help with palindrome program

[gethelpcpp123]

I posted yesterday my program for the palindrome and it consisted of the last parts because I was only checking whether a sentence is a palindrome or not.However, I have to check if a whole sentence is a palindrome or not,like:Madam I'm adam..ignoring spaces and punctutations.The basic idea is to copy the first char array into another by using 2 nested for loops as shown here. However, I'm not getting correct results.why?
void main()
{
cout<<"Enter the sentence you want to check if it is palindrome or not"<<endl;

char sentence1[100];
char sentence[100];

cin.getline(sentence1,100);


int length1=strlen(sentence1);
bool palindrome=true;
int j=0;
for(int i=0;i<length1;i++)
{
if(isalpha(sentence1[i]))//if the charecter is alphanumeric then copy it to other array
{for(j;j<length1;j++)
sentence[j]=sentence1[i];
}
}
int length=strlen(sentence);

for (int j=0;j<(length/2);j++)
{
if(toupper(sentence[j])!=tolower(sentence[length-j-1]))
{
palindrome=false;
break;
}
else
palindrome=true;
}
if(palindrome==true)
cout<<"This is a palindrome"<<endl;
else
cout<<"This is not a palindrome"<<endl;

}

[unkn00wn]

use "string" and ignore white spaces !

[gethelpcpp123]

how? i should alsoshould ignore punctuations

[gethelpcpp123]

i think the compiler is not really knowing what is the length of sentence.I mean it might be taking it as 100 not the real length

[gethelpcpp123]

please help me.