Question

I’m using the book “Programming Principles and Practice using C++” to learn programming and one of the exercises is looping through the characters a-z using a while-loop.

Now, I have programmed with C++ and other languages before, so I decided to try to use as few lines as possible (while still using a while-loop). However, this came with the cost of my output being kind of messed up.

Code:

#include <iostream>
int main(){
    char ch = 'a';
    while(ch <= 'z')
        std::cout << ch << '\t' << (int) ch++ << std::endl;
    return 0;
}

Output:

b   97
c   98
d   99
e   100
...
x   119
y   120
z   121
{   122

Now I do realize that this could’ve been done with a for-loop instead, and I did that as well (it worked). But I still don’t get what’s wrong with this code, and it’s really annoying me.

It appears as if I’ve told it to output “ch+1”, since it prints out ‘b’ where it should print ‘a’. The incrementing isn’t done until after the integer value of ch has been put into the out-stream (post-increment). And even if I had incremented it earlier, at least the printed characters and their integer values should correspond.

Any ideas of why this code isn’t working?

Answer 1

Short answer is that you have a bit of Undefined Behavior because you’re both modifying and using the value of the variable ch, via separate sub-expressions, in the same expression.

The Right Thing(TM) to do here is to use the most practical loop for the task at hand:

#include <iostream>
int main(){
    for(char ch = 'a'; ch <= 'z'; ++a){
        std::cout << ch << '\t' << ch+0 << std::endl;
    }
}