assigning char by char to c++ string

Question

I have a text file in which the data is stored in following format:-

aabb:aabb aacc:aacc aadd:aadd bbdd:bbdd bbaa:bbaa

I am reading from the file line by line and trying to separate the words on either side of ':'. I am taking each line into a string line. I am assigning the word left to ':', char by char in the string w and the word right to ':', char by char to string m. But the problem is the the string w and m become empty after both the loops are executed. Why are string w and m empty?

int flag; string line, w, m; ifstream fin; fin.open("files/file2.txt",ios::in); if (fin.is_open()) { while (getline(fin,line)) { for (int i=0; i<line.length(); i++) { if (line[i] == ':') { flag = i+1; break; } else w[i] = line[i]; } for(int i=flag,k=1; i<line.length(); i++,k++) { m[k] = line[i]; } cout<<w<<'\n'; cout<<m<<'\n'; } fin.close(); }

Thank you for your help.

why are string w and m empty?

Arjun
– Arjun

2018-01-12 18:58:19 +00:00
Commented Jan 12, 2018 at 18:58 — Arjun
– Arjun, Commented Jan 12, 2018 at 18:58
Try w += line[i]; and m += line[i];

Eljay
– Eljay

2018-01-12 18:58:22 +00:00
Commented Jan 12, 2018 at 18:58 — Eljay
– Eljay, Commented Jan 12, 2018 at 18:58

Sergey Kalinichenko · Accepted Answer · 2018-01-12 19:00:19Z

The problem with your code is that you are assigning characters to positions of m and w that do not exist yet: both strings are initially empty, so applying [] to them causes undefined behavior.

Since you are adding characters to the end of your strings, use append instead of []:

w.append(1, line[i]); ... m.append(1, line[i]);

or +=:

w += line[i]; ... m += line[i];

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