20: Input From a File

Reading and Using a diff

Use filesystem

Version 1: Get the Data

View Source file ex20_v1.cpp Only

#include <fmt/core.h>
#include <fstream>
#include <vector>

using std::vector, std::string, std::ifstream, std::ios;
using fmt::println;

int chunk_number(const string& line, size_t start, size_t end) {
  string chunk = line.substr(start, end - start);
  return stoi(chunk);
}

vector<int> parse_line(const string& line) {
  vector<int> result;
  size_t start = 0;
  size_t end = line.find(",", start);

  while(end != string::npos) {
    int as_number = chunk_number(line, start, end);
    result.push_back(as_number);

    start = end + 1;
    end = line.find(",", start);
  }

  // need to get the final chunk
  int as_number = chunk_number(line, start, end);
  result.push_back(as_number);

  return result;
}

int main(int argc, char* argv[]) {
  string filename{argv[1]};
  ifstream in_file(filename, std::ios::binary);

  if(!in_file.is_open()) {
    println("failed to open {}", filename);
    return 1;
  }

  string line;

  while(in_file) {
    getline(in_file, line);

    if(line == "") break;

    vector<int> row = parse_line(line);

    for(int cell : row) {
      println("number: {}", cell);
    }
  }
}

Version 2: Vector of Vector Tables

View Source file ex20_v1_v2.diff Only

--- ex20_v1.cpp 2026-04-12 10:36:26.108174362 -0400
+++ ex20_v2.cpp 2026-04-12 10:36:49.666521626 -0400
@@ -3,7 +3,7 @@
 #include <vector>
 
 using std::vector, std::string, std::ifstream, std::ios;
-using fmt::println;
+using fmt::println, fmt::print;
 
 int chunk_number(const string& line, size_t start, size_t end) {
   string chunk = line.substr(start, end - start);
@@ -33,6 +33,7 @@
 int main(int argc, char* argv[]) {
   string filename{argv[1]};
   ifstream in_file(filename, std::ios::binary);
+  vector<vector<int>> table;
 
   if(!in_file.is_open()) {
     println("failed to open {}", filename);
@@ -47,9 +48,14 @@
     if(line == "") break;
 
     vector<int> row = parse_line(line);
+    table.push_back(row);
+  }
 
+  for(vector<int> row : table) {
     for(int cell : row) {
-      println("number: {}", cell);
+      print("{}\t", cell);
     }
+
+    print("\n");
   }
 }

Version 3: Type Aliases

View Source file ex20_v2_v3.diff Only

--- ex20_v2.cpp 2026-04-12 10:36:49.666521626 -0400
+++ ex20_v3.cpp 2026-04-12 10:36:57.531303795 -0400
@@ -5,13 +5,16 @@
 using std::vector, std::string, std::ifstream, std::ios;
 using fmt::println, fmt::print;
 
+using Row = vector<int>;
+using Table = vector<Row>;
+
 int chunk_number(const string& line, size_t start, size_t end) {
   string chunk = line.substr(start, end - start);
   return stoi(chunk);
 }
 
-vector<int> parse_line(const string& line) {
-  vector<int> result;
+Row parse_line(const string& line) {
+  Row result;
   size_t start = 0;
   size_t end = line.find(",", start);
 
@@ -32,8 +35,8 @@
 
 int main(int argc, char* argv[]) {
   string filename{argv[1]};
-  ifstream in_file(filename, std::ios::binary);
-  vector<vector<int>> table;
+  ifstream in_file(filename, ios::binary);
+  Table table;
 
   if(!in_file.is_open()) {
     println("failed to open {}", filename);
@@ -47,11 +50,11 @@
 
     if(line == "") break;
 
-    vector<int> row = parse_line(line);
+    Row row = parse_line(line);
     table.push_back(row);
   }
 
-  for(vector<int> row : table) {
+  for(Row row : table) {
     for(int cell : row) {
       print("{}\t", cell);
     }