12 hour to 24 hour time conversion - c++

Question

I am trying to write this c++ program from hackerrank but in my output all I am getting is a blank space.
The input string is in the form of HH:MM:SSpp where HH is an hour on two digits with leading zero, MM the minutes, SS the seconds and pp is either AM or PM.
#include <bits/stdc++.h>
#include<iostream>
#include<string>
using namespace std;
string timeConversion(string s)
{
string p;
int i,j;
if(s[8]==80){ // checking if it is AM or PM
int x = s[0]*10 + s[1] +12;
int y = x%10;
int z = x/10;
s[0]= z;
s[1]= y;
for(i=0;i<10;i++){
p[i]=s[i];
}
}
string newt= p.substr(0, p.size()-2); //removing last two characters
return newt;
}
int main()
{
ofstream fout(getenv("OUTPUT_PATH"));
string s;
getline(cin, s);
string result = timeConversion(s);
fout << result << "\n";
enter code here
fout.close();
return 0;
}
Is there some logical error? I know the other approach for this question but it would be great if anyone could help me with it.

Answer 1

The problem is with treating character digits (e.g. s[0]) as integer digits.
If you are certain you are dealing with digits, the way to do arithmetic with the characters is by subtracting the value of the character '0', like so: s[0] - '0'. The result will be the integral digit.

Answer 2

The main problem
In your timeConversion() function, you define a string p, which is initialized by the string default constructor to "".
Now for AM times, you skip the if and go directly to string newt= p.substr(0, p.size()-2);, which on an empty p will just create an empty newt string. So you return an empty value. Every time !
For PM times, you go into the if to do some transformations. Unfortunately p[i]=s[i]; doesn't do what you think. In fact it is out of bound access in the empty p string. And in the end, the length of p will still be 0 which will cause an empty value to be returned (in the best case).
The start of a solution
Initialize p at construction:
string p=s;
The code will then immediately work for AM strings. For PM strings, you still need to take into account uv_ 's answer related to ascii vs. binary math.
Here the result:
string timeConversion(string s)
{
string p=s;
int i,j;
if(s[8]=='P'){ // checking if it is AM or PM
int x =(s[0]-'0')*10 + (s[1]-'0') +12;
p[0]= x/10 +'0';
p[1]= x%10 +'0';
}
return p.substr(0, p.size()-2); //removing last two characters
}
Note: this assumes that the entry format will always be valid, and no space will be used instead of a leading 0.
Important Note: This code will fail on hackerrank, because it transforms 12:15:00pm into 24:15:00 and not in 12:15:00. Furthermore 12:00:00am will be tranformed in 12:00:00 instead of 00:00:00. More on wikipedia. Online demo about how to address these special cases

Answer 3

This code will work considering all test cases, just added two more conditions.
string timeConversion(string s)
{
string ans=s;
if(ans[8]=='P')
{
int x = (ans[0]-'0')*10+(ans[1]-'0')+12;
//cout<<x;
if(x!=24)
{
ans[0]=x/10+'0';
ans[1]=x%10+'0';
}
}
if(ans[8]=='A')
{
int y=(ans[0]-'0')*10+ans[1]-'0';
if(y==12)
{
ans[0]='0';
ans[1]='0';
}
}
return ans.substr(0, ans.size()-2);
}