C++ While loop not working correctly - c++

Question

I am using the following code to break apart a line of text.
A line of text, such as
"adduser john -u 2001 -g 1002 -p john123 -c Project Work"
is passed to the constructor.
I am trying to separate the text by the commands "-u, -g, -p, -c"
"john" will be saved as name,
"2001" will be saved as UID,
"1002" will be saved as GID, etc.
AccountInfo::AccountInfo(char* line){
_line = line;
char bufferLine[256];
unsigned int length1 = 0;
unsigned int tempLength1 = 0;
//find length of line of text
while (_line[length1] != '\0'){
length1++;
}
//separate the text by white space
// start at 8 because adduser is not a command
for (int i = 8; i < length1 + 1; i++){
bufferLine[tempLength1 + 1] = '\0';
printf(bufferLine);
if (_line[i] == ' ')
{
if (bufferLine[0] == '-')
{
//test only u and c commands for now
if (bufferLine[1] == 'u'){
bufferLine[0] = '\0'; //clear contents of array
tempLength1 = 0;
while (!_line[i] == '-'){
bufferLine[tempLength1] = _line[i];
i++;
tempLength1++;
}
bufferLine[tempLength1] = '\0';
printf(bufferLine);
printf("\n This is UID \n");
setUID((unsigned int)bufferLine);
}
else if (bufferLine[1] == 'c'){
bufferLine[0] = '\0';
tempLength1 = 0;
while (!_line[i] == '\0'){
bufferLine[tempLength1] = _line[i];
i++;
tempLength1++;
}
bufferLine[tempLength1] = '\0';
printf(bufferLine);
printf("\n this is gecos \n");
setGecos(bufferLine);
}
}
else{
//is name
bufferLine[tempLength1] = '\0';
setName(bufferLine);
printf("\n I am the user's name\n");
printf(bufferLine);
printf("\n");
}
bufferLine[0] = '\0'; //reset buffer line
tempLength1 = 0; // reset incrementation for buffer line
}
else{
bufferLine[tempLength1] = _line[i];
tempLength1++;
}
}
}
I am working on functionality of the -u command. It reaches the
if (bufferLine[1] == 'u') portion of the code, but always bypasses the while loop following it
while (!_line[i] == '-') I have tried changing the '-' to exit the while loop with ' ' and even any letter 'a' 'g' 'd', however nothing works except '\0'. This is only acceptable for the -c command, as everything after it should be printed.
The -c command works as it should, as does saving the name. However, none of the other commands will work because it does not make it to the loop.
I have been thinking about this problem for a while and I believe it should work, however the code never makes it through the while loop. Is there a problem with the syntax? Or is there something I am missing about nested loops?
Thanks

Answer 1

This expression
while (!_line[i] == '-'){
evaluates as (see operator precedence):
while ((!(_line[i])) == '-') { // because ! has higher precedence than ==
while (false == '-') { // because _line[i] == ' ', !' ' is false
while (false) {
You probably meant
while (_line[i] != '-') {

Answer 2

while (!_line[i] == '-') is essentially while ((!_line[i]) == '-') (note the operators precedence).
What you possibly want is while (! (_line[i] == '-')) { or, simpler, while (_line[i] != '-'){.

Answer 3

!_line[i] == '-'
! has lower precedence than == and is evaluated as
(!_line[i]) == '-'
and this will never be true
http://en.cppreference.com/w/cpp/language/operator_precedence