

Let’s then attempt to read in an int character by character.

int readNumber() {
    int result = 0;
    int c = getchar();
    while (c != '\n') {
        result = (result * 10) + (c - '0');
        c = getchar();
    }
    return result;
}

Error Codes

In C it is common to use bits to represent error codes. For example:

#define ERROR_1 1
#define ERROR_2 2
#define ERROR_3 4

Error Codes

How do you then “raise” an error code?

You can use the | operator to combine error codes.

Error Codes

#define NEGATIVE_ERROR 1
#define DIVIDE_BY_ZERO_ERROR 2
int error = 0;

int divide(int a, int b) {
    if (a < 0 | b < 0) {
        error |= NEGATIVE_ERROR;
        return -1;
    }
    else if (b == 0) {
        error |= DIVIDE_BY_ZERO_ERROR;
        return -1;
    }
    return a / b;
}

Error Codes

How do you then “check” for an error code?

You can use the & operator to check for an error code.

void checkError() {
    if (error & NEGATIVE_ERROR) {
        printf("Negative Error\n");
    }
    if (error & DIVIDE_BY_ZERO_ERROR) {
        printf("Divide by Zero Error\n");
    }
}

Feeding File Contents Through Standard In

How can you feed the contents of a file through standard in to a C program?

You can use the < operator to feed the contents of a file to a program.

./a.out < input.txt