I think you maybe misunderstand the meaning of index and size. There are some errors in your code, please see my comments below. I attached my code, which is base on yours. Please refer to it.

for (int k = 0; k < rowsb; k++)
{
sum = sum + matrixa[i * columnsa + k] * matrixb[k * columnsb + j];
matrixc[i * finalcolumn + k] = sum; //move this line out of this loop then use j replace k,...

Sorry, my English is not so good. I mean that using

one_dimensional_array[i*column + j]

instead of

two_dimensional_array[i][j]

for (int i=0; i<rowsa; i++)
{
for (int j=0; j<columnsb; j++)
{
float sum = 0.0;
for (int k=0; k<columnsa; k++)
{
//first, look it as a two-dimensions array,...

I have finished it, but to don't violate the rules of this website --"Don't give away code" , I can not give you all my source code. The following is the framework of program. hope it can help you.
...

The various combinations of the flags and the corresponding meaning as below:

ios_base Flags Meaning
in Reads (file must exist)
out ...

int len = strlen(sentenceHolder1);
for(int i = 0; i < len; i++)
{
if( sentenceHolder1[i] == look_for_character )
sentenceHolder1[i] = sentenceHolder1[0]; //I think this will be OK
}...

Could you post your source code.