C++ program if statements in function not executing. Seems Logically errored - c++

Question

I am making a program in C++, that takes the user entered time in U.S. standard time and converts it to military time. The body of the main code is executing fine, but the problem comes in the body of my function beginning with the if statements. I am wondering why this is occurring; am fairly new with c++. Here's my code if you have question feel free to ask or need explanation of what the program is supposed to be doing.
#include <iostream>
#include <string>
using namespace std;
void militaryConversion(string am_pmPart_st, string firstPartofTime, string secondPartofTime){
// Converts they obtained strings, but first we must concatenate the two parts into one string
string concatenatedTime;
int militaryTime;
cout << "test1" << endl;
concatenatedTime = firstPartofTime + secondPartofTime;
if(firstPartofTime == "12")
{
cout << "Corresponding military time is: " << concatenatedTime << " hours" << endl;
}
else if(am_pmPart_st == " am")
{
if (concatenatedTime.length() < 4){
cout << "Corresponding military time is: " << concatenatedTime << " hours"<< endl;
}
}
else if(am_pmPart_st == " pm")
{
int castedTime;
castedTime = stoi(concatenatedTime); //This is where we convert the string to int because its the only place it matters
militaryTime = castedTime + 1200;
cout << "Corresponding military time is: " << militaryTime << " hours" << endl;
}
}
int main()
{
char DELEMETER = ':';
char DELEMETER_sp = ' ';
string time, firstPartofTime, secondPartofTime, am_pmPart_st, loweredAM_PM;
cout << "Enter the time in the format of: HH:MM AM/PM ";
getline(cin, time);
firstPartofTime = time.substr(0, time.find(DELEMETER));
cout << "The first digits of time " << firstPartofTime << endl;
secondPartofTime = time.substr(time.find(DELEMETER) + 1, time.find(DELEMETER_sp)-1);
cout << "The second set of digits " << secondPartofTime << endl;
am_pmPart_st = time.substr(time.find(DELEMETER_sp), time.size());
cout << "The am/pm part is:" << am_pmPart_st << endl;
for(int i=0; am_pmPart_st[i]; i++) am_pmPart_st[i] = tolower(am_pmPart_st[i]); //Converts am/pm to lowercase
cout << am_pmPart_st << endl;
militaryConversion(am_pmPart_st, firstPartofTime, secondPartofTime);
}

Answer 1

First, your question is vague because it does say what is happening that should not be happening. However, I think I can see what is happening. When you check for the hour part of the time in the first condition, you check for "12" first. However, you never correct for am or pm within that 12. My recommendation would be to check for 12 inside the am (12 am == 0000 hours) and pm (12 pm == 1200 hours). In am you will need to check for 12 and subtract 1200 from the time, in pm you will need to check for 12 and not add 1200 to the time.

Answer 2

substr takes two parameters. The first is the start position and the second is the length. When you call secondPartofTime = time.substr(time.find(DELEMETER) + 1, time.find(DELEMETER_sp)-1); you are mistakenly passing the second parameter as the end position, not the length.
Instead, you can do:
int startPos = time.find(DELEMETER) + 1;
int endPos = time.find(DELEMETER_sp) - 1;
secondPartofTime = time.substr(startPos, endPos - startPos + 1);
Ideally, you should check the return values of find and handle the case when npos is returned so you don't crash on invalid user input.

Answer 3

Input which doesn't follow the HH:MM AM/PM form is creating a problem. (Exactly 5 characters for HH:MM (including colon))
You have an if-else based decision tree, where one if is not accompanied by an else. The lack of else is why your program is not giving any output
string.substr() has some issues as explained by MFisherKDX