Using the OR Operator with Strings in C++ - c++

Question

To start, I'm trying to make a GPA calculator for my class.
I know how to compare strings, so I'm good there. The issue I'm having is that I'm trying to set up a situation so when the user inputs anything other than a letter grade value, it will return an error message. I've set up two arrays, one that stores string values and another that stores integral values. The idea was to use the string array to store the entered grade letter inputs, then use those to determine the GPA value for each class. It would then store that value into the integral array. I hate to be obnoxious, but here's the code for the first section alone:
void gpaCalSetClassNum5(){
string mathWeight5;
string scienceWeight5;
string historyWeight5;
string englishWeight5;
string elective1Weight5;
string elective2Weight5;
string gpaClassSet5[] = {"null", "null", "null", "null", "null"};
int gpaClassSet5int[] = {};
cout << "Enter the grade value of each of your classes." << endl;
/////////////////////////
MATH:
cout << "Math" << endl;
cin >> gpaClassSet5[0];
if (gpaClassSet5[0] == "A") {
gpaClassSet5int[0] = 4;
} else if (gpaClassSet5[0] == "a") {
gpaClassSet5int[0] = 4;
} else if (gpaClassSet5[0] == "B") {
gpaClassSet5int[0] = 3;
} else if (gpaClassSet5[0] == "b") {
gpaClassSet5int[0] = 3;
} else if (gpaClassSet5[0] == "C") {
gpaClassSet5int[0] = 2;
} else if (gpaClassSet5[0] == "c") {
gpaClassSet5int[0] = 2;
} else if (gpaClassSet5[0] == "D") {
gpaClassSet5int[0] = 1;
} else if (gpaClassSet5[0] == "d") {
gpaClassSet5int[0] = 1;
} else if (gpaClassSet5[0] == "F") {
gpaClassSet5int[0] = 0;
} else if (gpaClassSet5[0] == "f") {
gpaClassSet5int[0] = 0;
} else if (gpaClassSet5[0] != ){
cout << "Did you enter a letter grade value?" << endl;
goto MATH;
}
cout << "You have selected " << gpaClassSet5[0] << ", or " << gpaClassSet5int[0] << endl;
cout << "Is this class weighted? Use Y/N." << endl;
cin >> mathWeight5;
if (mathWeight5 == "Y" || "y") {
gpaClassSet5int[0] = gpaClassSet5int[0] + 1;
}
I'm looking for a simplified version of this. Why can't I use something like:
if(gpaClassSet5[0] == "A" || "a"){
//stuff//
}
I'm in need of a simplified version because, like a switch, I'd like to use different inputs to do different things -- but ultimately have a default in case any of the values listed weren't entered.
How can I do this? How can I set up a switch in C++?
Sorry if this question is a little dumb, I'm getting into C++ and these self-made programs are really my only practice.
Full program code here: http://justpaste.it/ee4u

Answer 1

Because that's not how C++ is specified to work, you need to do the comparison twice:
if(gpaClassSet5[0] == "A" || gpaClassSet5[0] == "a")
The logical OR operation means "if the left-hand expression is true, or the right-hand expression is true". In your case with the code as in your question, the right-hand side expression will always be true as "a" is not zero (i.e. false).

Answer 2

Operator precedence says you can't do it this way. gpaClassSet5[0] == "A" || "a" is the same as (gpaClassSet5[0] == "A") || ("a").
If you don't want to write gpaClassSet5[0] twice, you could use regular expressions if you don't mind a performance hit
std::regex_match(gpaClassSet5[0], std::regex("A|a"));
This gets more sensible if you test against a lot of possible matches:
std::regex_match(gpaClassSet5[0], std::regex("A|Grade A|1|Excellent|Outstanding|Perfect|Perfect Score)"));
If you are not using C++11 (don't have std::regex), you can use boost::regex from boost.org.
Or you could solve your specific code example with more compact logic:
char gradeLetter = std::tolower(gpaClassSet5[0])
if (gradeLetter >= 'a' && gradeLetter <= 'd')
gpaClassSet5int[0] = 4-(gradeLetter -'a');
else if (gradeLetter == 'f')
gpaClassSet5int[0] = 0;
else
{
cout << "Did you enter a letter grade value?" << endl;
goto MATH;
}
And extract a function to get rid of the goto and make the code easier to read:
int ConvertLetterToNumericGrade(char gradeLetter)
{
char lower = std::tolower(gradeLetter);
if (lower >= 'a' && lower <= 'd')
return 4-(lower -'a');
if (lower == 'f')
return 0;
throw std::runtime_error("cannot convert invalid grade letter");
}
void gpaCalSetClassNum5()
{
...
while (true)
{
cin >> gpaClassSet5[0];
try { gpaClassSet5int[0] = ConvertLetterToNumericGrade(gpaClassSet5[0]); }
catch (const std::runtime_error& )
{
cout << "Did you enter a letter grade value?" << endl;
continue;
}
break;
}
...
}
And with a switch (which is not supported for strings, but is supported for char and wchar):
int ConvertLetterToNumericGrade(char gradeLetter)
{
switch (gradeLetter)
{
case 'a':
case 'A':
return 4;
case 'b':
case 'B':
return 3;
case 'c':
case 'C':
return 2;
case 'd':
case 'D':
return 1;
case 'f':
case 'F':
return 0;
default:
cout << "Did you enter a letter grade value?" << endl;
throw std::runtime_error("cannot convert invalid grade letter");
}
}
Resp.
int ConvertLetterToNumericGrade(char gradeLetter)
{
switch (std::tolower(gradeLetter))
{
case 'a': return 4;
case 'b': return 3;
case 'c': return 2;
case 'd': return 1;
case 'f': return 0;
default:
cout << "Did you enter a letter grade value?" << endl;
throw std::runtime_error("cannot convert invalid grade letter");
}
}