在 C++ 中显示不需要的数字

Question

Okay so I've been working on a school project with C++. I have to enter my name and when I do, the letters from my input shall be converted into integers. The letters have their own corresponding numbers from 1 to 26 for a to z accordingly. So I figured maybe I could use this:

int main(int argc, char** argv) {

    char lowletters[26] = {'a', 'b', 'c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
    char upletters[26]  = {'A', 'B', 'C', 'D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
    char fname[50], lname[50];
    int corrNums[26] = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26};

    cout << "Enter name: ";
    cin >> fname >> lname;

    for(int a = 0; a <= sizeof(fname)-1; a++){
        for(int b = 0; b < 26; b++){
            if(fname[a] == lowletters[b]){  
                cout<<corrNums[b]<<"\t";
            }
            if(fname[a] == upletters[b]){   
                cout<<corrNums[b]<<"\t";
            }
        }
    }

     for(int a = 0; a <= sizeof(lname)-1; a++){
        for(int b = 0; b < 26; b++){
            if(lname[a] == lowletters[b]){  
                cout<<corrNums[b]<<"\t";
            }
            if(lname[a] == upletters[b]){   
                cout<<corrNums[b]<<"\t";
            }
        }
    }

    return 0;   
}

So I used the first loop to get the characters from the fname and compare it to the lowercased and uppercased letters to find what letter it is. When it does, it would display the corrNums(corresponding numbers) with the value of b in the for loop so that it would match.

I run it, then at first glance I thought it display correctly. It does, but there are these numbers if you would notice at the end of the first and last name. Take a look:

Answer 1

The problem is that you use sizeof(fname) and sizeof(lname) (the -1 isn't particularly important here). sizeof() is a compile-time constant that does not correlate to the length of the input.

This is a problem is your uninitialized array has something in it that matches your upletters or lowletters array. Since you check the entire array, your program will print that also.

The solution is to only check the part of the array that has been imputed. You can do this with strlen() :

for(int a = 0; a < strlen(fname); a++){
    //...
}
//...
for(int a = 0; a < strlen(lname); a++){
    // ...
}

Of course, if you are allowed to use std::string (which many C++ classes do not), that is much easier:

std::string fname, lname;
//...
for(int a = 0; a < fname.size(); a++){
    //...
}
//...
for(int a = 0; a < lname.size(); a++){
    // ...
}

This is generally preferred because you can't overrun the buffer this way, but you can do what you want/what your professor wants you to do.

Answer 2

When you are working with char arrays you have to use strlen() . If you use std::string it will be automatically resized to fit your input with cin .

Your solution could be simplified by doing a bit of maths. No need for those arrays of letters and numbers. Just subtract the value of a char from 'a' or 'A' and add 1 to get the numerical value. Also you could do the printing inside a function to avoid duplicated code.

#include <iostream>
#include <string>

using namespace std;

void PrintName(std::string name)
{
    for (size_t a = 0; a < name.length(); a++)
    {
        char letter = name[a];
        if (letter >= 'A' && letter <= 'Z')
            cout << (letter - 'A' + 1) << "\t";
        else if (letter >= 'a' && letter <= 'z')
            cout << (letter - 'a' + 1) << "\t";
    }
}

int main(int argc, char** argv) {

    std::string fname;
    std::string lname;

    cout << "Enter name: ";
    cin >> fname >> lname;

    PrintName(fname);
    PrintName(lname);

    return 0;
}

If you have C++11 or above you can do range based looping instead, eg:

for (auto letter : lname)

Answer 3

I don't want to give a school assignment to you, but I'd be glad to point out a few helpful hints. Starting with: arrays are unnecessary when your array contents are mathematically reproducible:

• If ( fname[a]>='a' && fname[a]<='z' ) the matching index in both lowletters & corrNums is fname[a] - 'a'

• Likewise, if ( fname[a]>='A' && fname[a]<='Z' ) the matching index in upletters & corrNums is fname[a] - 'A'

• corrNums[i] = i+1, in every case

• You are producing the same results regardless of case, so you can use std::tolower to cut your checks in half