# 15.6. Parsing numbers¶

The next task is to convert the numbers in the file from strings to integers. When people write large numbers, they often use commas to group the digits, as in 1,750. Most of the time when computers write large numbers, they don’t include commas, and the built-in functions for reading numbers usually can’t handle them. That makes the conversion a little more difficult, but it also provides an opportunity to write a comma-stripping function, so that’s ok. Once we get rid of the commas, we can use the library function atoi to convert to integer. atoi is defined in the header file cstdlib.

To get rid of the commas, one option is to traverse the string and check whether each character is a digit. If so, we add it to the result string. At the end of the loop, the result string contains all the digits from the original string, in order.

 #include <iostream>
using namespace std;

int convertToInt (const string& s)
{
string digitString = "";

for (size_t i = 0; i < s.length(); i++) {
if (isdigit (s[i])) {
digitString += s[i];
}
}
return atoi (digitString.c_str());
}


The variable digitString is an example of an accumulator. It is similar to the counter we saw in Section [loopcount], except that instead of getting incremented, it gets accumulates one new character at a time, using string concatentation.

The expression

digitString += s[i];


is equivalent to

digitString = digitString + s[i];


Both statements add a single character onto the end of the existing string.

Since atoi takes a C string as a parameter, we have to convert digitString to a C string before passing it as an argument.

Try the function out for yourself! As you can see, this function can also be used to parse phone numbers!

Create the replace_with() function that takes a string “str”, a character to get rid of “olc_char”, and a character to replace it with “new_char”. It should return a new string that has replaces any occurances of old_char with new_char.