How do I compare two C-string pointers?

Question

I am trying to write code to check whether two C-string variables are the same barring the difference of capitalization. Given the two input books BOOKS, the program should return 1, and with Incorrect correct, it should return 0. My code doesn't print it accurately.

#include <iostream>
#include <cctype>
#include <cstring>
using namespace std;
int Judge_Char(const char* str1,const char* str2){
    char first[20],second[20];
    for(int i=0;i<20;i++){
        first[i]=str1[i];
        second[i]=str2[i]; //assigned pointers to variable
    }                      //bc I didn't know other ways to compare

    int k=0,l=0;
    for(k=0;first[k]!='\0';k++);
    for(l=0;second[l]!='\0';l++); //got the length of the chars here


    for(int i=0;i<k;i++){
        first[i]=toupper(first[i]);
    }                             //i converted them to same case here
    for(int i=0;i<l;i++){
        second[i]=toupper(second[i]);
    }

    for(int n=0;n<k;n++){
        for(int m=0;m<l;m++){
            if(first[n]==second[m]){
                return 1;           // i check whether they are same or not
            }
            else{
                return 0;
            }
        }
    }
}


int main()
{
    char a[20],b[20];
    cin>>a>>b;
    int flag=Judge_Char(a,b);
    cout<<flag<<endl;
    return 0;
}

Please call them c-strings. CString is an unrelated C++ class. learn.microsoft.com/en-us/cpp/atl-mfc-shared/… — Mooing Duck
– Mooing Duck, Commented Jun 4, 2020 at 15:22
You should not compare pointers. You should compare the data that the pointers point to. — Thomas Matthews
– Thomas Matthews, Commented Jun 4, 2020 at 16:53

Bathsheba · Accepted Answer · 2020-06-04 15:37:01Z

3

Two things annoy me about C++:

There is no standard definition for pi (yet we have an extremely sophisticated random number library some of which requires a value for pi to be defined for it to be implementable).
There is no standard way of comparing two strings on a case-insensitive basis. A subset of the general case compare problem is defined by isupper, islower, toupper, and tolower. Yes indeed a full locale-aware case-insensitive compare is beyond the scope of the C++ standard library, but one could be conceived in simpler terms.

In Windows you use ::_stricmp and #include <string.h>

In Unix you use strcasecmp and #include <strcasecmp>

An example where it's useful, even if it doesn't support the German SS, Norwegian Slashed o, &c. &c.

struct iLT
{
    bool operator()(const std::string& lhs, const std::string& rhs) const {
        return strcasecmp(lhs.c_str(), rhs.c_str()) < 0;
    }
};

typedef std::map<std::string, double, iLT> MapWithCaseInsensitiveKeys;

edited Jun 4, 2020 at 15:37

answered Jun 4, 2020 at 15:25

Bathsheba

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10 Comments

Konrad Rudolph Over a year ago

toupper and tolower are not actually sufficient (or indeed useful) for implementing a correct case insensitive comparison.

Bathsheba Over a year ago

@KonradRudolph: Go on, spill the beans?

NathanOliver Over a year ago

C++20 adds std::pi_v

Mark Ransom Over a year ago

C++ is fundamentally incapable of doing toupper and tolower properly on Unicode.

Konrad Rudolph Over a year ago

@Bathsheba The functions are not bijective. For instance, correct_stricmp("straße", "STRASSE") == 0, a fact which can’t be expressed in terms of case conversion.

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