Ошибка expression cannot be used as a function - ErrorsMaster.ru - большая энциклопедия ошибок и их решений

I created a quick method in my program to compute the distance between two points using the distance formula, here’s the code:

#include <iostream>
#include <cmath>

using namespace std;

int distanceFormula(int x1, int y1, int x2, int y2) {
    double d = sqrt((x1-x2)^2(y1-y2)^2);
    return d;
}

it gives me a compiler error on the line where I declare the d variable saying that

error: expression cannot be used as a function.

What does this mean? And what am I doing wrong?

Gilfoyle

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asked Feb 7, 2014 at 1:24

Be careful, (x1-x2)^2 will not do an exponent of 2 here.
See http://www.cplusplus.com/reference/cmath/pow/.

Second, you probably forgot a + in your expression:

int distanceFormula(int x1, int y1, int x2, int y2) {
    double d = sqrt(pow(x1-x2, 2) + pow(y1-y2, 2));
    return d;
}

answered Feb 7, 2014 at 1:27

Mr_PouetMr_Pouet

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The compiler error is because 2(y1-y2) is invalid syntax.

In this case 2 (or perhaps (x1-x2)^2) is the «expression» and (y1-y2) is taken as a function call argument list; this grammar production is simply not allowed.

Compare the following form where a binary operator (*) is introduced, which in turn makes the parser treat the subsequent (y1-y2) as an expression (bounded by grouping parenthesis) and not a function call. While it won’t do what is desired, as ^ is not exponentiation and the resulting equation is nonsense, it should parse and compile.

sqrt((x1-x2)^2*(y1-y2)^2)

answered Feb 7, 2014 at 1:33

user2864740user2864740

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expression cannot be used as a function

expression cannot be used as a function

Hello,

I am exploring Mersenne Twister implementation to use it as a wrapped class that can be reused as a dll for other implementations like C#. I was trying this class and cannot figure out what is wrong. The code listed below and also available @ cpp.sh/4hte. I will appreciate any guidance help with fixing this class.

It gives me error
In member function ‘double Random::GenerateNext()’: 19:19: error: expression cannot be used as a function 20:2: warning: control reaches end of non-void function [-Wreturn-type]

#include <random> 
#include <iostream>
#include <iomanip>
#include <string>
#include <map>


class Random {
    std::mt19937* gen;
    std::uniform_real_distribution<>* distr;
  public:
	Random (int seed, double min, double max)
	{
		gen = new std::mt19937(seed);
		distr = new std::uniform_real_distribution<>(min,max);
	};
    double GenerateNext() 
	{
		return distr(gen);
	};
};

int main()
{
    double a = 1.0;
    double b = 2147483647.0;
	int seed = 999999999;
    Random x(seed, a, b);
	std::cout << std::fixed << std::setprecision(10) << x.GenerateNext() << std::endl;
	std::cout << std::fixed << std::setprecision(10) << x.GenerateNext() << std::endl;
	std::cout << std::fixed << std::setprecision(10) << x.GenerateNext() << std::endl;
}

Well, you have ponters, not objects, remember. You need to dereference pointers first.

Why do you use pointer semantic at all?

class Random 
{
    std::mt19937 gen;
    std::uniform_real_distribution<> distr;
  public:
    Random (int seed, double min, double max) : gen(seed), distr(min, max) {}
    double GenerateNext() 
    {
        return distr(gen);
    }
};

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#include <iarduino_Pressure_BMP.h>   // Подключаем библиотеку для работы с датчиками BMP180 или BMP280
iarduino_Pressure_BMP sensor(0x76);        // Создаём объект sensor для работы с датчиком адрес которого на шине I2C установлен по умолчанию.
#include "FastLED.h"

#define NUM_LEDS 114 // 4*7*4 +2  Количество светодиодов
#define COLOR_ORDER BRG  // Порядок цвета для ленты
#define DATA_PIN 6  // Вывод для данных
#define BRI_PIN 3  // Вывод сенсора

CRGB leds[NUM_LEDS]; // Определение СД ленты
                    // 0,0,0,0
                    // 1,1,1,1
                    //  1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728
byte digits[12][28] = {{0,0,0,0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1},  // Digit 0
                       {0,0,0,0,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1},   // Digit 1
                       {1,1,1,1,1,1,1,1,1,1,1,1,0,0,0,0,1,1,1,1,1,1,1,1,0,0,0,0},   // Digit 2
                       {1,1,1,1,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,1,1,1,1,1,1,1,1},   // Digit 3
                       {1,1,1,1,1,1,1,1,0,0,0,0,1,1,1,1,0,0,0,0,0,0,0,0,1,1,1,1},   // Digit 4
                       {1,1,1,1,0,0,0,0,1,1,1,1,1,1,1,1,0,0,0,0,1,1,1,1,1,1,1,1},   // Digit 5
                       {1,1,1,1,0,0,0,0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1},   // Digit 6
                       {0,0,0,0,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1},   // Digit 7
                       {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1},   // Digit 8
                       {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,0,0,0,0,1,1,1,1,1,1,1,1},   // Digit 9 | Массив числе на 7 сегментах
                       {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0},   // Digit *0
                       {0,0,0,0,0,0,0,0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,0,0,0,0}};  // Digit C
// сигнальный провод подключен к 2 пину на Arduino
bool Dot = false;  //состояние точек
int last_digit = 0;
// int ledColor = 0x0000FF; // Используемый цвет (in hex)
long ledColor = CRGB::DarkOrchid; // Используемый цвет (in hex)
//long ledColor = CRGB::MediumVioletRed;
//Случайные цвета
long ColorTable[16] = {
  CRGB::Amethyst,
  CRGB::Aqua,
  CRGB::Blue,
  CRGB::Chartreuse,
  CRGB::DarkGreen,
  CRGB::DarkMagenta,
  CRGB::DarkOrange,
  CRGB::DeepPink,
  CRGB::Fuchsia,
  CRGB::Gold,
  CRGB::GreenYellow,
  CRGB::LightCoral,
  CRGB::Tomato,
  CRGB::Salmon,
  CRGB::Red,
  CRGB::Orchid
};                       
void setup(){
    Serial.begin(9600);
    delay(1000);
    sensor.begin();                  // Инициируем работу с датчиком (начальная высота по умолчанию = 0 метров)
 LEDS.addLeds<WS2812B, DATA_PIN, COLOR_ORDER>(leds, NUM_LEDS); // Выбор типа ленты
 LEDS.setBrightness(55); // Установка яркости
}

// Convert temp to array needet for display
void TempToArray(){
//  tmElements_t tm;
//  RTC.read(tm);

//  if (tm.Second != 27) {
//    TempShow = false;
//    return;
//  }
/*
 if  (time.seconds !=20||time.seconds !=50){
    TempShow = false;
    return;
    }
*/
/*
//  TempShow = true;
//  int t = RTC.temperature();
   int t = sensor.temperature;
//  int celsius = (t / 4.0) * 100;
   int celsius = t;
*/
    int celsius = sensor.temperature;
    
//  Serial.print("Temp is: ");Serial.println(celsius);
//  Serial.println(sensor.temperature);
  Serial.println(celsius);
 
  int cursor = 114; // last led number
    
    leds[57]=0x000000;
    leds[56]=0x000000;
      
  for(int i=1;i<=4;i++){
    int digit = celsius % 10; // get last digit in time
    if (i==1){
      Serial.print("Digit 4 is : ");Serial.print(digit);Serial.print(" ");

      cursor = 86;
      
      for(int k=0; k<=27;k++){
        Serial.print(digits[11][k]);
        if (digits[11][k]== 1){leds[cursor]=ledColor;}
         else if (digits[11][k]==0){leds[cursor]=0x000000;};
         cursor ++;
        };
      Serial.println();
    }
    else if (i==2){
      Serial.print("Digit 3 is : ");Serial.print(digit);Serial.print(" ");

      cursor =58;
      
      for(int k=0; k<=27;k++){
        Serial.print(digits[10][k]);
        if (digits[10][k]== 1){leds[cursor]=ledColor;}
         else if (digits[10][k]==0){leds[cursor]=0x000000;};
         cursor ++;
        };
      Serial.println();
      }
    else if (i==3){
      Serial.print("Digit 2 is : ");Serial.print(digit);Serial.print(" ");
      cursor =28;
      for(int k=0; k<=27;k++){
        Serial.print(digits[digit][k]);
        if (digits[digit][k]== 1){leds[cursor]=ledColor;}
         else if (digits[digit][k]==0){leds[cursor]=0x000000;};
         cursor ++;
        };
      Serial.println();
      }
    else if (i==4){
      Serial.print("Digit 1 is : ");Serial.print(digit);Serial.print(" ");
      cursor =0;
      for(int k=0; k<=27;k++){
        Serial.print(digits[digit][k]);
        if (digits[digit][k]== 1){leds[cursor]=ledColor;}
         else if (digits[digit][k]==0){leds[cursor]=0x000000;};
         cursor ++;
        };
      Serial.println();
      }
    celsius /= 10;
  };
};

void loop (){
TempToArray();
FastLED.show();

}

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The essence of the program: get a map, where the values are char from the passed string, and the key is the number of these values in the string

using namespace std;

map<char, int> is_merge(const string& s) {
    map<char, int> sCount {};
    for (auto lp : s) {
        if (find_if(sCount.begin(), sCount.end(), lp) != sCount.end()) {
            sCount[lp] += 1;
        } else {
            sCount.insert(make_pair(lp, 0));
        }
    }

    return sCount;
}

int main()
{
    string test = "aba";

    map <char, int> res = is_merge(test);

    for (auto lp : res) {
        cout << lp.first << ":" << lp.second << endl;
    }

    return 0;
}

But an error occurs in the console: /usr/include/c++/12/bits/predefined_ops.h:318:30: error: expression cannot be used as a function 318 | { return bool(_M_pred(*__it)); } | ~~~~~~~^~~~~~~

>Solution :

std::find_if takes a predicate not a value. Hence the error that lp is not a callable. To find a key in a map you should use std::map::find because it is O(logn) compared to O(n) for std::find/std::find_if (as a rule of thumb you can remember: If a container has a member function that does the same as a generic algorithm the member function is at least as effcient, often better).

However, there is not need to check if the key is present via find. The function can be this:

map<char, int> is_merge(const string& s) {
    map<char, int> sCount {};
    for (auto lp : s) {
         ++sCount[lp];
    }

    return sCount;
}

std::map::operator[] already does insert an element when none is found for the given key. You don’t need to do that yourself.

PS: And if you do call insert then there is no need for std::make_pair : sCount.insert({lp, 0});. std::make_pair is for when you need to deduce the type of the pair from arguments to std::make_pair, but you don’t need that here.

PPS: And if you do use std::find you need to consider that the element type of your map is std::pair<const char, int>, not char.

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Arduino Forum

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expression cannot be used as a function

>Solution :

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