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

Question

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;
}

http://ideone.com/53laPq All fine for me. You're probably calling it by providing a string literal, don't do this. That's read-only memory. — Hatted Rooster, Nov 17 '16 at 15:02
Not directly related to your problem: don't use magic numbers. `65` -> `'A'`, `90` -> `'Z'` etc. — Jabberwocky, Nov 17 '16 at 15:07
Possible duplicate of [Why is this string reversal C code causing a segmentation fault?](http://stackoverflow.com/questions/1614723/why-is-this-string-reversal-c-code-causing-a-segmentation-fault) — Jabberwocky, Nov 17 '16 at 15:10

score 3 · Accepted Answer · edited May 23 '17 at 12:24

3

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?

edited May 23 '17 at 12:24

Community

1
1

answered Nov 17 '16 at 15:09

Marco A.

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it works can you please send me a guide on the whole `*` meaning? – Rokni Nov 17 '16 at 15:11
@Rokni Added a link in case you want to learn more about string literals and why they're read-only – Marco A. Nov 17 '16 at 15:13

score 2 · Answer 2 · answered Nov 17 '16 at 15:10

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);

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

2 Answers2