Trebuchet with Dart (1)

The entire narrative is at Day 1 of AoC 2023, but in essence we are tasked with decoding a file with obfuscated values. Each line of the file has a value we need to extract, and the answer is the sum of all of these values.

The correct value for each line is the first and last numeric digit in that line. For example, the line tr3b2c4et has a value of 34, and the line tr4e has a value of 44.

Implementation

This problem boils down to how we can find the first and last numbers in a string. The simplest way to do this is to have two for loops, one starting from the beginning and incrementing, and one starting from the end and decrementing. Each for loop will break once it hits a number.

The question, then, is how do we know that a string is a number? We can do this through the tryParse function, defined for the int class. More specifically, given a string str, int.tryParse(str) will return an int if str is an integer, otherwise it will return null. Therefore, we know a character is an integer if passing it through this method results in a non-null value.

// final calibration value
int calibration = 0;

// search from the beginning for first number
for (int i = 0; i < text.length; i++) {
    int? value = int.tryParse(text[i]);
    if (value != null) {
        calibration += 10 * value;
        break;
    }
}

Here we see the first for loop. Notice that since there is a possibility that value is null, we have to define it with the int? type to reflect that.

Now, to tackle the problem of reading input. We can read input from a file by using dart’s dart:io package along with the File object:

import 'dart:io';

final file = File('day1.in');
int values = 0;
List<String> lines = file.readAsLinesSync();

This reads the lines from the file called day1.in, which is our input file. These lines are read into a List<String>. If we wrap our previous logic in a function, then loop over each line, we get our final code:

import 'dart:io';

int getValues(String text) {
    // final calibration value
    int calibration = 0;

    // search from the beginning for first number
    for (int i = 0; i < text.length; i++) {
        int? value = int.tryParse(text[i]);
        if (value != null) {
            calibration += 10 * value;
            break;
        }
    }

    // search from the end for first number
    for (int i = text.length - 1; i >= 0; i--) {
        int? value = int.tryParse(text[i]);
        if (value != null) {
            calibration += value;
            break;
        }
    }

    return calibration;
}

void main() {
    final file = File('day1.in');
    int values = 0;
    List<String> lines = file.readAsLinesSync();

    for (String line in lines) {
        values += getValues(line);
    }

    print(values);
}