invalid conversion from ‘const char*’ to ‘char’

Solution 1:

use single quotes

str[ i ] = ' ';

In C++, the token " " is a string literal which represents an array of two characters: the value of a space in the character set (eg, the value 32 in ascii) and a zero. On the other hand, the token ' ' represents a single character with the value of a space (usually 32). Note that in C, the token ' ' represents an integer with the value of a space. (In C, sizeof ' ' == sizeof(int), while in C++, sizeof ' ' == sizeof(char) == 1.)

Solution 2:

Single char literals are obtained with single quotes:

str[i] = ' ';

A literal with double-quotes is a full string literal (a null-terminated array of char), but you're only replacing a single char.