I am new to C++ (coming from a C# background) and am trying to learn how to convert a string to an int.
I got it working by using a stringstream and outputting it into a double, like so:
const char* inputIndex = "5+2";
double number = 0;
stringstream ss(inputIndex);
ss >> number;
// number = 5
This works great. The problem I'm having is that the strings I'm parsing start with a number, but may have other, not digit characters after the digits (e.g. "5+2", "9-(3+2)", etc). The stringstream parses the digits at the beginning and stops when it encounters a non-digit, like I need it to.
The problem comes when I want to know how many characters were used to parse into the number. For example, if I parse 25+2, I want to know that two characters were used to parse 25, so that I can advance the string pointer.
So far, I got it working by clearing the stringstream, inputting the parsed number back into it, and reading the length of the resulting string:
ss.str("");
ss << number;
inputIndex += ss.str().length();
While this does work, it seems really hacky to me (though that might just be because I'm coming from something like C#), and I have a feeling that might cause a memory leak because the str() creates a copy of the string.
Is there any other way to do this, or should I stick with what I have?
Thanks.
You can use std::stringstream::tellg() to find out the current get position in the input stream. Store this value in a variable before you extract from the stream. Then get the position again after you extract from the stream. The difference between these two values is the number of characters extracted.
double x = 3435;
std::stringstream ss;
ss << x;
double y;
std::streampos pos = ss.tellg();
ss >> y;
std::cout << (ss.tellg() - pos) << " characters extracted" << std::endl;
The solution above using tellg() will fail on modern compilers (such as gcc-4.6).
The reason for this is that tellg() really shows the position of the cursor, which is now out of scope. See eg "file stream tellg/tellp and gcc-4.6 is this a bug?"
Therefore you need to also test for eof() (meaning the entire input was consumed).
Related
An input file is entered with the following data:
Juan Dela Cruz 150.50 5
'Juan Dela Cruz' is a name that I would like to assign to string A,
'150.50' is a number I would like to assign to float B
and 5 is a number I would like to assign to int C.
If I try cin, it is delimited by the spaces in between.
If I use getline, it's getting the whole line as a string.
What would be the correct syntax for this?
If we analyze the string, then we can make the following observation. At the very end, we have an integer. In front of the integer we have a space. And in front of that the float value. And again in fron of that a space.
So, we can simply look from the back of the string for the 2nd last space. This can easily be achieved by
size_t position = lineFromeFile.rfind(' ', lineFromeFile.rfind(' ')-1);
We need a nested statement of rfind please see here, version no 3.
Then we build a substring with the name. From start of the string up to the found position.
For the numbers, we put the rest of the original string into an std::istringstream and then simply extract from there.
Please see the following simple code, which has just a few lines of code.
#include <iostream>
#include <string>
#include <cctype>
#include <sstream>
int main() {
// This is the string that we read via getline or whatever
std::string lineFromeFile("Juan Dela Cruz 150.50 5");
// Let's search for the 2nd last space
size_t position = lineFromeFile.rfind(' ', lineFromeFile.rfind(' ')-1);
// Get the name as a substring from the original string
std::string name = lineFromeFile.substr(0, position);
// Put the numbers in a istringstream for better extraction
std::istringstream iss(lineFromeFile.substr(position));
// Get the rest of the values
float fValue;
int iValue;
iss >> fValue >> iValue;
// Show result to use
std::cout << "\nName:\t" << name << "\nFloat:\t" << fValue << "\nInt:\t" << iValue << '\n';
return 0;
}
Probably simplest in this case would be to read whole line into string and then parse it with regex:
const std::regex reg("\\s*(\\S.*)\\s+(\\d+(\\.\\d+)?)\\s+(\\d+)\\s*");
std::smatch match;
if (std::regex_match( input, match, reg)) {
auto A = match[1];
auto B = std::stof( match[2] );
auto C = std::stoi( match[4] );
} else {
// error invalid format
}
Live example
As always when the input does not (or sometimes does not) match a strict enough syntax, read the whole line and then apply the rules which to a human are "obvious".
In this case (quoting comment by john):
Read the whole string as a single line. Then analyze the string to work out where the breaks are between A, B and C. Then convert each part to the type you require.
Specifically, you probably want to use reverse searching functions (e.g. https://en.cppreference.com/w/cpp/string/byte/strrchr ), because the last parts of the input seem the most strictly formatted, i.e. easiest to parse. The rest is then the unpredictable part at the start.
either try inputting the different data type in different lines and then use line breaks to input different data types or use the distinction to differentiate different data types like adding a . or comma
use the same symbol after each data package, for example, Juan Dela Cruz;150.50;5 then you can check for a ; and separate your string there.
If you want to use the same input format you could use digits as an indicator to separate them
Ok, just to be up front, this IS homework, but it isn't due for another week, and I'm not entirely sure the final details of the assignment. Long story short, without knowing what concepts he'll introduce in class, I decided to take a crack at the assignment, but I've run into a problem. Part of what I need to do for the homework is read individual characters from an input file, and then, given the character's position within its containing word, repeat the character across the screen. The problem I'm having is, the words in the text file are single words, each on a different line in the file. Since I'm not sure we'll get to use <string> for this assignment, I was wondering if there is any way to identify the end of the line without using <string>.
Right now, I'm using a simple ifstream fin; to pull the chars out. I just can't figure out how to get it to recognize the end of one word and the beginning of another. For the sake of including code, the following is all that I've got so far. I was hoping it would display some sort of endl character, but it just prints all the words out run together style.
ifstream fin;
char charIn;
fin.open("Animals.dat");
fin >> charIn;
while(!fin.eof()){
cout << charIn;
fin >> charIn;
}
A few things I forgot to include originally:
I must process each character as it is input (my loop to print it out needs to run before I read in the next char and increase my counter). Also, the length of the words in 'Animals.dat' vary which keeps me from being able to just set a number of iterations. We also haven't covered fin.get() or .getline() so those are off limits as well.
Honestly, I can't imagine this is impossible, but given the restraints, if it is, I'm not too upset. I mostly thought it was a fun problem to sit on for a while.
Why not use an array of chars? You can try it as follow:
#define MAX_WORD_NUM 20
#define MAX_STR_LEN 40 //I think 40 is big enough to hold one word.
char words[MAX_WROD_NUM][MAX_STR_LEN];
Then you can input a word to the words.
cin >> words[i];
The >> operator ignores whitespace, so you'll never get the newline character. You can use c-strings (arrays of characters) even if the <string> class is not allowed:
ifstream fin;
char animal[64];
fin.open("Animals.dat");
while(fin >> animal) {
cout << animal << endl;
}
When reading characters from a c-string (which is what animal is above), the last character is always 0, sometimes represented '\0' or NULL. This is what you check for when iterating over characters in a word. For example:
c = animal[0];
for(int i = 1; c != 0 && i < 64; i++)
{
// do something with c
c = animal[i];
}
This might have appeared before, but I couldn't understand how to extract formatted data. Below is my code to extract all text between string "[87]" and "[90]" in a text file.
Apparently, the position of [87] and [90] is the same as indicated in the output.
void ExtractWebContent::filterContent(){
string str, str1;
string positionOfCurrency1 = "[87]";
string positionOfCurrency2 = "[90]";
size_t positionOfText1, positionOfText2;
ifstream reading;
reading.open("file_Currency.txt");
while (!reading.eof()){
getline (reading, str);
positionOfText1 = str.find(positionOfCurrency1);
positionOfText2 = str.find(positionOfCurrency2);
cout << "positionOfCurrency1 " << positionOfText1 << endl;
cout << "positionOfCurrency2 " << positionOfText2 << endl;
//str1= str.substr (positionOfText);
cout << "String" << str1 << endl;
}
reading.close();
An Update on the currency file:
[79]More »Brent slips to $102 on worries about euro zone economy
Market Data
* Currencies
CAPTION: Currencies
Name Price Change % Chg
[80]USD/SGD
1.2606 -0.00 -0.13%
USD/SGD [81]USDSGD=X
[82]EUR/SGD
1.5242 0.00 +0.11%
EUR/SGD [83]EURSGD=X
That really depends on what 'extracting data means'. In simple cases you can just read the file into a string and then use string member functions (especially find and substr) to extract the segment you are interested in. If you are interested in data per line getline is the way to go for line extraction. Apply find and substr as before to get the segment.
Sometimes a simple find wont get you far and you will need a regular expression to do easily get to the parts you are interested in.
Often simple parsers evolve and soon outgrow even regular expressions. This often signals time for the very large hammer of C++ parsing Boost.Spirit.
Boost.Tokenizer can be helpful for parsing out a string, but it gets a little trickier if those delimiters have to be bracketed numbers like you have them. With the delimieters as described, a regex is probably adequate.
All that does is concatenate the output of reading and the strings "[1]" and "[2]". I'm guessing this code resulted from a rather literal extrapolation of similar code using scanf. scanf (as well as the rest of C) still works in C++, so if that works for you I would use it.
That said, there are various levels of sophistication at which you can do this. Using regexes is one of the most powerful/flexible ways, but it might be overkill. The quickest way in my opinion is just to do something like:
Find index of substring "[1]", i1
Find index of substring "[2]", i2
get substring between i1+3 and i2.
In code, supposing std::string line has the text:
size_t i1 = line.find("[1]");
size_t i2 = line.find("[2]");
std::string out(line.substr(i1+3, i2));
Warning: no error checking.
I am looking into code at work. I am having following code. In following code what is the meaning of the last statement?
bOptMask = true;
std::string strMask;
strMask.append(optarg);
std::stringstream(strMask) >> std::hex >> iMask >> std::dec;
In addition to the above question: I have string input and I need to know how to convert it to an integer using C++ streams as above instead of atoi().
The problem I am facing is if I give input
strOutput.append(optarg);
cout << "Received option for optarg is " << optarg << endl;
std::stringstream(strOutput) >> m_ivalue ;
cout << "Received option for value is " << m_ivalue << endl;
For the above code, if I am running with argument "a" I am having an output with first line as "a" and a second line output as 0. I am not sure why, can any one explain?
The last statement creates a temporary stringstream and then uses it to parse the string as hexadecimal format into iMask.
There are flaws with it though, as there is no way to check that the streaming succeeded, and the last stream achieves nothing as you are dealing with a temporary.
Better would be to create the stringstream as a non-temporary, ideally using istringstream as you are only using it to parse string to int, and then checking whether the conversion succeeds.
std::istringstream iss( strMask );
iss >> std::hex;
if(!( iss >> iMask ))
{
// handle the error
}
You only need to set the mode back to decimal if your stringstream is now about to parse a decimal integer. If it is going to parse more hex ones you can just read those in too, eg if you have a bunch of them from a file.
How you handle errors is up to you.
std::hex and std::dec are part of the <iomanip> part of streams that indicate the way text should be formatted. hex means "hexadecimal" and dec means "decimal". The default is to use decimal for integers and hexadecimal for pointers. For reasons unknown to me there is no such thing as a hex representation for printing float or double, i.e. no "hexadecimal point" although C99 sort-of supports it.
The code takes the string optarg and, treating it as hex, converts it to an integer and stores it in iMask.
If you remove the std::hex modifier you can parse the input as decimal. However, I usually use boost's lexical_cast for this. For example:
int iMask = boost::lexical_cast< int >( strMask );
This code uses manipulators to set the stream to expect integers to be read in base 16 (hexadecimal, using the digits 0123456789ABCDEF), then extracts a hexadecimal number from the string, storing it in iMask, and uses another manipulator to set the string stream back to the default of expecting integers to be written in decimal form.
I have a program that allows the user to enter a level number, and then it plays that level:
char lvlinput[4];
std::cin.getline(lvlinput, 4)
char param_str[20] = "levelplayer.exe "
strcat_s(param_str, 20, lvlinput);
system(param_str);
And the level data is stored in folders \001, \002, \003, etc., etc. However, I have no way of telling whether the user entered three digits, ie: 1, 01, or 001. And all of the folders are listed as three digit numbers. I can't just check the length of the lvlinput string because it's an array, so How could I make sure the user entered three digits?
Why not use std::string?
This makes storage, concatenation, and modification much easier.
If you need a c-style string after, use: my_string.c_str()
Here is a hint: To make your input 3 characters long, use std::insert to prefix your number with 0's.
You are really asking the wrong question. Investigate the C++ std::string class and then come back here.
Eh? Why do they need to enter 3 digits? Why not just pad it if they don't? If you really want to check that they entered 3 digits, use strlen. But what I recommend you do is atoi their input, and then sprintf(cmd, "levelplayer.exe %03d", lvlinput_as_integer)
Here's how you could do this in C++:
std::string lvlinput;
std::getline(std::cin, lvlinput);
if (lvlinput.size() > 3) { // if the input is too long, there's nothing we can do
throw std::exception("input string too long");
}
while (lvlinput.size() < 3) { // if it is too short, we can fix it by prepending zeroes
lvlinput = "0" + lvlinput;
}
std::string param_str = "levelplayer.exe ";
param_str += lvlinput;
system(param_str.c_str());
You've got a nice string class which takes care of concatenation, length and all those other fiddly things for you. So use it.
Note that I use std::getline instead of cin.getline. The latter writes the input to a char array, while the former writes to a proper string.
What do you mean you can't check the length of the string? getline generates a NULL terminated c-string so just use strlen(lvlinput).
Neil told you where you should start, your code might look like this.
std::string level, game = "levelplayer.exe ";
std::cout << "Enter the level number : ";
std::cin >> level;
if(level.size() != 3)
{
// Error!
}
else
{
// if you have more processing, it goes here :)
game += level;
std::system(game.c_str());
}
You can check the length of your NULL terminated string that getline returns by using:
int len = strlen(lvlinput);
This works because getline returns a NULL-terminated string.
However, this is besides the point to your problem. If you want to stay away from std::string (and there isn't any particular reason why you should in this case), then you should just convert the string to an integer, and use the integer to construct the command that goes to the system file:
char lvlinput[4];
std::cincin.getline(lvlinput, 4);
char param_str[20];
snprintf(param_str, 20, "levelplayer.exe %03d", atoi(lvlinput));
system(param_str);