non-standard syntax; use '&' to create a pointer to member C++ - c++

Question

I have a problem running my small piece of code.
I have a vector who reads in temperature from User Input. I want to process the data a bit. Giving out which kind of day it was (hot day, summer day, etc...) and which day was the hottest.
But everytime I want to get the maximum temperature with the function maxNumber I get 2 errors which I dont understand:
non-standard syntax; use '&' to create a pointer to member
"! =": Function overload cannot be resolved
Please help! Thx very much
Code:
#include <iostream>
#include <vector>
std::string klimaTag(float a) {
if (a >= 25 and a < 30) {
return "Sommertag";
}
else if (a >= 30 and a < 35) {
return "Heißer Tag";
}
else if (a >= 35) {
return "Wüstentag";
}
else {
return "Normaltag";
}
}
float maxNumber(std::vector<float> &a) {
float current_max = 0;
for (int i = a.begin; i != a.end; i++) {
if (a.at(i) > current_max) {
current_max = a.at(i);
}
return current_max;
}
}
int main()
{
std::vector<float> temperatures;
float current_temp;
for (int i = 0; i < 5; i++) {
std::cout << "Hoechsttemp für Tag " << i << " eingeben: ";
std::cin >> current_temp;
temperatures.push_back(current_temp);
}
for (int i = 0; i < 5; i++) {
std::cout << "Tag " << i + 1 << " ist ein " << klimaTag(temperatures.at(i)) << std::endl;
}
std::cout << maxNumber(temperatures);
}

Answer 1

First of all begin() and end() are method, so you're getting that error because you're trying to reference a function so it should require an address operator.
Second is begin() and end() return an iterator and not an index, and you're clearing trying to access like index.
Third thing is you're always returning after the first cycle as your return is inside it and should be outside.
To loop correctly your array is enough to use the for range loop
float maxNumber(std::vector<float> &a) {
float current_max = 0;
for(const auto& element : a) {
if(element > current_max){
current_max = element;
}
}
return current_max;
}
if you want to use the old way you can always do
float maxNumber(std::vector<float> &a) {
float current_max = 0;
for(int i=0; i<a.size(); ++a) {
if(a.at(i) > current_max){
current_max = a.at(i);
}
}
return current_max;
}

Answer 2

a.end refers to the function address, you need to invoke the function itself with a.end() hence the complaint about the non standard syntax, as if you meant to take the function address you could call &a.end. Same with a.begin.

Answer 3

To use the for loop as you described, you need int i = 0; i < a.size(). Moreover the return statement is inside the for loop in maxNumber. That's not what you want (it exits at the first loop cycle), put it outside.
Begin and end are actually methods for working with iterators. To use them you should change a bit the loop.

Answer 4

begin and end are functions which need to be called like functions: begin() and end().
However, you can use range-based for loop like:
float maxNumber(std::vector<float> &a) {
float current_max = 0;
for (auto const i : a) {
if (i > current_max) {
current_max = i;
}
}
return current_max;
}
Please, note where I put your return statement. It is outside of the for loop.