Why am I not getting desired output when using fast scan function?

Question

I read in geeksforgeeks and added the template to it so that it can work for every integer data types such as long, long long and int.

#include <bits/stdc++.h>
using namespace std;

template<typename T>
void fs(T& x)     // For faster scanning of input
{  
    int n = 1;
    char c = getchar();
    x = 0;
    for (; (c < 48 || c>57) && c != '-'; c = getchar());
    if (c == '-') {
        n = -1;
        c = getchar();
    }
    for (; (c < 47 && c < 58); c = getchar())
        x = (x << 1) + (x << 3) + c - 48;
    x = n * x;
}

int main() 
{
    int test;
    fs(test);
    cout << test;
    return 0;
}

But when I tried executing it, the output is shown as 0 and not the input number. Is the above code wrong?

Unrelated: [What is a magic number, and why is it bad?](https://stackoverflow.com/questions/47882/what-is-a-magic-number-and-why-is-it-bad) — user4581301, Jun 30 '19 at 06:01
Notice that `` is *not* a [standard C++ header](https://en.cppreference.com/w/cpp/header) (just an ugly implementation detail). So don't use it. Also, read [*How to debug small programs*](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) — Basile Starynkevitch, Jun 30 '19 at 06:07
Please get a [good C++ book](https://stackoverflow.com/a/388282/) and start reading if you want to learn C++ systematically. — L. F., Jun 30 '19 at 09:41

bruno · Accepted Answer · 2019-06-30T14:53:08.687

4

the output is shown as 0 and not the input number.

the test in

for (;(c<47 && c<58);c=getchar())

is not the right one and must be

for (;(c>47 && c<58);c=getchar())
//      ^

Because of the error when you enter for instance 123 none of the for do something and x stay equals to 0.

After the correction, compilation and execution (also adding <<endl in the cout):

pi@raspberrypi:/tmp $ g++ -pedantic -Wall -Wextra cc.cc
pi@raspberrypi:/tmp $ ./a.out
123
123
pi@raspberrypi:/tmp $ ./a.out
-123
-123
pi@raspberrypi:/tmp $

Out of that to use the codes is not readable nor portable, better to do

for(;(c<'0' || c>'9') && c!='-';c=getchar()); 
...
for (;(c>='0' && c<='9');c=getchar())

and these for are more readable being while.

As signaled by @Basile Starynkevitch you can also use std::isdigit to check if c is a digit or not, you would not have had your mistake using it.

Warning, if you reach EOF in the first for the code loops without ending, it is more secure to add && (c != EOF) in it and for that to type c with int rather than char.

I also encourage you to no use #include <bits/stdc++.h>, see Why should I not #include <bits/stdc++.h>?

edited Jun 30 '19 at 14:53

answered Jun 30 '19 at 05:51

bruno

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1

Both are same... – Nick Jun 30 '19 at 05:56
@Nick what you mean ? with `c>47` the code runs as expected – bruno Jun 30 '19 at 05:57
1

@nick Note the switched `<`-`>` in `c>47` – user4581301 Jun 30 '19 at 06:02
Oh sorry thanks..silly mistakes – Nick Jun 30 '19 at 06:04
`!isdigit(c)` is better than `c < '0' || c > '9'` – Basile Starynkevitch Jun 30 '19 at 06:09
@BasileStarynkevitch I wanted to be closer to the original code, I edited my answer to speak about it, after all the OP would not have made the mistake using it ;-) – bruno Jun 30 '19 at 06:37
1

`c` should be an int. – Retired Ninja Jun 30 '19 at 14:29
@RetiredNinja you are right, I read wrongly the code and supposed it was already, I edit my answer – bruno Jun 30 '19 at 14:51

Why am I not getting desired output when using fast scan function?

1 Answers1