# After class questions of Chapter 2 of essential C + +

Time：2020-11-15

## Exercises 2.2 and 2.3

### Header file

``````#pragma once

#include<iostream>
#include<vector>

void Pentagonal(const std::vector<int> &inputs, int num);
inline bool IsValidNum(const int num, const std::vector<int> &v) {
if (num <= 0 || num >= 1024 || num <= v.size()) {
return false;
}
return true;
}``````

### CC file

``````#include "practice_2_2.h"

void Pentagonal(std::vector<int> &inputs, int num) {
//First judge whether num is legal
if (!IsValidNum(num, inputs)) {
return;
}
for (int i = inputs.size() + 1; i < num + 1;i++) {
int value = i * (3 * i - 1) / 2;
inputs.push_back(value);
}
}

void PrintElements(const std::vector<int> * inputs) {
for (int ele : *inputs) {
std::cout << ele << std::endl;
}
}

int main() {
std::vector<int> v;
Pentagonal(v, 10);
Pentagonal(v, 20);
PrintElements(&v);
//Return 0 indicates that the program was successfully run
return 0;
}``````

## Exercise 2.4

### Header file

``````#pragma once
#include<iostream>
#include<vector>

const std::vector<int> * GetPentagonal(int num);

int GetValueFromIndex(int index, const std::vector<int> * v);``````

### CC file

``````#include "practice_2_4.h"
#include<string>

/*
The reason why I started with the following is wrong because I just declared one
Pointer to vector < int >, however, this pointer does not point to any
Object, so if you directly call the - > size() function, you will definitely report an error
*/
// const std::vector<int> * GetPentagonal(int num) {
//// local static object pointer
//     static std::vector<int> * v;
//     if (num <= 0 || num > 1024) {
//         return v;
//     }
//     for (int i = v -> size() + 1; i < num + 1; i++) {
//         std::cout << "point" << i << std::endl;
//         v -> push_back(i * (3 * i - 1) / 2);
//     }
//     return v;
// }

const std::vector<int> * GetPentagonal(int num) {
//A local static object initializes an object directly
static std::vector<int> v;
if (num <= 0 || num > 1024) {
return &v;
}
for (int i = v.size() + 1; i < num + 1; i++) {
v.push_back(i * (3 * i - 1) / 2);
}
return &v;
}

int GetValueFromIndex(int index, const std::vector<int> * v) {
//Determine whether it is a null pointer
if (v == 0) {
return -1;
std::cout << "Error : v is nullptr" << std::endl;
}
//Judge whether the index exceeds
if (index >= v -> size()) {
return -1;
std::cout << "Error : index too large!" << std::endl;
}
return (*v)[index];
}

int main() {
const std::vector<int> *v = GetPentagonal(10);
//Determine whether it is a null pointer
if (v == 0) {
std::cout << "Error: v is nullptr" << std::endl;
}
std::cout << GetValueFromIndex(5, v) << std::endl;
//Make sure that the next null pointer is 0
//Note that pointers can be compared to 0. In general, objects cannot be declared as pointers compared to 0
std::string* check = nullptr;
if (check == 0) {
std::cout << "the value of nullptr is 0" << std::endl;
}
return 0;
}``````

## Exercise 2.5 / 2.6

### Header file

``````#pragma once
#include<iostream>
#include<vector>

int max(int a, int b);
float max(float a, float b);
std::string max(const std::string &a, const std::string &b);
int max(const std::vector<int> &v);
float max(const std::vector<float> &v);
std::string max(const std::vector<std::string> &v);
Int max (const int * arr()); // arrays can only be declared as pointers?
template <typename T> T TemplateMax(T a, T b);``````

### CC file

``````#include "practice_2_5.h"
#include<string.h>

int max(int a, int b) {
return (a > b) ? a : b;
}
float max(float a, float b) {
return (a > b) ? a : b;
}
std::string max(const std::string &a, const std::string &b) {
std::cout << "a:" << a << "," << "b:" << b << std::endl;
int bigger = a.compare(b);
return (bigger > 0) ? a : b;
}

template <typename T>
T TemplateMax(T a, T b) {
return (a > b) ? a : b;
}

int main() {
std::cout << max(1,2) << std::endl;
std::cout << max(1.2f, 2.4f) << std::endl;
std::cout << max("abc", "abcd") << std::endl;
std::cout << max(110, 112) << std::endl;
return 0;
}``````

Take out first

## Java’s functional programming is like this!

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