I'm having an issue trying to check against multiple possibilities in an if statement.
The user inputs a string, and then I check that string against multiple possibilities.
if (theString == "Seven" || "seven" || "7")
{
theInt = 7;
cout << "You chose: " << theInt << endl;
}
else if (theString == "Six" || "six" || "6")
{
theInt = 6;
cout << "You chose: " << theInt << endl;
}
So there's just a quick example of what I'm trying to accomplish. In my program, these if statements are in a function, and I am using #include [string]. (I'm not even sure if "6" or "7" is possible, but I can't even test my code right now :( So right now in my code, if the user input 6, my program would run and assign a value of 7 to theInt. Any ideas?
Thanks.
You cannot compare a variable against multiple values like that in C++ you should be doing:
if (theString == "Seven" || theString == "seven" || theString == "7")
{
theInt = 7;
cout << "You chose: " << theInt << endl;
}
else if (theString == "Six" || theString == "six" || theString == "6")
{
theInt = 6;
cout << "You chose: " << theInt << endl;
}
I suppose that the type of the variable theString is std::string. Otherwise at least this comparison
theString == "Seven"
does not make sense,
The condition in the if statement
if (theString == "Seven" || "seven" || "7")
is equivalent to
if ( ( theString == "Seven" ) || ( "seven" ) || ( "7" ) )
and always yields true because at least the address of the string literal "seven" is not equal to zero. So this subexpression ( "seven" ) provides that the whole expression will be equal to true.
You should write
if (theString == "Seven" || theString == "seven" || theString == "7")
But it would be better at first to convert the string to upper or lower case.
For example
#include <algorithm>
#include <string>
#include <cstring>
//...
std::transform(theString.begin(), theString.end(), theString.begin(),
[](char c) { return std::toupper((unsigned char)c); });
if (theString == "SEVEN" || theString == "7")
{
theInt = 7;
cout << "You chose: " << theInt << endl;
}
else if ( theString == "SIX" || theString == "6" )
{
theInt = 6;
cout << "You chose: " << theInt << endl;
}
Using std::set and c++11 you can do one liner with similar syntax as your.
check this:
#include <iostream>
#include <string>
#include <set>
int main()
{
if( (std::set<std::string>{"Seven", "seven", "7"}).count("seven") )
{
std::cout << "foo\n";
}
std::string theString("6");
if( (std::set<std::string>{"Six", "six", "6"}).count(theString) )
{
std::cout << "bar\n";
}
}
update 2017-04-22 -- added code example below
This is a classic example of identifying a derived requirement found during implementation. I suggest you consider writing a function to support it.
change from
if (theString == "Seven" || "seven" || "7")
{
//....
(which is not valid c++ because the if condition is always true)
change to
if (0 == compare(theString, "Seven", "seven", "7")
{
//....
and declare and implement something like
// return 0 when theString matches at least one patX
// else return -1
int compare(const std::string& theString,
const char* pat0, // no default, must provide
const char* pat1 = nullptr,
const char* pat2 = nullptr,
const char* pat3 = nullptr,
const char* pat4 = nullptr
/* add as many patX as you desire */)
{
if (0 == theString.compare(pat0)) return 0; // found a match
//
// ignore nullptr patterns
if (nullptr != pat1) && (0 == theString.compare(pat1)) {
return(0);
}
if(nullptr != pat2) && (0 == theString.compare(pat2)) {
return(0);
}
// ...
// when get to end with no match
return (-1); // indicate no match to any of patterns
}
I actually prefer the following. The above is somewhat like strstr(), where this uses more functionality of std::string
int compare(const std::string& theString, // what you are given
const std::string& patterns) // concatenated list of search patterns
{
//..
}
This you invoke as
if (0 == compare(theString, "Seven seven SEVEN 7")
{
//
The implementation must tease apart space delimited patterns .. but this is not difficult, and can easily be implemented in a loop, so no limit to how many comparisons you wish to test.
When should you consider creating a new function in support of a new derived rerquirement?
It is my practice to create the new function when I can identify 3 or more uses. Good luck.
update 2017-04-22
Found some code I wrote a few years ago, fixed it up, added demos ...
Code compiles and seems to run, but very little testing.
- I created minimal packaging -- a dummy namepace (dtb - for d___'s tool box), and a dumy class (T471_t - test 471)
dtb::T471_t provides private methods for your review.
size_t compare(const std::string s, std::string patterns)
size_t grep(const std::string pfn, const std::string patterns, std::ostream& an_ostream = std::cout)
-- uses compare()
size_t cpuinfoGet()
-- uses grep
-- "wc < /proc/cpuinfo" (a 'well known file' on ubuntu) reports 54 lines on my 2 core machine, more cores, more lines
size_t coreCountGet()
-- uses grep()
-- creates nullDev to suppress grep normal output
#include <chrono>
#include <fstream>
#include <iostream>
#include <sstream>
#include <string>
#include <cassert>
// 'compressed' chrono access --------------vvvvvvv
typedef std::chrono::high_resolution_clock HRClk_t; // std-chrono-hi-res-clk
typedef HRClk_t::time_point Time_t; // std-chrono-hi-res-clk-time-point
typedef std::chrono::microseconds US_t; // std-chrono-microseconds
using namespace std::chrono_literals; // support suffixes like 100ms
// examples:
//
// Time_t start_us = HRClk_t::now();
//
// auto duration_us = std::chrono::duration_cast<US_t>(HRClk_t::now() - start_us);
// auto count_us = duration_us.count();
// or
// std::cout << " complete " << duration_us.count() << " us" << std::endl;
namespace dtb
{
class T471_t
{
const std::string dashLine = (" --------------------------------------------------------------");
public:
T471_t() = default;
~T471_t() = default;
int exec()
{
std::cout << "\n cpuinfoGet()\n" << dashLine << std::endl;
(void)cpuinfoGet(); // uses grep which uses compare
std::cout << dashLine << std::endl;
// count of lines which contain "processor" in linux file "/proc/cpuinfo"
std::cout << "\n\n " << coreCountGet()
<< " cores on this system. (coreCountGet())\n\n" << std::endl;
return(0);
}
private: // methods
// returns std::string::npos when none of the patterns found,
// else returns index of earliest found patterns of space delimited substr in
size_t compare (const std::string& s,
std::string patterns) // pass by value
{
size_t found = std::string::npos;
size_t patCount = 0;
std::stringstream ssPat(patterns + ' '); // load patterns into ram based stringstream
// ^^^^^ -- getline() can cause eof() in this
const char delim = ' '; // see getline()
do
{
if(0 == patterns.size()) break; // no patterns to search for, kick out
if(0 == s.size()) break; // no string in which to search, kick out
do {
std::string pat;
(void)std::getline(ssPat, pat, delim); // extract 1 space delimited pattern
if(false == ssPat.good())
{
if(ssPat.eof()) break; // quitely exit, a normal op
// let user know of patten problem
std::cerr << "\n err pattern extract: " << patterns
<< " (" << patCount << ')' << std::endl;
break;
}
patCount += 1;
//trimLeadingWhiteSpace(patterns); // tbr
//trimTrailingWhiteSpace(patterns); // tbr
if(0 == patterns.size()) break; // no more patterns
// search s for pat
found = s.find(pat);
if(found != std::string::npos) break; // one of the patterns found in s
} while(1); // repeat until 1 found or all patterns tried
}while(0);
return(found);
} // size_t compare (const std::string& s, std::string patterns)
size_t grep(const std::string pfn,
const std::string patterns, // concatenated list of search patterns
std::ostream& an_ostream = std::cout) // redirectable
{
size_t foundCount = 0;
an_ostream << " grep (" << pfn << ", [" << patterns
<< "] )" << std::endl;
do
{
std::ifstream infile(pfn);
if(!infile.is_open())
{
an_ostream << pfn << " not found.\n" << std::endl; // tbr - std::cerr?
break; // skip over file op's (no close needed)
}
do
{
if(infile.eof()) break; // file empty?
std::string lineIn;
(void)getline(infile, lineIn); // default delimiter is \n
if (0 == lineIn.size()) continue; // empty line?
size_t found = compare(lineIn, patterns); // any of patterns in lineIn?
if(std::string::npos != found) // found at least one pattern
{
an_ostream << " " << lineIn << std::endl; // found it, print it
foundCount += 1;
}
// else no pattern found - continue until eof of inFil
} while(1);
infile.close();
}while(0);
return(foundCount);
} // size_t grep(const std::string pfn, const std::string patterns, std::ostream& an_ostream = std::cout)
// space delimited list of ---^^^^^^^^
size_t cpuinfoGet()
{
size_t count = grep("/proc/cpuinfo", // pfn
"bogomips model name processor"); // patterns to search for
std::cout << "\n info lines: " << count << std::endl;
return(count);
} // size_t cpuinfoGet(void)
size_t coreCountGet()
{
// create a no-ouptput output
std::ofstream nullDev; // and do not open
nullDev.setstate(std::ios_base::badbit); // set error, ignore errors, do not close
size_t retVal = grep(std::string("/proc/cpuinfo"),
std::string("processor"), // line count of "processor" is core count
nullDev); // nullDev -- no output
return(retVal);
} // size_t coreCountGet()(void)
}; // class T471_t
} // namespace dtb
int main(int /*argc*/, char** /*argv[]*/)
{
Time_t start_us = HRClk_t::now();
int retVal = -1;
{
dtb::T471_t t471;
retVal = t471.exec();
}
auto duration_us = std::chrono::duration_cast<US_t>(HRClk_t::now() - start_us);
std::cout << " FINI " << duration_us.count() << " us" << std::endl;
return(retVal);
}
Output:
cpuinfoGet()
--------------------------------------------------------------
grep (/proc/cpuinfo, [bogomips model name processor] )
processor : 0
model : 75
model name : AMD Athlon(tm) 64 X2 Dual Core Processor 5000+
bogomips : 4809.67
processor : 1
model : 75
model name : AMD Athlon(tm) 64 X2 Dual Core Processor 5000+
bogomips : 4809.67
info lines: 8
--------------------------------------------------------------
2 cores on this system. (coreCountGet())
FINI 829 us
Sometimes, data can be a better solution than code.
std::map<std::string, int> values;
values["Seven"]=7;
values["seven"]=7;
values["7"]=7;
values["Six"]=6;
values["six"]=6;
values["6"]=6;
std::string input;
std::cin >> input;
std::cout << values[input];
As Vlad noted, you might be better of converting to lowercase first. This answer is just the straightforward conversion of your code to data. Note that this answer will use a default value of 0 for missing strings; your code left out such a default.
来源：https://stackoverflow.com/questions/43484761/c-multiple-strings-inside-an-if-statement