Word boundaries and reliable text tests

Define a word before counting one

Here a word is a maximal consecutive group of non-space characters. Only the ordinary space character separates words; tabs are outside this exercise’s format. Repeated, leading, and trailing spaces do not create extra words.

Counting spaces and adding one fails on an empty line or repeated spaces. Instead, count transitions into a word. Start insideWord as false. A space sets it false; a non-space while outside increments the count and sets it true.

#include <iostream>
#include <string>

int main() {
    std::string line;
    std::getline(std::cin, line);
    int words = 0;
    bool insideWord = false;
    int n = static_cast<int>(line.size());
    for (int i = 0; i < n; ++i) {
        if (line[i] == ' ') {
            insideWord = false;
        } else if (!insideWord) {
            ++words;
            insideWord = true;
        }
    }
    std::cout << words << '\n';
    return 0;
}

A neighbour rule describes the same event

Index i begins a word exactly when line[i] is not a space and either i == 0 or line[i − 1] is a space. The short-circuit order ensures that index −1 is never accessed.

The state-flag solution remembers the previous situation, while the neighbour solution reads it from the sequence. Both count starts, not every non-space character. Both scan the line once.

Draw the smallest failing sequence

For an array or string failure, write the active length and every valid index. Calculate i − 1, i + 1, and n − 1 − i explicitly. A program that does not crash can still access invalid or inactive cells.

Test empty input when allowed, one character, repeated characters, a match at index zero, a missing target, and leading or repeated spaces. For arrays, include all-negative extrema and tied maxima. For matrices, include odd/even dimensions and nonsquare shapes.

Compile with C++23 and warnings. Where supported, address and undefined-behavior sanitizers can expose invalid operations on executed paths, but do not replace reasoning about bounds. A warning about signed and unsigned sizes deserves an explanation, not a random cast.