我需要 C++ 变更案例的帮助

Question

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string sentence =""; 
    cin >> sentence; //aab
    int i;
    
    for (i=0;i=sentence.length();i++){ 
        if (i<=65 && i>=90) {
            sentence = sentence[i] + 32;
        }
        else if (i<=97 && i>=122){                //i=0,
            sentence = sentence [i]-32;
        }
        
        
    }

    cout << sentence;
    return 0;
}

When I enter this code for changing cases of letters it keeps asking me to enter more although I have only one cin in the code why does that happen?

Answer 1

Problem one is inadvertent assignment. Look at your loop condition:

for (i=0;i=sentence.length();i++)

That assigns i rather than comparing it, resulting in an infinite loop. Use < instead of = :

for (i=0; i < sentence.length(); i++)

Problem two is you're comparing the position in the string to the character ranges rather than the character itself, and the comparison is backwards and can never be true:

if (i<=65 && i>=90)

Should be:

if (sentence[i] >= 65 && sentence[i] <= 90)

Same for the lower case range.

Finally, you don't want to change the whole sentence to one character, just that character:

sentence = sentence[i] + 32;

Should be:

sentence[i] = sentence[i] + 32;

Again, same for the lower case range.

With these changes, it seems to work, at least for single words. If you want to do entire sentences, I'd recommend using std::getline(std::cin, sentence); rather than cin >> sentence; .