Substring

Learn to take part of a string with .substr().

The substr method

To take only part of a string you use the substr method. It is called as s.substr(start, len); start is the starting index and len is how many characters to take. Indices are zero-based.

For example std::string s = "Computer"; s.substr(0, 4) returns "Comp". If you omit the second parameter, s.substr(start) returns everything from the start index to the end of the string.

Single-parameter substr

If you pass only the start index to substr, you get the rest of the string from that index on. std::string s = "Programming"; s.substr(7) returns "ming".

This short form is handy for separating file extensions or suffixes. Negative indices are not supported in C++; the index must be between 0 and s.length().

Exercise Print the first four characters

The text variable is set to "Computer". Take the first four characters with text.substr(0, 4) into the part variable and print it with std::cout. The output must be Comp.

Show example solution

#include <iostream>
#include <string>

int main() {
    std::string text = "Computer";
    std::string part = text.substr(0, 4);
    std::cout << part << std::endl;
    return 0;
}

Test yourself

What does std::string s = "Computer"; s.substr(4, 3) return?
```
std::string s = "Computer";
s.substr(4, 3)
```
- "Com"
- "ute"
- "Comp"
- "uter"
Show answer

"ute" — From index 4 take 3 characters: "ute".

Write the substr start parameter

std::string s = "Welcome";
std::string part = s.substr(____, 3);

Show answer

std::string s = "Welcome";
std::string part = s.substr(0, 3);

Match each substr call to its result

s.substr(0, 3) on "Welcome"

"Wel"
s.substr(3) on "Welcome"

"come"
s.substr(0, 1) on "Welcome"

"W"
s.substr(4, 2) on "Welcome"

"om"

Next lesson: String search

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