Just plain stuck

[GoPredsGo]

I have been working on this program for 3 days and keep getting the same one error code, I was hoping some fresh eyes could give my code a look and maybe give some advice on what to do. Please see code and error below:

// Friedmann Chapter 7
// This programs produces an output of a word in ICAO form


#include <iostream>
#include <string>
#include <iomanip>
#include <cctype>

using namespace std;

// Function prototype
void displayChar (char, char,char, int&);

int main()
{
string instring;
char letter;
char string [26];
int len, ipos;


// Prompt for word input

cout << "Please type a word you would like to convert to ICAO spelling" << endl;

// Input of word and output of ICAO Spelling

cin >> instring; // Read the instring
cout << "The ICAO spelling is: ";
len = instring.length();

ipos = 0;

while (ipos <= len-1)
{
letter = instring.at(ipos);
putchar (toupper(letter));

if (isalpha(letter) == 0)
cout << "Will skip character " << letter << endl;
else
displayChar(letter);
ipos = ipos + 1;


switch (letter)
{
case 'A':cout << "Alpha";
break;
case 'B':cout << "Bravo";
break;
case 'C':cout << "Charlie";
break;
case 'D':cout << "Delta";
break;
case 'E':cout << "Echo";
break;
case 'F':cout << "Foxtrot";
break;
case 'G':cout << "Golf";
break;
case 'H':cout << "Hotel";
break;
case 'I':cout << "India";
break;
case 'J':cout << "Juliet";
break;
case 'K':cout << "Kilo";
break;
case 'L':cout << "Lima";
break;
case 'M':cout << "Mike";
break;
case 'N':cout << "November";
break;
case 'O':cout << "Oscar";
break;
case 'P':cout << "Papa";
break;
case 'Q':cout << "Quebec";
break;
case 'R':cout << "Romeo";
break;
case 'S':cout << "Sierra";
break;
case 'T':cout << "Tango";
break;
case 'U':cout << "Uniforms";
break;
case 'V':cout << "Victor";
break;
case 'W':cout << "Whiskey";
break;
case 'X':cout << "X-ray";
break;
case 'Y':cout << "Yankee";
break;
case 'Z':cout << "Zulu";
break;
default : cout << " is not a valid letter";

}

ipos = ipos + 1;

}


cout << endl << endl;
cin.get();
cin.get();

return 0;

}








Error:

1>------ Build started: Project: Friedmann_Chap.7, Configuration: Debug Win32 ------
1> Friedmann_Chapter7.cpp
1>c:\users\friedmann\desktop\c++ projects\friedmann_chap.7\friedmann_chap.7\friedmann_chapter7.cpp(43): error C2660: 'displayChar' : function does not take 1 arguments
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Please help me! Thanks

[AbstractionAnon]

Line 13: You have a forward declaration for the function displayChar which says the function takes 4 arguments.
Line 43: You call displayChar passing only one argument. The call must match the forward declaration.

Also, the implementation of displayChar does not exist in your program.

PLEASE USE CODE TAGS (the <> formatting button) when posting code.