CODEWITHSUNDEEP
c++char
Rokni
Rokni

Reputation: 65

c++ How to change a character in char* in for loop

Hey guys i got an assignment that i cant seem to solve. I get a char* string and i need to make all upper case letters (everything is in ASCII) to lower case using bit operations. I'm adding my code but it keeps crashing.

#include <iostream>
#include <cstring>

using namespace std;

void convertToLower(char* string)
{
   for (unsigned int i = 0; i < strlen(string); i++)
   {
       if (string[i] >= 65 && string[i] <= 90)
       {
           string[i] |= 32;
       }
   }
   cout << string << endl;
}
int main()
{
   convertToLower("Hello");
   return 0;
}

Upvotes: 0

Views: 1911

Answers (2)

Marco A.
Marco A.

Reputation: 43662

You're trying to modify a string literal i.e. read-only memory: that's the reason for your crash (or undefined behavior to be more precise)

void convertToLower(char* str)
{
   for (unsigned int i = 0; i < strlen(str); i++)
   {
       if (str[i] >= 65 && str[i] <= 90)
       {
           str[i] |= 32;
       }
   }
   cout << str << endl;
}

int main() {

    char arr[] = "STRING";
    convertToLower(arr); // Fine

    char *readonly = "READONLY";
    convertToLower(readonly); // Nope
}

I suppose a recent compiler should also have warned you of this (Wwritable-strings).

Read more about this issue here: Why are string literals const?

Upvotes: 3

Hatted Rooster
Hatted Rooster

Reputation: 36503

convertToLower("Hello");

"Hello" is of type const char[]. Conversion from const char* to char* has been illegal since C++11 so your compiler should've told you something about that if you're using a modern one, at least a warning. You can't overwrite read-only memory (const). Attempting to do so leads to undefined behaviour. Make a copy of it and pass that in:

char copy[] = "Hello";
convertToLower(copy);

Upvotes: 2

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