Linear interpolation is not working as expected - c++11

Question

I have a map that represents the values of a coefficient Y for a given range of temperatures. I'm trying to get the coeff_Y whenever the input key designTempfalls anywhere between the upper and lower limits of keys. I was able to get the three cases: a) when the value of the input designTemp is below the first key then coeff_Y is the first value, b) if the value of the input designTemp is beyond the last key then coeff_Y is the last value and c) if designTemp matches a key then the coeff_Y becomes the corresponding value. The case if the key falls anywhere within the key range is not working. The code showing the failed attempt of interpolation is shown below. Please note that I'm not a programmer, I'm a piping engineer just trying to write my own programs and trying to become proficient at coding with C++. Also, if there is any better solution please show so.
`cout << "\n Enter design temp. in degF: ";
float designTemp;
cin.clear(); cin.ignore(10000, '\n'); cin >> designTemp;
map<float, float> ferriticsteels_Y = { {900, 0.4}, {950, 0.5}, {1000, 0.7} };
if (ferriticsteels_Y.find(designTemp) != ferriticsteels_Y.end())
{
float coeff_Y = ferriticsteels_Y[designTemp];
cout << "\n Y: " << coeff_Y << endl;
}
if (designTemp < ferriticsteels_Y.begin()->first)
{
float coeff_Y = ferriticsteels_Y.begin()->second;
cout << "\n Y: " << coeff_Y << endl;
}
if (designTemp > ferriticsteels_Y.rbegin()->first)
{
float coeff_Y = ferriticsteels_Y.rbegin()->second;
cout << "\n Y: " << coeff_Y << endl;
}
auto lower = ferriticsteels_Y.lower_bound(designTemp) == ferriticsteels_Y.begin() ? ferriticsteels_Y.begin() : --(ferriticsteels_Y.lower_bound(designTemp));
auto upper = ferriticsteels_Y.upper_bound(designTemp);
float coeff_Y = lower->second + (upper->second - lower->second) * float(designTemp - lower->first)/fabs(upper->first - lower->first);
time_t rawtime_end;
struct tm * timeinfo_end;
time(&rawtime_end);
timeinfo_end = localtime(&rawtime_end);
cout << "\n" << asctime(timeinfo_end);
cout << "\nEnter any character and hit enter to exit: ";
char ans;
//cin.clear(); cin.ignore(numeric_limits<streamsize>::max(), '\n'); cin >> ans;...giving error at 'max()'
cin.clear(); cin.ignore(10000, '\n'); cin >> ans;
return 0;}`

Answer 1

It works. I just was making stupid mistake. It only required to revise the nesting of the if-statements and to add a cout for looking the interpolated value at the last else. Below is the code which works as expected:
#include "../../std_lib_facilities.h"
#include <Windows.h>
#include <map>
int main()
{
SetConsoleTitle(TEXT("PipeTran™_v0.1"));
system("CLS");
system("color F1");
time_t rawtime_start;
struct tm * timeinfo_start;
time(&rawtime_start);
timeinfo_start = localtime(&rawtime_start);
printf(asctime(timeinfo_start));
cout << "\n Enter design temp. in degF: ";
float designTemp;
cin >> designTemp;
map<float, float> ferriticsteels_Y = { { 900, 0.4 },{ 950, 0.5 },{ 1000, 0.7 } };
if (ferriticsteels_Y.find(designTemp) != ferriticsteels_Y.end()) {
float coeff_Y = ferriticsteels_Y[designTemp];
cout << "\n Y: " << coeff_Y << endl;
}
else if (designTemp < ferriticsteels_Y.begin()->first) {
float coeff_Y = ferriticsteels_Y.begin()->second;
cout << "\n Y: " << coeff_Y << endl;
}
else if (designTemp > ferriticsteels_Y.rbegin()->first) {
float coeff_Y = ferriticsteels_Y.rbegin()->second;
cout << "\n Y: " << coeff_Y << endl;
}
else {
auto lower = ferriticsteels_Y.lower_bound(designTemp) == ferriticsteels_Y.begin() ? ferriticsteels_Y.begin() : --(ferriticsteels_Y.lower_bound(designTemp));
auto upper = ferriticsteels_Y.upper_bound(designTemp);
float coeff_Y = lower->second + (upper->second - lower->second) * float(designTemp - lower->first) / fabs(upper->first - lower->first);
cout << "\n Y: " << coeff_Y << endl;
}
time_t rawtime_end;
struct tm * timeinfo_end;
time(&rawtime_end);
timeinfo_end = localtime(&rawtime_end);
cout << "\n" << asctime(timeinfo_end);
cout << "\nEnter any character and hit enter to exit: ";
char ans;
cin.clear(); cin.ignore(10000, '\n'); cin >> ans;
return 0;
}