find substring in a string with loops in c++

Question

The code will get two string from user and checks the string , includes substring as the second input or not.

    string st1;
    string subst1;
    string message = " ";
    cout << "Enter string and subst:";
    cin >> st1;
    cin >> subst1;

    for (int a=0; a < st1.length(); a++) {
        if (st1[a] == subst1[0]) {
            for (int k = 0; k < subst1.length(); k++) {
                if (st1[a + k] == subst1[k])
                    message = "True";
                else
                    message = "False";
            }
        }
    }
    cout << message;

This code does not work inputs like "alice" and "ba". The output should be false but when I execute the code program directly ended

If your code never evaluates str1[a] == subst1[0] as true then message will not get set to false and subsequently gets left as the single white space. It would work for say, Alice and Abba. — AntiElephant
– AntiElephant, Commented Oct 31, 2015 at 22:45

Jean-Claude Colette · Accepted Answer · 2015-10-31 23:33:41Z

1

Because in some cases a + k exceeds the length of the string st1:

if (st1[a + k] == subst1[k]) {
    message = "True";
}

before executing this statement, verify if a + k < st1.length()

but another remark: when message becomes False you must stop comparison else the variable message might be again True.

edited Oct 31, 2015 at 23:33

answered Oct 31, 2015 at 22:55

Jean-Claude Colette

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Comments

Christophe · Accepted Answer · 2015-10-31 23:07:39Z

0

Why not use find():

string st1, subst1, message=" ";
cout<<"Enter string and subst:";
cin>>st1>>subst1;
if (st1.find(subst)==string::npos) 
    message="Not found"; 
else message ="Found";

If you're not allowed to use this approach, then I propose you the following:

string st1, subst1, message=" ";
bool found=false; 
cout<<"Enter string and subst:";
cin>>st1 >>subst1;

for (int a=0; !found && a<st1.length();a++) {
    if (st1[a]==subst1[0]) {
        found = true;  // there's some hope 
        for (int k=0; found && k<subst1.length(); k++) {
           if (a+k>=st1.length() || st1[a+k]!=subst1[k])  // oops! might overflow or fail
                 found = false;   // this will also end the inner loop
        }
    }
}
message = found ?  "True":"False";
cout<< message<<endl;

The principle is that for a compare to be successfull, all chars must be equal. But if a single char fails, you shall stop the failed comparison.

Here a live demo

edited Oct 31, 2015 at 23:07

answered Oct 31, 2015 at 22:48

Christophe

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2 Comments

karacaoglanb Over a year ago

I found this solution but I want to code the solution with using loops

Christophe Over a year ago

@moreeff I supspected it, so I've edited to give also a loop approach. The trick for the compare to be successfull is that ALL chars must be equal, but the first char that is different means it failed.

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